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Extron Glossary of Terms
Audio

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A/D - Analog to Digital (converter). A device that converts an analog signal to a digital value.

AC coupled - A circuit design that does not pass the DC component of a signal, therefore it ignores DC offsets.

AC-3 - See "Dolby® Digital."

Active crossover - A circuit that separates the audio signal into the appropriate frequency bands for the woofer, midrange, and tweeter. An active crossover is placed in the signal path ahead of the amplifier, where a passive crossover is placed between the amplifier and the speaker.

ADC - Analog to Digital Converter. A device that converts analog signals to digital signals.

AES3 - A digital audio standard defined by the Audio Engineering Society. The standard specifies several basic physical interconnections between devices:
- Balanced - 3-conductor, 110 ohm cabling with an XLR connector, typically referred to as “AES/EBU audio.”
- Unbalanced - 2-conductor, 75 ohm coaxial cable with an RCA connector, typically used in consumer audio applications. In many consumer products such as DVD players and A/V receivers, this is often referred to as a “digital coaxial” connection type.
- AES-3id - A professional version of the 2-conductor 75 ohm coaxial cable terminated with a BNC connector. AES3 unbalanced and AES-3id audio can be switched or routed using a video switcher with a minimum of 150 MHz (-3 dB, fully loaded) video bandwidth.
- Optical – Plastic optical fiber using an F05 style connector, typically used in consumer audio applications. In many consumer products, this is often referred to as a “digital optical” connection type. TOSLINK is the most common implementation of this connection type.

ALC - Automatic Level Control. In audio recording, a circuit used to control the volume or level of the recorded signal automatically without distortion due to overload. Sometimes called Automatic Gain Control (AGC), or Automatic Volume Control (AVC).

AM - Amplitude Modulation. A method of radio transmission, by which the information part of the signal causes the amplitude of a carrier frequency to vary without affecting the frequency.

Ambient Sound List - Any environmental or background sound that exists before a new sound source is added. For example, in a school classroom, ambient sound may come from an adjacent hallway or playground, HVAC system, room lights, or another classroom. Ambient sound must be taken into consideration when designing a sound support system.

Amp - Ampere. The international base unit of electrical current that represents the rate flow of electric charges through a conductor. Symbolized by “A.” 1 amp is equal to the steady current produced by 1 volt applied across a resistance of 1 ohm.

Amplifier - An electronic device used to increase the voltage amplitude of a signal.

Amplifier classifications - Audio amplifiers are typically described by “class”. There are four primary classes used in A/V system designs: A, B, A/B, and D.

Amplitude - The level or strength of a signal as measured by the height of its waveform. Electronic waveforms can be displayed and measured on an oscilloscope.

Amplitude Modulation – AM - Amplitude modulation is also employed in fiber optics applications, in which light acts as a carrier signal with its amplitude varying in accordance to the signal being conveyed.

Analog - A continuously varying action or movement that takes time to change from one position to another. Standard audio and video signals are analog. An analog signal has an infinite number of levels between its highest and lowest value (unlike digital, in which changes are in steps).

Analog control - A method using continuously varying voltage levels to provide control of equipment.

Anechoic chamber - An acoustic space without echo or reverberation. Often used for the acoustic testing of microphones and loudspeakers.

Artifacts - Any error in the perception or representation of any visual or aural information introduced by the involved equipment. Image artifacts appear as deviations from the original in the delivered image in video streaming systems.

Asynchronous - Intermittent, not synchronized or continuous. A conversational type of communication that allows the parties at each end to talk when they like instead of at a prescribed time. Used in videoconferencing.

ATM - Asynchronous Transfer Mode. A standardized digital data transmission technology that is a cell-based switching technique which uses asynchronous time division multiplexing. This is the core protocol used over the SONET/SDH backbone of the ISDN (Integrated Services Digital Network).

Attenuate/Attenuation - To reduce the amplitude (strength) or current of a signal.

Audio - Of or concerning the electronic transmission of sound, specifically the electrical currents representing a sound. (CF. Sound)

Audio follow - A term used when audio is tied to other signals, such as video, and they are switched together. The opposite of “breakaway.”

Audio frequency - Frequencies within the range of human hearing, about 20 Hz to 20 kHz.

Audio summing amplifier - A device that converts two-channel stereo audio signals into balanced/unbalanced one-channel mono audio signals.

Auto-input switching - The feature that enables a product to detect which input has an active sync signal and switch to that input.

AWG - American Wire Gauge. A standard measurement for wire conductor diameter.

Balanced audio - The audio signal that is carried on three wires (or five wires for stereo pair), with two of them carrying the same signal but with reversed polarity, and a third wire for shielding. Since the two signal wires would pick up virtually identical noise from outside (common mode noise), and that noise can be canceled out at the receiving end by a differential amplifier, the balanced audio is much less susceptible to hum and interference from long cable runs.

Band pass filter - A filter that allows a specific range to pass. The bandpass frequencies are normally associated with frequencies that define the half power points, i.e. the -3 dB points. In multi-driver speaker systems, for example, the mid-range driver may be fed by a bandpass filter.

Band reject filter - A filter that combines the characteristics of a low pass and a high pass filter that is used to block a narrow band of frequencies, while allowing frequencies above or below this band to pass. (i.e. notch filter)

Bandwidth - The total range of frequencies required to pass a specific signal without significant distortion or loss of data. In analog terms, the lower and upper frequency limits are defined as the half power, or -3 dB signal strength drop, compared to the signal strength of the middle frequency, or the maximum signal strength of any frequency, expressed as xx Hz to xx kHz (or MHz) @ -3 dB. In digital terms, it is the maximum bit rate at a specified error rate, expressed in bits per second (bps). A device's bandwidth should be wider than the highest possible bandwidth of the signals it may handle. (In general, the wider the bandwidth, the better the performance. However, bandwidth that is too wide may pass excessive noise with the signal.)

Baseband - A prime signal such as composite video, component video, and audio with its own path but is not modulated onto a carrier signal or combined with other signals on a path. An unmodulated signal or band of signals. The video signal seen on a waveform monitor is a baseband video signal.

Binary - A numbering system using base-2. Each digit is represented by a 1 or a 0 (on or off).

Binary code - A coding system using the digits 0 and 1 to represent a letter, numeral, or other character in a computer. For example: the character “A” in ASCII code becomes 0100 0001 in binary.

Bit - The shortened form of “binary digit” (0 or 1). A bit is the smallest unit of information in a computer.

Bit depth - The number of bits per pixel. Bit depth determines the number of shades of gray or variations of color that can be displayed by a computer monitor. For example, a monitor with a bit depth of 1 can display only black and white; a monitor with a bit depth of 16 can display 65,536 different colors; a monitor with a bit depth of 24 can display 16,777,216 colors

Bit Rate - The number of bits that are conveyed or processed per unit of time. The bit rate is quantified using the bits per second (bit/s or bps) unit, often in conjunction with an SI prefix such as kilo- (kbit/s or kbps), mega- (Mbit/s or Mbps), giga- (Gbit/s or Gbps).

Blu-ray Disc - An optical disc storage medium developed by Sony as the replacement for DVD. Blu-ray is capable of storing high-definition video, audio, and data with a capacity of 50GB per disc. Blu-ray players are backward-compatible with standard DVDs and audio CDs.

Boot/Boot-up/Bootstrap - The initialization process a system goes through after power comes on. It may also occur as part of resetting. To start a new beginning, you “pull yourself up by your bootstraps.”

Breakaway - The ability to separate audio and video signals for switching them independently. For example, the audio and video signals from one source may break away and be switched to two different destinations. This is the opposite of the term “audio follow.”

Bridging (or Bridged) Audio - Some stereo amplifiers are designed to allow “bridging” or combining the power output of two channels into one channel. Bridging allows the amplifier to drive one speaker with more power than the amp could produce for two speakers. Not all amplifier designs allow bridging, however. NEVER attempt bridging of an amplifier without first consulting the manufacturer’s documentation and instructions.

Buffer - 1) In electronics, a circuit such as a unity gain amplifier used to isolate the signal source from the load. A buffer can be used for digital or analog signals. 2) In digital systems, a region of memory used to temporarily hold data while it is being delivered from one process to another.

Bus - A path for transporting voltages, signals, or a ground between the different sections of an electronic device, such as a data bus between a CPU and memory or a peripheral device. Its width is determined by the number of lines (conductors) that make up the bus, and its speed (data transfer rate) is determined by the circuits that drive the lines.

Butterworth filter - A filter characterized by smooth response at all frequencies and -6 dB per octave decrease from the specified cutoff frequencies. Butterworth filters are maximally flat; that is, they pass the selected band of frequencies (the pass band) without distortion.

Capacitance - The ability to store an electrical charge.

Captive screw connector - A connector that uses a screw to hold a stripped wire end.

CAT 5 - Category 5. Describes the network cabling standard that consists of four unshielded twisted pairs of copper wire terminated by RJ-45 connectors. CAT 5 cabling supports data rates up to 100 Mbps. CAT 5 is based on the EIA/TIA 568 Commercial Building Telecommunications Wiring Standard.

Class A - The output transistor(s) always have current flowing through them. This method of operation is pure, but inefficient. Class A amplifiers rarely exceed 20% efficiency in terms of power consumed (converted to heat) versus power delivered to the load.

Class A/B - A combination of Class A and Class B amplifier designs that corrects the inefficiency of Class A amplifier designs and allows a small amount of current to continually flow through the output transistors at all times. This alleviates most of the cross-over distortion at the expense of efficiency. An A/B amplifier is still more efficient (60 to 65%) than a Class A amplifier.

Class B - Somewhat more efficient than Class A. Class B amplifiers utilizing two drive elements operating in a push-pull configuration. On the positive excursion of the signal, the upper element supplies power to the load while the lower is turned off. During negative going signal excursions, the opposite operation occurs. This increases operating efficiency, but the exchange from ON to OFF causes a switching error condition commonly called cross-over distortion.

Class D - Also known as a switching amplifier, Class D amplifiers utilize output transistors which are either completely turned on or completely turned off (they’re operating in switch mode). Class D amplifiers operate either in the fully ON-region or fully OFF-region. Class D amplifiers reach efficiencies as high as 90%. This is of great importance to portable applications relying on battery power and for the lowest production of heat.

Clipping - Cutting off the peaks (or excursions) of a signal. A form of distortion that occurs when the signal excursions exceed the limits of the circuit.

Clipping level - An electronic limit to avoid overdriving an audio or video signal.

Clock rate - The rate at which analog audio is sampled and converted to a digital signal. Clock rate is important in digital audio recording and processing systems. When samples are not output at the correct time relative to other samples, a condition called “clock jitter” occurs. Clock jitter can also arise when digital audio is run through several audio products. When each product runs on its own clock, compensating for small differences between the clocks can cause output errors. For instance, even if both clocks are at exactly the same frequency, they will almost certainly not be in phase. If the clock rate of the input digital stream and the playback unit differ (44.1 KHz and 48 KHz, for instance), the playback unit has no choice but to perform a sample rate conversion. If they are the same, the playback unit may use sample rate conversion to oversample the input (for example, 88.2 or 96 kHz), then pick the samples that “line up” with its own clock.

CMR - Common Mode Rejection. A measure of how well a differential amplifier rejects a signal that appears simultaneously and in phase at both input terminals. As a specification, CMR is expressed as a dB ratio at a given frequency.

CMRR - Common Mode Rejection Ratio. (1) For a differential amplifier, the ratio of the differential gain to the common mode gain. (2) Expressed in dB, it is the ratio of common mode input voltage to output voltage. (3) For an operational amplifier, the ratio of the change in input offset voltage to the change in common mode voltage.

Coaxial cable - A two-conductor wire in which one conductor completely wraps the other, with the two separated by insulation. Constant impedance transmission cable. Example: 75 ohm, type RG-59 cable used for video signals. Abbreviated as coax.

Coaxial speaker - A type of speaker design in which a high frequency driver (typically a tweeter) is placed inside a low or mid frequency driver.

CobraNet - A trademark of Peak Audio, CobraNet is network technology for the transmission of digital audio, video, and control signals over 100Mbps Ethernet networks.

Codec - (1) Coder/decoder. A device that converts analog video and audio signals into a digital format for transmission over telecommunications facilities and also converts received digital signals back into analog format. It may also dial up the connection, like a modem for teleconferencing. (2) Compressor/decompressor. Codecs can be implemented in software, hardware, or a combination of both. Some popular codecs for computer video include MPEG, QuickTime, and Video for Windows.

Compression - The art and science of reducing the amount of data required to represent a picture or a stream of pictures and sound before sending or storing it. Compression systems are designed to eliminate redundant or repeated information to the desired data level while allowing the original information to be reproduced to the desired quality.

Compression (Audio) - Compression is commonly used to keep mic levels within an acceptable range for maximum intelligibility. Though a compressor effectively makes louder portions of a signal softer, it is used to make softer sounds louder. This is achieved by reducing the dynamic range, then raising the output level of the compressor (referred to as "make-up gain"), or by increasing the input signal, then preventing clipping by reducing the louder portions of the signal. Compression is also used to protect a system or a signal chain from overload.

Compression artifacts - Compacting of a digital signal, particularly when a high compression ratio is used, may result in small errors when the signal is decompressed. These errors are known as artifacts, or unwanted defects. The artifacts may resemble noise (or edge busyness) or may cause parts of the picture, particularly fast moving portions, to be displayed as distorted or incomplete.

Compressor - A compressor regulates the level of an input signal by reducing or compressing its dynamic range above a user-defined threshold. Also see "Compression (Audio)."

Constant voltage system - The common name given to the interface between amplifiers and speakers in a distributed audio system. Several voltages are used, but the most common are 70.7 V (commonly shortened to 70 V) in the US, and 100 V in Europe. “Constant voltage” refers to the characteristic that whether the total output of the amplifier is 5 watts or 50 watts or 500 watts, the maximum output voltage is always a constant of 70.7 V. The voltage stays the same regardless of the load, so the output current varies but not the voltage.

Contact closure - The momentary connection of two conductors to complete an electrical circuit. Often used to switch inputs on switchers.

Continuous power - The continuous power specification can be used to describe the output of an amplifier and is typically stated at “x watts (rms) into y ohms from 20 Hz to 20K Hz at z% THD (total harmonic distortion)”. For example, the Extron MPA 122 amplifier is specified as 11 watts (rms) per channel into 4 ohms at 1% THD.

Crest factor - The ratio of peak value of a signal divided by the rms value of the signal. The crest factor of the audio program determines the required headroom needed in the audio system.

Crossover distortion - A type of distortion that occurs in push-pull class AB or class B amplifiers. It happens during the time that one side of the output stage shuts off, and the other turns on.

Crossover network - An electrical circuit that combines high pass, low pass, and bandpass filters to divide the audio frequency spectrum, 20 to 20,000 Hz, into ranges suitable for low frequencies (woofer), mid-range, and high frequencies (tweeters).

Crosstalk - Caused by interference between two signals, usually from an adjacent channel, which adds an undesired signal to the desired signal. Crosstalk is caused by magnetic induction or capacitive coupling, and can occur when there is a grounding problem or improper cable shielding. Video symptoms include noise and ghosting, while audio symptoms include signal leakage.

Current - The flow of electricity, and the rate at which it flows. Also see "Amp."

D/A - Digital to analog.

DAC - Digital to analog converter.

Damping factor - The measurement of a power amplifier’s ability to control the motion of a speaker’s cone after a signal disappears. The higher the number, the better the damping factor.

DAT - Digital Audio Tape. A method developed by Sony and Hewlett-Packard for recording large amounts of information in digital form on a small cassette tape.

Data Compression - A mathematical algorithm for compressing or encoding data to fit within given bandwidth requirements for transmission or storage.

Data Compression Ratio - The ratio representing the data output from a compression system relative to the original data. A computer-science term used to quantify the reduction in data-representation size produced by a data compression algorithm.

dB (Decibel) - The standard unit used to express gain or loss of power between two values. A decibel is 10 times the logarithm of a ratio of two power values. When comparing voltage or pressure, the values in the ratio are squared or the log is multiplied by 20 instead of 10. An extension is placed behind the ‘dB’ when one of those values is a fixed reference (i.e. dBV, dBu, dBSPL).

dB per octave - How quickly a crossover or filter attenuates signals (decreases their power) outside its passband (those frequencies intended to pass through without attenuation); expressed in decibels per octave. Crossover and filter slopes are designed as first order (attenuates signals slowly, cutting output by 6 dB per octave); second order (12 dB per octave); third order (18 dB per octave); and fourth order (24 dB per octave). The steeper the slope the quicker the attenuation.

dBm - dB referenced to 1 milliwatt. To convert into an equivalent voltage level, the impedance must be specified. For example, 0 dBm into 600 ohms gives an equivalent voltage level of 0.775V, or 0 dBu; however, 0 dBm into 50 ohms, for instance yields an equivalent voltage of 0.24 V. Since modern audio engineering is concerned with voltage levels, as opposed to power levels in the early years of telephone, the convention of using a reference level of 0 dBm is academic. But in the A/V industry, many people still refer to 0.775Vrms (600 r) as 0 dBm, which should be more accurately called 0dBu.

dBSPL - dB referenced 20 micro pascals (0.00002 PA). 0dBSPL is a scale used to express acoustic energy, that is as loud as sound is. For example, when a sound is described as being “110 dB,” the measurement is expressing the sound pressure level of the source. Benchmarks include: 30-40 dBSPL - ambient room noise; 50-70 dBSPL - normal conversation; 110-120 dBSPL - rock concert; 130-140 dBSPL - painful sound.

dBu - dB unterminated. 0 dBu is a voltage reference point equal to 0.775Vrms. [This reference originally was labeled dBv (lower case) but was too often confused with dBV (upper case), so it was changed to dBu (for unterminated).] +4 dBu is a standard pro audio voltage reference level equal to 1.23Vrms. XLR and captive screw audio connectors are commonly used in this equipment.

dBV - dB referenced to 1.0 Vrms. -10dBV is a standard audio line level for consumer and some professional audio use, equal to 0.316 Vrms. RCA audio connectors are a good indicator of units operating at -10 dBV levels.

DC - Direct Current. The flow of electrons in one direction.

DC coupled - A circuit that passes both AC and DC components of a signal, and therefore is sensitive to DC offsets. Also see "AC coupled."

DC offset - Refers to the degree to which a DC voltage is skewed away from a zero or baseline value.

Decoder - 1) In analog video, a device used to separate the RGBS (red, green, blue and sync) signals from a composite video signal. Also known as an NTSC decoder. 2) In digital systems, a device which does the reverse of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode. Video over IP decoders accept IP data streams and output an analog or digital video signal. 3) In control systems, the device in a synchronizer or programmer which reads the encoded signal and turns it into a form of control.

Delay - A basic DSP process in which the output of the input signal is delayed by a specified time (called the delay time).

Dielectric - Insulating material in coaxial cables between center conductor and outer conductor.

Differential audio - See "Balanced audio."

Digital - A system of data or image values in the form of discrete, non-continuous codes, such as binary. When data is in a digital format, it can be processed, stored (recorded), and reproduced easily while maintaining its original integrity.

Digital signal - An electrical signal which possesses two distinct states (on/off, positive/negative); typically represented by “0” or “1”.

Digitization - The transformation of an analog signal into digital information.

DIP switches - Small switches that are used to change settings on printers, computers, interfaces, switchers, modems, etc. They are designed to fit in a DIP (Dual Inline Package) space on a circuit board.

Dither - (1) The process of filling a gap between two pixels with another pixel having an average value of the two to minimize the difference or add detail to smooth the result. (2) In audio, a process that deliberately adds a tiny amount of noise to a signal in order to mask unwanted sounds introduced when the signal’s original bit depth is reduced. Dithering is recommended when transferring audio to a device that uses a lower bit depth.

DMM - Digital Multimeter. A test and measurement device, typically handheld, that combines measurement tools for voltage, amperage, resistance, and other common electrical and electronic measurement needs.

Dolby noise reduction - A patented noise reduction technique from Dolby Labs that raises the volume of sound track elements most likely to be affected by inherent noise during recording and then lowers them again during playback so that the noise seems lower in relation to the wanted elements of the audio recording.

Dolby® Digital - A digital audio encoding and decoding technology utilized for DVDs, Blu-ray Discs, video games, and many cable and satellite television services. Also referred to as “AC-3.” Dolby Digital can transmit mono or standard two-channel stereo audio, as well as 5.1 channel surround sound (left front, center front, right front, left rear, right rear, and sub-woofer).

Dolby® Digital Plus - A digital audio compression technology designed as an optional codec for use with Blu-ray Disc. Dolby Digital Plus is an extension of the earlier Dolby Digital format and supports up to 13 audio channels, although Blu-ray Disc is limited to 8 discrete channels. The extra audio channels are often used to support multiple languages.

Dolby® TrueHD - An advanced, lossless multi-channel audio encoder and decoder technology intended primarily for high-definition content and is optional for Blu-ray Disc; support for TrueHD is also optional in the HDMI 1.3 specification. TrueHD supports up to 8 discrete audio channels at 96 kHz sampling, or up to 6 channels at 192 kHz sampling. Since TrueHD is optional for Blu-ray Disc, discs encoded with a TrueHD audio track must also include a separate 2-channel digital audio track.

DRM – Digital Rights Management - A generic term for technologies such as content scrambling in cable or satellite television transmission, HDCP, and DPCP that can be used to control the access to, or reproduction of, copyrighted, commercially-available content. DRM is used primarily to prevent piracy, the unauthorized duplication and distribution of copyrighted material. However, DRM often also governs how content can be used. Commercially-available DVDs and Blu-ray Discs, for example, are typically licensed for personal use in a residential environment. Use of such content in a public venue, such as a school or business setting, without express consent or licensing by the copyright holder, is typically in violation of the media’s license.

DSP - Digital Signal Processor. A specialized CPU or circuit designed to process signals such as audio and video which have been converted to digital form. DSP is used to process sound, video, and images in a variety of ways.

DTS® Digital Surround - A digital audio encoding and decoding technology from DTS, Inc. that delivers 5.1 channels of surround sound. It is an optional surround sound format for DVDs but is mandatory for Blu-ray Disc. DTS Digital Surround has also been used in some LaserDisc releases as well as CDs, and is also featured in some video games.

DTS-HD High Resolution Audio - An extension to the DTS Digital Surround format that offers up to 7.1 channels at 24-bit resolution and 96 kHz sampling. DTS-HD High Resolution Audio is an optional surround sound format for Blu-ray Disc.

DTS-HD Master Audio - A lossless audio encoder/decoder technology from DTS, Inc. DTS-HD Master Audio allows a bit-for-bit representation of a movie’s original studio master soundtrack and supports up to 8 audio channels. Support for DTS-HD Master Audio is optional in the HDMI 1.3 specification released in 2006, and is also optional for Blu-ray Disc.

DVD-Audio - A digital format for delivering high-fidelity audio content on DVD – Digital Video Discs. DVD-Audio is a standalone format intended for audio only and is not used for the audio portion of DVD video content. DVD-Audio is similar in application to SACD, although to maintain compatibility with DVD players, the format is not capable of the very high sampling rates found in SACD. Support for DVD-Audio was added to the HDMI 1.1 specification in 2004.

Dynamic range - The highest and lowest potential signal levels on a given device. Also applies to fiber optic applications in terms of the ratio between the most – or strongest – and least – or weakest – observable optical signals.

Dynamic transducer - Technical description of a loudspeaker or dynamic microphone. A dynamic transducer uses a paper, plastic, fabric, or metal cone which is driven by a voice coil that moves back and forth through a magnetic field produced by the audio signal.

Echo cancellation - A DSP technique that filters unwanted signals caused by echoes from the main audio source.

Echo suppression - Used in telephone networks to reduce the impact of echo and thereby improve voice quality. The impact of echo suppression is usually to limit data transmission to one direction at a time. In order to provide for duplex operation, modems request telephone systems to turn off echo suppression.

Edge-to-edge - Describes common density factors in loudspeaker layout designs. Edge-to-edge density places the speakers such that the outside edges of their sound cones just touch one another.

Embedded data - Digital technologies such as SDI, HD-SDI, and HDMI, can carry variety of other data along with the primary video data, including audio, control, metadata such as content title or other identifying information, or other ancillary information. These data are said to be embedded, as they travel with the primary signal from the source device to the destination.

EMI - Electromagnetic Interference. Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics/electrical equipment.

Encryption - To manipulate information into a coded form that cannot be read without a device that will unscramble the code. Video signals are also scrambled in cable and pay-TV systems so that the viewer must pay to receive the program after it has been electronically deciphered.

EQ (Audio) - Equalizer. An audio signal processor used to add or attenuate frequencies in order to change the character of the resulting sound. EQ’s are properly used with a reference signal (see pink noise) and a real- time analyzer to ensure that the sound from the speakers closely matches the original sound.

Error Concealment - A method of concealing and hiding the impact of data lost during transmission. In video streaming systems, error concealment prevents lost network packets from disrupting a video frame or sequence of video frames.

Error Correction - A method of detection errors and reconstructing the original information using extra, redundant information sent along with the original data.

Error detection and correction - The ability to detect errors caused by interference or other factors during the transmission of a signal, and then reconstruct the received signal so that it is a faithful reproduction of the original signal, without errors. A process known as Forward Error Correction is often used to allow the receiver to detect and correct some errors without having to “ask” the transmitter to send additional data.

Error propagation - A single error experienced, produces a knock on effect to sequential information. In video streaming solutions decoding products should provide a method by which a single error encountered effects only a small area of a picture and should not affect an entire frame or sequential frames of video.

Ethernet - A Local Area Network (LAN) standard officially known as IEEE 802.3. Ethernet and other LAN technologies are used for interconnecting computers, printers, workstations, terminals, servers, etc. within the same building or campus. Ethernet operates over twisted pair and over coaxial cable at speeds starting at 10Mbps. For LAN interconnectivity, Ethernet is a physical link and data link protocol reflecting the two lowest layers of the OSI Reference Model.

Far end - In videoconferencing, the party or group you are connecting to at the distant site.

Farad - Unit of measurement for capacitance which stores one coulomb of electrical charge when one volt is applied. More commonly, stated in picofarads, or one-millionth of one-millionth (10e-12) of a farad.

FCC - Federal Communications Commission. The US governmental agency that controls and makes all policy for the use of broadcast airwaves.

Feedback - (also known as Larson effect) The phenomenon where the sound from a loudspeaker is picked up by the microphone feeding it, and then reamplified again and again. The resulting loop between audio input (the microphone) and audio output (the loudspeaker) results in a scream or squeal emitted from the loudspeaker. Feedback is determined by the resonant frequencies in the amplifier and loudspeakers, the acoustics of the room, the design of the microphone, and the relative positions of the microphone and loudspeaker.

FFT - Fast Fourier Transform. Converts analog waveforms into a form that can be easily analyzed for DSP applications.

Filter - In general, a filter accepts the desired and rejects the undesired. Every filter has a specific purpose. In electronics, for example, if you have some high frequency noise mixed with the signal that you want, then a lowpass filter is used to pass the signal and reject the (high frequency) noise. In software, a filter allows the application to open a file of a specific format. Also see "Band pass filter", "High pass filter", and "Low pass filter."

FireWire™ - A trademark of Apple. Also known as 1394 or IEEE-1394. A data communication standard used primarily with digital camcorders, 1394 FireWire manages the data transfer and tape transport control processes when transferring DV (digital video) to your computer or DV editing system. FireWire supports data transfer rates of 100 to 400 Mbps.

Flat (response) - A theoretical ideal for audio components, especially speakers, that represents a frequency response that does not deviate from a flat line over the audible frequency spectrum. A flat response, though ideal, is impossible in real world listening due to the speaker itself and its interactions with the room and various surfaces within the room. All speakers will fluctuate above and below the ideal flat response, but speakers that stay within two or three dB of a flat response are considered very linear and very nearly flat in their response.

Fletcher-Munson effect - Also referred to as “equal loudness contours.” Fletcher and Munson, researchers at Bell Labs, first measured the sensitivity of human hearing at various volumes and frequencies. Fletcher and Munson found that human hearing is dependent on loudness, and that the ear is most sensitive in the range of 3 kHz to 4 kHz. Sensitivity falls off rapidly at lower frequencies and somewhat more slowly at higher frequencies. Sounds in frequencies below and above this range need to be louder (more powerful) in order to be heard clearly. The loudness control found on audio reproduction systems is designed to compensate for the Fletcher-Munson effect.

Flutter - Pitch variations heard as a fast wavering or wobbling caused by an audiotape or CD moving at varying speeds. Also see "Wow."

FM - Frequency Modulation. A method of combining an information signal with a carrier signal so that it may be transmitted. FM radio is frequency modulated. Audio is encoded on the carrier by varying the frequency in response to the audio.

Frequency - The number of times a particular event happens per a given time. In A/V, the number of complete cycles per second of a musical tone or electronic signal, expressed in Hertz (Hz).

Frequency Division Multiplexing – FDM - The combining of two or more signals into a single carrier signal for transmission through FM – frequency modulation. Each signal modulates the carrier signal at a different region of the frequency spectrum.

Frequency domain - The means of representing a signal as a plot of amplitude (normally on the vertical axis) versus frequency (normally on the horizontal axis). A spectrum analyzer represents signals in the frequency domain. Also see "Time domain."

Frequency range (audio) - The range of frequencies between high and low end points; for example, in audio, the frequency range of the human ear is said to be 20 Hz to 20,000 Hz. Individual speaker elements like woofers, midranges and tweeters serve different frequency ranges within the overall audio frequency range.

Frequency response - The frequency range over which signals are reproduced within a stated amplitude range. Generally expressed in Hz vs. dB. For example: 100 - 5000 Hz +/- 3dB means that the device can handle a frequency range of 100 to 5000 Hz with a possible deviation in amplitude within that frequency range of +3 to -3dB.

Full duplex - The ability of a device or line to transmit data simultaneously in both directions.

Full Duplex Operation - When data is being both sent and received simultaneously. (i.e., sound cards, network interface cards, hubs)

Fundamental rejection - Usually expressed in dB, the amount by which a total harmonic distortion (THD+N) analyzer rejects the fundamental component of the input signal. The lowest measurable distortion of THD+N analyzer is limited by the fundamental rejection, along with several other attributes. Also see "THD" and "THD+N."

Gain - (1) A general term for an increase in signal power or voltage produced by an amplifier. The amount of gain is usually expressed in decibels above a reference level. Opposite of attenuation. (2) The amplification of a signal, unit, or system. Expressed in the unit of measurement appropriate to the signal or system. (3) In fiber optics applications, the measurement of back reflections using an OTDR - Optical Time Domain Reflectometer, due to a mismatch in core sizes between adjoining optical fibers.

Graphic equalizer - An active electronic device which allows the adjustment of narrow ‘bands’ of audio frequencies to correct for recording and listening room deficiencies. Graphic equalizers generally have sliding levers, known as faders, representing the different ranges of frequencies between 20 and 20,000 Hz. (See EQ-Audio)

Ground - Electrical connection of a circuit to a point designated as having zero potential.

Ground loop - A potential system grounding problem that may produce symptoms that appear as sync noise and cause a horizontal bar (hum bar) to roll vertically on the video image. A ground loop occurs when some devices in a system are not connected to the same electrical ground as the other devices. This can create a voltage potential difference between ground on the pieces of equipment.

Group delay - The amount of delay or rate of phase shift with respect to frequency though a device or cable.

Half duplex - Data or audio transmission that can occur in two directions over a single line, but only one direction at a time.

Harmonics (in music: overtones) - Multiples of an original frequency that add to and modify the original frequency. A pure sine wave is free of harmonics. When harmonics occur in electronic signals, it adds distortion to the original signal, causing undesirable results.

Hertz (Hz) - A unit of frequency; describing the number cycles per second. 1Hz = 1 cycle/second…1MHz = 1 Million cycles/second

High fidelity - Hi fi, accurate, and faithful reproduction of the original. Absence of distortion or enhancements.

High impedance - Hi Z or high Z. A relative term that is different for each application. In video, when the signal is not terminated it has a Hi Z load. Hi Z is typically 800 to 10k ohms or greater.

High pass filter - A circuit that discriminates between high and low frequencies and allows only the high frequencies to pass. Also called a “low cut filter.”

Huffman Coding - A method of entropy encoding used in lossless data compression where the most frequently occurring values use the shortest codes.

Hum - The coupling of an unwanted frequency into other electrical signals. In audio, hum can be heard; in video, it can appear as waves or bars in the picture. Often it is an audible disturbance caused by the power supply, or an improper ground.

IEEE 1394 - Institute of Electrical and Electronics Engineers standard 1394. Also known as FireWire (a trademark of Apple) and i.Link (a trademark of Sony), IEEE 1394 is a serial digital format that handles a wide range of data. IEEE 1394 offers peer-to-peer interface capability, so it does not require computer support.

Impedance - The opposition or load to a signal, measured in ohms and abbreviated W or Z. In video, typical low impedance circuits (low Z) are 600 ohms or less and high impedance circuits (high Z) may be 10 k ohms or greater. Video termination impedance is 75 ohms. Also see "High impedance" and "Low impedance."

Impedance matching - Circuits that generate audio or video signals are designed to work with a certain load (impedance). When connecting devices in a system, it is important that the impedance specifications are adhered to. If the impedance of the load is not matched to that of the source, there could be undesirable results, such as loss or distortion of the original signal, reflections, etc.

IMUX - Inverse multiplexer. A unit that combines multiple low bandwidth digital phone lines into a single high bandwidth call.

Inductor - An electrical component that opposes changes in current flow and stores electrical energy as a magnetic field. Although all wires have inductive properties, an inductor component is usually a coil of wire. Transformers use this same principle.

Input sensitivity - The minimum input level signal required to output a specified output level.

ISDN - Integrated Services Digital Network. An international communications standard for sending voice, video, and data digitally over telephone lines. ISDN uses special wires and can transfer at rates of 64,000 bits per second. Another version called B-ISDN uses fiber optics and can transfer at 1.5 megabits per second.

ITU - International Telecommunication Union. Formerly known as the CCIR (Comité Consultatif International des Radiocommunications) or International Radio Consultative Committee. A global organization responsible for establishing television standards.

kHz - Kilohertz. One thousand cycles per second (hertz).

Layer 2 Switch - Layer 2 switches support functions of the 2nd layer of the ISO model. Layer 2 switches provide hardware switching. They are capable of switching packets between devices connected to the switch. A table is built in the switch based on the physical MAC address of the connected devices. A Layer 2 switch does not examine IP packets.

Level - The relative intensity of an audio or video source.

Limiter - Audio circuit or device that prevents the signal from reaching a level where audible distortion can occur.

Line level - Audio signal industry-referenced at 600 ohms, 0 dB. Consumer systems may use a different reference.

Line out - Audio output. In consumer systems, this may be 10,000-50,000 ohms, at -10 dB or -20 dB.

Lip Sync - A technical term for matching lip movements seen in a video picture with voice. Audio and video is synchronised when lip sync is maintained.

Listening plane - In acoustics, the intersection of the audio field (the horizontal plane) with the listener’s ear.

Logarithm - An exponent used in mathematical equations to express the level of a variable quantity (or, the power to which a number must be raised to produce a specific result).

Lossless - When using compression to reduce text and/or graphic files, some techniques discard data in the process. Methods that compress files without losing data are called lossless.

Lossy - A term to describe compression techniques that throw away data as part of the process. The more data loss, the smaller the file, and the lower the quality (grainy or jagged edged) of the image. Lossy compression methods include JPEG and MPEG. Note: with JPEG, high means high compression (greater loss) and low means low compression (less loss).

Lossy Compression - Method which discards (loses) some of the data, in order to achieve its goal, with the result that decompressing the data yields content that is different from the original, though similar enough to be useful in some way.

Loudness - The sound pressure level (SPL) of a standard sound. Loudness is the perception of the strength or weakness of a sound wave resulting from the amount of sound pressure level produced. Sound waves that have more intensity or larger variations in air pressure produce louder sounds. Low-intensity sound waves with smaller fluctuations in air pressure produce quieter sounds.

Loudspeaker - An electro-acoustic transducer that converts electrical audio signals at its input to audible sound waves at its output.

Low impedance - The condition where the source or load is at a lower impedance than the characteristic impedance of the cable. Low source impedance is common; low load impedance is usually a fault condition. Example: 30-600 ohms.

Low pass filter - A circuit that discriminates between low frequencies and high frequencies and allows only the low frequencies to pass. For example, a low pass filter design for a subwoofer, set at 80 Hz, would allow the audio signals below 80 Hz to pass through to the speaker, and attenuate the portion of the signal above 80Hz.

LPCM - Linear PCM - A specific method of pulse code modulation that is used to represent an analog waveform as a sequence of amplitude values. LPCM has been defined as part of the DVD and Blu-ray Disc standards, and is also used by HDMI. Also see "PCM – Pulse Code Modulation."

Mathematically Lossless Compression - Allows the exact original data to be reconstructed from the compressed data. Data compacting in mathematically lossless processes is between 2:1 and 3:1. The term lossless is in contrast to lossy compression, which only allows an approximation of the original data to be reconstructed in exchange for better compression rates.

Matrix - An electronic device that accepts and distributes video (and/or audio) signals selected from multiple inputs and multiple outputs. Also see "Matrix switcher."

Matrix mixer - Similar to a matrix switcher, but with additional signal processing features, such as equalization (EQ), compression, and level/gain controls on the inputs and outputs.

Matrix switcher - A means of selecting an input source and connecting it to one or more outputs. Like a regular switcher, but with multiple inputs and multiple outputs.

MB - Megabyte. A megabyte is actually 1,048,576 bytes, or roughly 1 million bytes.

Mbps - Megabits per second. One million bits per second; a unit of measurement for data transmission.

Media player - A software application used for the playback of audio and video files.

Mid-range - The range of audio frequencies, 250 Hz to 5000 Hz, to which the human ear is most sensitive. Mid-range frequencies give sound its energy.

Milli - m. Abbreviation for one one-thousandth. Example: 1 ms = 1 millisecond or 1/1000 second.

MIPS - Million Instructions Per Second. The rate at which a computer executes instructions.

Modulation - The process of adding an information signal to a carrier frequency to allow it to be transmitted. Thus, the carrier is modulated by the information signal, as in a modem.

Momentary contact - A non-latching contact closure that lasts as long as it is held in place.

Momentary switch - A switch that returns to its normal circuit condition when the actuating force is removed.

MPEG - Moving Picture Experts Group. A standards committee under the auspices of the International Standards Organization working on algorithm standards that allow digital compression, storage and transmission of moving image information such as motion video, CD-quality audio, and control data at CD-ROM bandwidth. The MPEG algorithm provides inter-frame compression of video images and can have an effective compression rate of 100:1 to 200:1.

MPEG-2 - The second generation standard for video compression of audio and video applying the discrete cosine transform. The standard includes a combination of lossy video and audio compression methods which permit storage and transmission of movies using currently available storage media and transmission bandwidth. Commonly used for digital television transmission, DVD, and other similar equipment.

MPEG-4 - A patented collection of methods defining compression of audio and visual (AV) digital data. Uses of MPEG-4 include compression of AV data for web (streaming media) and CD distribution, voice (telephone, videophone) and broadcast television applications. MPEG-4 absorbs many of the features of MPEG-1 and MPEG-2 and other related standards, adding new features such as (extended) VRML support for 3D rendering, object-oriented composite files (including audio, video and VRML objects), support for externally-specified Digital Rights Management and various types of interactivity.

MTS - Multichannel Television Sound. One of the first stereo sound systems developed for television. MTS consists of two independent singles each carrying a discrete channel. One channel provides stereo sound by providing left/right channel difference signals relative to transmitted mono audio track. The second carrier carries the Secondary Audio Program (SAP) which is used for a second language or for Descriptive Video, a descriptive commentary for the vision impaired.

Multipoint - When more than two locations are connected for a videoconference using a bridge. Usually multipoint switching is done by video-follow-audio, such that the person speaking is automatically seen by the other conference site(s).

MUX - Short for multiplexer. A device that combine multiple signals for transmission over a single line. The signals are demultiplexed (DEMUX’d), or separated, at the receiving end.

NAB - National Association of Broadcasters, the body that sets standards for US broadcast stations. www.nab.org

Near end - In videoconferencing, the party or group at the local end of the connection.

Noise - Any unwanted signal that adversely affects the quality of the picture or sound.

Noise criteria - Abbreviated “NC”. An NC level is a standard that describes the relative loudness of a room or space, across a range of frequencies. The NC level illustrates the extent to which ambient noise interferes with speech intelligibility. NC should be considered for any project where excessive noise would be irritating to the users, especially where speech intelligibility is important.

Noise gate - An audio processing device that reduces background noise by muting a sound signal when it falls below a pre-determined level, and restores it when the level increases again. For example, in a board room where multiple microphones are installed, a noise gate may be used to mute the microphones of those not currently speaking, in order to eliminate disturbing background noise in the sound support system.

Noise reduction - Any technique for reducing the amount of undesired noise in an audio signal. A common technique is called “companding”, a circuit that combines a COMPressor and an exPANDer. The signal is compressed before recording (which maximizes the signal to noise ratio), then expanded during played back. As the signal is expanded, noise tends to be “pushed down,” resulting in a quieter signal.

Noise-canceling - A microphone designed to cancel ambient noise so that it will not be broadcast or recorded. The housing of the microphone allows noise to reach both sides of the diaphragm simultaneously, one side canceling the other out. A close speaking voice strikes the diaphragm on only one side, generating a stronger signal with less background noise.

Nominal level - The signal strength level at which a product is designed to operate for optimized performance. Most of the specifications are tested at the nominal level and all of the built-in signal meter reference levels (e.g. 0dB) are set at the nominal level. A device’s input nominal level should match the nominal level of the source device it is connected to, while the output nominal level of a device should match the nominal level of the destination device it is connected to. The most commonly used nominal levels for video and audio are: video, 1Vp-p (video with sync), 0.7Vp-p (video without sync), 0.3Vp-p (chroma); audio, +4dBu (1.23V, pro audio line level), -10dBV (316mV, consumer audio line level), -60dBV (1mV, mic level).

Normally closed contact - Relay or switch contacts that are closed when the relay is not activated.

Normally open contact - Relay or switch contacts that are open when the relay is not activated.

Nyquist frequency - The highest frequency that can be reproduced accurately when a signal is digitally encoded at a given sample rate. Theoretically, the Nyquist frequency is half of the sampling rate. For example, when a digital recording uses a sampling rate of 44.1kHz, the Nyquist frequency is 22.050kHz. If a signal being sampled contains frequency components that are above the Nyquist limit, aliasing (stair-stepping) will be introduced in the digital representation of the signal unless those frequencies are filtered out prior to digital encoding.

Octave - A doubling or halving of a frequency. For example, 200 Hz is one octave higher than 100 Hz; 50 Hz is one octave lower.

Ohm - The unit of electrical resistance, transmitting a current of 1 amp when subjected to a potential difference of 1 volt. Represented by W or Z.

Oscilloscope - A test device that allows measurement of electronic signals by displaying the waveform on a CRT screen.

Packet - A block of data that is transmitted over a network in a packetswitched system. A packet is also referred to as a frame or datagram.

Packet jitter - The term jitter is used as a measure of the variability over time of the packet latency across a network. In real-time applications such as VoIP and video, variation in the rate at which packets in a stream are received that can cause quality degradation. Video decoders must account for jitter which may be experienced delivering packets across a network.

Packet loss - Occurs when one or more packets of data traveling across a computer network fail to reach their destination. Packet loss is distinguished as one of the three main error types encountered in digital communications; the other two being bit error and spurious packets caused due to noise. Packet loss is typically experienced in the real world as a random burst of packet loss.

Parametric equalizer - A type of audio equalizer having several “parameters” for control of various filters that can be applied to audio signals. Parametric equalizers are most widely used in situations where very fine control over the audio signal is desired, and provide control over gain, Q (Quality factor, a measure of a resonant system), and frequency. (See EQ-Audio)

Passive crossover - A crossover network that divides audio frequencies without any active amplification or buffering components and which uses only resistors, capacitors and inductors.

PCM – Pulse Code Modulation - The digital representation of an analog audio signal. PCM is the standard form of digital audio in computers and the compact disc (CD) “red book” format, as well as the standard used for the audio portion of digital video recording.

Peak - The highest level of signal strength, as determined by the height of the signal’s waveform.

Peak-to-peak - Abbreviated “p-p.” The difference in amplitude (voltage, for example) between the most positive and the most negative excursions (peaks) of a signal.

Phantom power - A standardized method of providing power to condenser microphones using the two signal leads of a balanced audio connection. An international standard, IEC 60268-15, defines three DC voltages, 48 V, 24 V, and 12 V. In professional applications, 48 V Phantom power is the most common.

Phase - The relative timing of one signal to another, usually expressed in degrees of shift.

Phone plug - A small, round audio plug used as a speaker connector. Also called 1/4” phone plug.

Phono plug - A plug most often used with line level audio signals. Also known as an RCA plug.

Pigtail - A short length of cable with one end terminated with a connector and the other end spliced or hard-wired to existing cable or equipment.

Pink noise - A type of random noise that contains an equal amount of energy per octave. The bands 100-200, 800-1600, and 3000-6000 all contain the same amount of energy. Since pink noise is based on octaves rather than individual frequencies, there is no increase in energy in the high octaves. Because of this, and because Real Time Analyzers (RTA) tend to look at octave or 1/3 octave ranges, pink noise is very useful for measuring the frequency response of audio equipment, as well as for determining room response for sound reinforcement applications.

Plug - In A/V and fiber optics, this is also known as the male connector.

Plug-in - A program of data that enhances, or adds to, the operation of a parent program. Software decoders often use a plug-in provided in media players.

Point-to-point - A videoconference between two locations, like a telephone call. Also see "Multipoint."

Polarity - The positive and negative orientation of a signal. Polarity usually refers to the direction or a level with respect to a reference (e.g. positive sync polarity means that sync occurs when the signal is going in the positive direction).

Potentiometer - A variable resistor. Potentiometers typically have three terminals: the two end terminals, across which the entire resistance appears, and a third terminal, the “wiper”, which moves to a different spot on the resistor as the shaft is turned. In this manner, the resistance between the wiper and one end terminal gets smaller while, at the same time, the resistance between the wiper and the other end gets larger. This allows the potentiometer to be used as a variable voltage divider, for use in attenuators such as volume controls or tone controls.

Power (electrical) - The dissipation of heat by passing a current through a resistance. Measured in watts [W], Power [P] is expressed by ohm’s law from the three variables: voltage [E] current [I] and resistance [R]. That is, P = I2 x R, or, P = E2/R or P = E x I.

Preamp - Preamplifier. An electronic circuit that raises a weak signal high enough to be fed into an amplifier.

Processing amplifier - An electronic device that stabilizes, changes, or rebuilds signals.

Propagation delay - The amount of time that passes between when a signal is transmitted and when it is received at the opposite end of a processor, amplifier, or cable.

Pseudorandom noise - A noise that satisfies one or more of the standard tests for statistical randomness. Although it seems to lack any definite pattern, pseudorandom noise consists of a sequence of pulses that will usually repeat itself, albeit after a long time or a long sequence of pulses.

Pulse code modulation - A method used to convert an analog signal into noise-free digital data that can be stored and manipulated by computer. PCM takes an 8-bit sample of a 4kHz bandwidth 8000 times a second, which gives 16K of data per second.

Pulse width modulation - A powerful technique for controlling analog circuits with a processor’s digital outputs. PWM is employed in a wide variety of applications, including high power switching amplifiers. By controlling analog circuits digitally, system costs and power consumption can be drastically reduced.

Q-factor (Quality factor) - A measure of the “quality” of a resonant system. Resonant systems respond to frequencies close to the natural frequency much more strongly than they respond to other frequencies. On a graph of response versus frequency, the bandwidth is defined as the 3 dB change in level besides the center frequency.

Quantization - The process of sampling an analog signal to convert its values into digital data.

Quantizing error - Inaccuracies in the digital representation of an analog signal. These errors occur as a result of limitations in the resolution of the digitizing process.

Quantizing noise - The noise (deviation of a signal from its original or correct value) that results from the quantization process. In serial digital video, it is a granular type of noise that occurs only in the presence of a signal.

Rack unit - As defined by the Electronics Industries Association (EIA). In reference to product rack height, a unit is a universal measurement: 1.75 inches or 44 mm. 1U (Unit) high refers to 1.75 inches, 2U high refers to 3.5 inches, etc.

Random noise - Also known as “white noise.” A noise signal that never repeats and has a flat frequency spectrum. Random noise can also be a digital pattern that never repeats. Random noise is generally considered to have a Gaussian amplitude distribution, but numerically generated noise can also have a flat amplitude distribution. The amplitude of random noise is normally measured as the rms value.

RC network - An electrical network that is constructed using a resistor and capacitor in parallel, and acts as an effective high-pass filter.

RCA plug - A connector type most often used with line level audio signals and composite video. (Phono)

Real-time analyzer - A device that measures room acoustics in real time, typically using pink noise. Also see "Pink noise."

Relay - A device that acts like a switch and is controlled by a current. The relay switch contacts and then controls another circuit to pass a signal. Most relays are either solid state or electromagnetic.

Resistance - The opposition to the flow of electric current. Also see "Ohm" and "Power (electrical)."

Resistor - An electrical component that limits the flow of current via resistance or impedance. It allows a specific amount of current to flow, as determined by the voltage applied to the resistor.

Resonance - (1) Electronically, the condition where the inductive reactance and capacitive reactance are equal. (2) Acoustically, an intense and prolonged sound produced by a sympathetic vibration.

Reverberation - The persistence of sound in an enclosed space, as a result of multiple reflections after the sound source has stopped. The decaying residual signal that remains after a sound occurs, created by multiple reflections as the original sound wave bounces off walls, furniture, and other non-absorbing barriers within a room or other acoustical environment. A room with very little reverberation is called a “dead” room, which is the opposite of a “live” acoustic space which is very reflective.

Reverberation time - The length of time required for the sound field to collapse, after the sound source has stopped.

RF - Radio frequency. A range of frequencies used for electromagnetic transmission (e.g., radio and TV).

RFI - Radio Frequency Interference. High frequency interference from transmissions such as telephones, microwaves, and television stations.

Ripple - Generally referring to the wavelike variations in the amplitude response of a filter.

Rise time - The time required for a signal to go from 10% to 90% of its maximum amplitude level.

rms - An acronym for “root mean square.” Used in audio to help rate the continuous power output of an amplifier or input capability of speakers. This is the preferred method for comparing anything in audio applications.

Room mode - An acoustical resonance in a room caused by parallel wall surfaces. Any set of parallel walls will establish a series of standing waves, the lowest one of which has the wall spacing as a half-wavelength. These sound waves interfere with one another to produce a series of places where the SPL is high and another series of places between then where the SPL is very low. It is as if the sound wave were stationary between the two surfaces.

RTP - Real-time Transport Protocol, an IETF standard for streaming realtime multimedia over IP in packets.

RTSP - Real Time Streaming Protocol. A network control protocol designed for use in entertainment and communications systems to control streaming media servers.

S/N ratio - See "Signal to Noise ratio."

S/PDIF – Sony/Philips Digital Interconnect Format - A data protocol for compressed or uncompressed digital audio co-developed by Sony and Philips Electronics and now part of the larger AES/EBU audio standard. S/PDIF is often misconstrued as a connection type; however, S/PDIF audio can be found in products using either a 75 ohm coaxial connection or a TOSLINK fiber optic connection. S/PDIF is commonly found in Compact Disc and DVD players.

SACD – Super Audio Compact Disc - A very high fidelity, read-only optical disc format for both two-channel stereo and 5.0 (no sub-woofer) or 5.1 surround sound audio. SACD can store up to 10 times as much data as a standard audio CD, up to 7.95 GB. Support for SACD audio was added to the HDMI 1.2 specification in 2005.

Sample rate - The rate at which an analog signal is sampled. It is frequently expressed as kilosamples/sec (kS/s) or Megasamples/sec (MS/s. For example, 44.1 kHz is the standard sample rate for compact disks; 48 kHz is often used with digital audio tape (DAT) recording. A higher sample rate allows a higher frequency response. In order to accurately reconstruct a sound, the sample rate must be at least twice the highest frequency in the sound.

SAP - Second Audio Program. A feature of MTS television stereo audio, SAP permits the delivery of a second soundtrack for enhanced services such as second language or descriptive video services (DVS) for the vision impaired.

Sensitivity - A standard way to rate audio devices like microphones, loudspeakers, and amplifiers. For loudspeakers, sensitivity is rated by applying 1 watt of power to the speaker and measuring sound pressure level (SPL) at one meter. For microphones, sensitivity is expressed as the minimum input signal required to produce a standard output level. For a power amplifier, sensitivity is rated as the input level required to produce 1 watt of power output into a specified load impedance, typically 4 ohms or 8 ohms.

Signal Noise - A random fluctuation in an electrical signal, a characteristic of all electronic circuits.

Signal to Noise ratio - Also stated as "S/N ratio". The ratio is expressed in decibels as a ratio between the audio or video signal level and that of the noise accompanying the signal. The higher the S/N ratio, the better the quality of the sound or picture.

Single-ended - An unbalanced circuit where one side of the circuit or transmission line is grounded. Single-ended audio is unbalanced audio.

Slew rate - The ability of audio equipment to reproduce fast changes in amplitude. Measured in volts per microsecond, this specification is most commonly associated with amplifiers, but applies to most types of audio products. In amplifiers, a low slew rate “softens” the attack of a signal, “smearing” the transients and sounding “mushy.” Since high frequencies change in amplitude the fastest, this is where slew rate is most critical. An amp with a higher slew rate will sound “tighter” and more dynamic.

Software Decoder - A software decoder provides a means to decode audio/video streams in software without requiring use of a dedicated hardware appliance. Software decoders are typically used on a PCs using a browser page, media player or special purpose application.

Sound level meter - An instrument designed to measure sound pressure level.

Speaker (conical) coverage - Ceiling speakers, much like a spot light, generally project audio in a conical coverage pattern. As with any device featuring a conical coverage pattern, the higher it’s mounted, the larger the circle of coverage. As the area of coverage increases, audio amplification wattage must also increase to maintain the same sound pressure level per unit of area of coverage.

Speaker polarity - Loudspeakers of all types have a positive and a negative polarity. A speaker cone that moves forward (out) when a positive voltage is applied is said to have “positive polarity”; conversely, a speaker cone that moves backward (in) when a negative voltage is applied is said to have “negative polarity.” Speakers must be wired in such a way to ensure that they are “in phase”, that is, all speaker cones are moving in the same direction, in or out, at the same time. Audio test generators, such as the Extron VTG 300 and VTG 400, include a test tone (typically a “click” or “pop”) that’s used to check speaker polarity

Spectrum analyzer - An device used to measure and analyze the frequency spectrum of an input signal, usually amplitude (vertical) vs. frequency (horizontal).

Stereo - A process of using separate audio signals on separate channels for the left and right audio, thereby giving depth, or dimension to the sound.

Subwoofer - A loudspeaker designed to reproduce only the very low end of the audio frequency range, typically one or two octaves between 20 Hz and 80 to 100 Hz.

Summing amplifier - A device which combines the left and right channel audio signals into a single mono channel. It is useful in multi-speaker mono paging systems, or in large stadium and church environments.

Sweep - In audio, a sequence of puretone frequencies used to generate a frequency response curve.

Switcher - (1) A device that allows a selection between more than one source, such as video cameras, VCRs, etc. In audio/video, switchers are a means of connecting an input source to an output device or a system. Also see “Matrix switcher.” (2) A term often used to describe a special effects generator; a unit that allows the operator to switch between video camera signals. Switchers are often used in industrial or security applications to switch between video cameras that view certain areas for display on a monitor, or system of display devices. These kinds of switchers do not have sync generators. Also see "Matrix switcher."

Synchronisation - Timekeeping which requires the coordination of events to operate a system in unison. Synchronisation in video systems can refer to a number of items. Lip-sync is the synchronisation of audio and video. Genlock refers to alignment of vertical sync in video signals. Framesync or framelock refers to the alignment of video frames in systems with multiple video sources.

Teleconferencing - A meeting between people at two or more locations who can communicate by audio and/or visual devices often via telephone and/or closed-circuit television. Also see "Videoconferencing."

Telepresence - A set of technologies which allow a person to feel as if they were present, to give the appearance that they were present, or to have an effect, at a location other than their true location. Telepresence solutions include the delivery of audio, video, data and computer graphic information over IP networks using video over IP encoders and decoders.

Temporal - Relating to the sequence of time or to a particular time.

Temporal Resolution - A measurement of elements occurring in time. Example: the temporal resolution of video may be 50 or 60 frames per second.

THD - Total Harmonic Distortion. The amount of internally generated noise in a receiver that varies from the ideal sound wave. Since the perfect audio wave is physically impossible, the variation from the ideal wave is the percentage referred to as THD.

THD+N - Total Harmonic Distortion plus Noise. THD+n is a specification that includes both harmonic distortion of the sine wave and non-harmonic noise. THD+N tells the user what amount of hum, noise, and interference has been added to the audio signal by the equipment through which it is passing.

Third octave - A term that refers to frequencies spaced three octaves apart. For example, the third-octave above 1 kHz is 8 kHz. Third-octave filters, typically in a bank between 20 Hz and 20 kHz, are useful because they have a good correlation to the subjective response of the human ear. Filtering broad spectrum pink noise through a third-octave filter is often used to characterize acoustical systems, such as loudspeakers, or for the measurement of absorption or damping of acoustical materials.

THX - Originally part of Lucasfilm, Ltd., THX embodies a series of certification programs for equipment, software, production environments, and presentation venues with the objective of delivering optimum sound and video.

Time division multiplexing – TDM - A digital transmission scheme where the channel is divided into two or more time slots or subchannels, such that the subchannels are taking turns in the bit stream. Multiple digital signals are multiplexed into a serial digital stream. The serial digital stream is transmitted to the receiver where it is de-multiplexed into the individual digital signals

Time domain - A means of representing a signal on a graph of amplitude (usually on the vertical axis) versus time (usually on the horizontal axis). An oscilloscope produces a time domain representation of a signal. Also see "Frequency domain."

Toneburst - A group of short duration audio frequencies, from 20 Hz to 20 kHz, used to evaluate the behavior of a listening environment in response to loudspeakers. Toneburst is used to identify room reflections caused by resonance or standing waves. Also see "Room mode."

TOSLINK - An optical fiber connection standard for digital audio developed by Toshiba. TOSLINK is very commonly used for audio output from Compact Disc and DVD players, as well as some game consoles. A generic name, “EIAJ optical,” is sometimes used to describe this standard.

Transducer - In audio, a device to convert sound to electrical energy or vice versa. Microphones and loudspeakers are two type of transducers. Microphones convert sound into electrical energy; loudspeakers convert electrical energy back into sound.

Transform - A transform is a method applied to convert a data set from one domain to another. The rationale for transforming the data into a new domain is typically to make handling and processing the information easier. One common example is the RGB to YUV color space transformation. Imagery collected from the real-world using sensors in done so in an RGB color space. The RGB information is then transformed to a component YUV domain allowing independent processing of luminance and color information.

Transformation - A change or alteration. In the context of still image compression, a picture frame is input as a fixed resolution of rows and columns of pixels and a transformation into a frequency domain applying the Discrete-Cosine Transform.

Transformer taps - A transformer is a simple device that “transforms” electricity from one voltage to another. Transformers are added to conventional, 8 ohm loudspeakers to adapt them for use in constant voltage, multi-speaker distributed audio systems. The transformers are “tapped”, that is, designed with several different output points, to allow for different output wattages from the 70 V or 100 V constant voltage input. Taps are typically spaced at 2x increments, for example, 2 watts, 4 watts, 8 watts, 16 watts, etc.

Transistor - An electronic component that acts as a valve, or switch, allowing one signal (voltage or current) to control another.

Transmitter - A device that converts from one signal type to another for transmission.

Transport Stream - A defined package for delivering data. Transport Streams are multiplexes of audio, video and other content which are usually broadcast over-the-air, although they can be streamed over IP networks too.

Tweeter - A loudspeaker designed to reproduce high-pitched or treble sounds in the range of 4Hz to 20 kHz.

UDP (User Datagram Protocol) - A connectionless protocol providing “best effort” delivery of packets across networks. UDP is frequently used in real-time streaming applications were best effort delivery is acceptable and the network devices and applications manage data flow control and errors.

Unbalanced audio - An audio output where one of the two output terminals is at ground potential.

USB - Universal Serial Bus. USB was developed by seven PC and telecom industry leaders (Compaq, DEC, IBM, Intel, Microsoft, NEC, and Northern Telecom). The goal was easy plug-and-play expansion outside the box, requiring no additional circuit cards. Up to 127 external computer devices may be added through a USB hub, which may be conveniently located in a keyboard or monitor. USB devices can be attached or detached without removing computer power. The number of devices being designed for USB continues to grow, from keyboards, mice, and printers to scanners, digital cameras, and ZIP drives.

USB 1.1 - Universal Serial Bus 1.1. An external bus standard that supports data transfer rates of 12Mbps and 1.5Mpbs. A single USB port can connect up to 127 peripheral devices. Also see "USB 2.0."

USB 2.0 - Universal Serial Bus 2.0. An external bus standard that supports data transfer rates up to 480Mbps, 12Mpbs, and 1.5Mpbs. USB 2.0 is an extension of USB 1.1 and is backward compatible with USB 1.1, using the same cables and connectors. Also see "USB 1.1."

VCA - Voltage Control Amplifier. An amplifier whose output is controlled by varying its voltage rather than by direct resistance (as with a potentiometer). VCA’s are typically used in audio mixers; instead of the signal being directly manipulated by the fader (and being subject to inconsistencies in the fader itself), the fader controls a voltage that corresponds to a certain sound level.

VCO - Voltage Control Oscillator. Voltage-controlled oscillators are commonly found in wireless systems, frequency synthesizers, and other devices that must be able to tune across a band of frequencies.

Videoconferencing - Conducting a live conference between two or more locations using video cameras, microphones, and video monitors. The participants can be seen as well as heard. Referred to as a virtual conference room. Also see "Teleconferencing."

Video-follow-audio - In videoconferencing, when the video source switches automatically to show the person speaking, regardless of the location.

Volt - The electrical potential difference or electromotive force that will cause current of 1 ampere to flow through the resistance of 1 ohm. Symbolized by “V.”

Voltage - The electrical potential difference or electromotive force expressed in volts. Also see "Volt."

VU - Volume Unit. A unit that is designed to measure perceived loudness changes in audio. 100 VU is 100 percent of the audio that is supposed to be present. VU is measured on a VU meter.

VU meter - Volume Unit meter. For audio systems or recorders, a VU meter is a device that indicates the relative levels of the audio being recorded or played. It is usually calibrated to show a maximum recording level to avoid tape saturation and distortion

Watt - A unit of electrical power used to indicate the rate of energy produced or consumed by an electrical device. One watt is one joule of energy per second. Also see "Power (electrical)."

Waveform - A display of a signal (on an oscilloscope) that shows the magnitude of current or voltage signals with respect to time. By displaying the waveform of a signal on an oscilloscope, the time between cycles can be measured and its frequency can be calculated.

Waveform monitor - A special oscilloscope used to display and analyze electrical (voltage or current) signals.

Waveguide - An acoustic device built into a loudspeaker enclosure that improves the efficiency of the speaker by confining the movement of a sound wave to travel over a desired path. In brief, a waveguide is a tube-like structure, straight or folded, that couples the motion of the loudspeaker cone to the motion of the air in the tube. This allows a small speaker driver to create clear sound, without distortion, even at the high volume levels required for low frequency reproduction.

Wavelength - The distance from one peak to the next between identical points in adjacent waves of electromagnetic signals propagated in space or along a wire. Wavelength is usually specified in meters, centimeters, or millimeters. In the case of infrared, visible light, ultraviolet, and gamma radiation, the wavelength is usually specified in nanometers (10e-9 meter) or Angstroms (10e-10 meter). Wavelength is inversely related to frequency. The higher the frequency of the signal, the shorter the wavelength.

Weighting filter - A special type of band-limiting filter used in measuring audio loudness levels that “weights”, or gives more attention to, certain frequency bands. Common weighting filter designs include: A-weighting, a wide bandpass filter, centered at 2.5 kHz, that mimics the way we hear (see “Fletcher-Munson Curve”); and C-weighting, generally “flat” frequency response with -3 dB attenuation at 31.5 Hz and 8 kHz.

White noise - Noise with random amplitude (strength) over a wide frequency range. Used to test speakers for resonance and sensitivity. Low levels of white noise can be used to cover up other random noises, for example, in an open office environment.

Wideband - A relative term indicating a high bandwidth.

Woofer - A loudspeaker designed to reproduce low frequencies.

Wow - A low pitch audio artifact caused by speed fluctuation in the playback device. Also see "Flutter."

XLR connector - XLR connectorA type of audio connector featuring three leads: two for the signal and one for overall system grounding. A secure connector often found on high quality audio and video equipment. Also called a “cannon connect

Z - A symbol for impedance.



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