The INLINE Technical Notes have been designed as an educational and reference tool for anyone requiring a basic knowledge of audio/video systems integration issues, especially as they relate to high resolution computer video signals. Major topics include:
This publication may be used as a stand alone reference, or in conjunction with a computer based presentation on computer interfacing. The charts and graphics in Technical Notes mirror the screens in the computer based screen show, while the text provides the basis of a complete Computer Interfacing Seminar as presented by INLINE staff at major conventions and association meetings throughout the world.
TABLE OF CONTENTS
EVOLUTION
Communications technology has evolved rapidly over the last two decades. Audio equipment, video equipment, high resolution computer graphics, multimedia, and data display devices are being tied together into comprehensive training and presentation systems. These environments, often found in boardrooms, training centers, classrooms, conference halls and convention centers, facilitate the communication process by offering an exciting blend of sounds and images which respond interactively to the needs of both audience and presenter.
In order to achieve a successful melding of all the high tech equipment provided by different manufacturers, it is important to have a basic understanding of the various types of audio and video signals. This will help A/V systems designers and end users alike to specify the correct equipment for each new communications installation, helping insure that all equipment is chosen to work well together, and avoiding potential problems with equipment incompatibility.
AUDIO
As shown in the previous slide, audio signals are the oldest
type of signal used for presentations. Current audio technology has advanced
to the stage where the only practical restrictions on audio signals are the
perceptive capabilities of the human ear. The typical human can hear audio
frequencies as low as 20 Hz and as high as 20 KHz (20,000 Hz). Most audio
equipment, from microphones and mixers to amplifiers and speakers, operate in
this general range, with some equipment functioning from 10 Hz on the bottom
end to 30 KHz and higher at the top end of the spectrum. This range of
frequencies is commonly referred to as the bandwidth of a
particular piece of equipment.
Audio amplifier specifications usually include figures for bandwidth (or
frequency response), power, and distortion levels. Bandwidth specifications
for audio signals and equipment are very similar to video signal bandwidth
specs, except that the frequency range for video signals is much wider,
corresponding to a higher bandwidth.
BROADCAST VIDEO
The first video signals devised were broadcast type video
signals, well known around the world as a powerful visual communications
medium. RF video signals as carried over the air include both audio and video
components and are outside the purview of this document. We will mainly
discuss the video signal once it has been demodulated from a RF (radio
frequency) signal into a composite video or S-Video (Y/C) signal.
Interestingly enough, broadcast video signals vary throughout the globe, with
most countries using one of the three major video formats: NTSC, PAL, or
SECAM. While there are actually scores of different video standards, most are
simply variations on the three major formats. In the United States the NTSC
(National Television Standards Committee) color system has been adopted,
whereas European and other nations use the PAL or SECAM standards. These
standards were developed decades ago and have yet to be updated. When compared
to most computer graphics signals, these video standards offer low resolution
and a relatively low bandwidth of around 5.5 MHz (5,500,000 Hz).
The NTSC color standard has a horizontal frequency of 15.75 KHz, which means
that 15,750 horizontal lines are drawn every second. The best theoretical
maximum picture resolution for NTSC is roughly 525 horizontal lines by 525
vertical lines. In practical application, the net viewable resolution is
further decreased by the real world limitations of television transmission,
storage, and display equipment. Diagram 1.4 charts the visual resolution of
various devices and standards.
IDTV
The first successor to the current television standards is
IDTV which stands for Improved Definition Television. IDTV is achieved through
a technique called scan doubling in which the number of scan lines is doubled.
Even though scan doubled IDTV images do not actually add greater resolution to
a video picture, their non-interlaced images are much more appealing to the
eye, especially on large screen displays. Scan doubling eliminates visible
scan lines to create a solid, film-like image. For more information on scan
doubling see page 39.
HDTV
The long awaited HDTV (High Definition Television) standard
promises a next generation solution to replace NTSC. Although
the current plans for HDTV as outlined by the FCC allow for several different
possible HDTV resolutions, some interlaced and others with progressive scans,
all of these modes provide higher resolutions than the current NTSC standard
with some proposed formats offering as many as 1050, 1125 or even 1920 lines of
resolution. This higher number of lines also corresponds to a higher scan rate
than NTSC and greater bandwidth requirements for all equipment in the video
recording and playback chain.
COMPUTER VIDEO SIGNALS
While television signals are generally combined together into
a single composite or Y/C signal containing all color components, computer
signals generally keep all color signals separate to insure highest resolution.
There are dozens of different types of computer video signals in use by the
various computer manufacturers. Since the introduction of the personal
computer, IBM has introduced many computer video standards from low resolution
MDA and CGA to high resolution S-VGA and XGA. Each new video standard was
released to improve the resolution, number of colors, and speed of the video
display. Apple computers have a similar line up of video standards, and other
manufacturers have an additional set of video standards, each with their own
special video output connectors and signal characteristics.
HIGH RESOLUTION GRAPHICS
Images created by high resolution video cards and graphic
workstations now approach photo-realistic quality. Resolutions of 1280 x 1024
are not uncommon, along with high vertical refresh rates which eliminate
picture flicker. In order to reproduce subtle shading and achieve realistic
imagery, the newest high resolution graphic sources may also offer a vast
selection of colors, from 32 thousand colors to over 16 million colors. With
horizontal scan rates in excess of 64 KHz and bandwidths in excess of 100 MHz,
high resolution computer graphics require careful interfacing to the data
display system. In many cases, improper selection of interfacing and data
display equipment will result in a poor display of these high resolution
images, or perhaps no image display at all!
SUMMARY
Signals used for visual communications have shown a distinct
trend over the past thirty years, beginning with low resolution / low bandwidth
requirements / limited color availability and moving to ever higher
resolutions, higher bandwidths, and photo-realistic color. While broadcast
video sources initially offered a greater range of color display than early
computer graphic signals, many computer graphic cards are now capable of
generating millions of colors. Computer graphics video sources currently offer
much higher resolution than broadcast video signals, however, broadcast video
promises to offer higher resolution in the future when new broadcast standards
such as HDTV are implemented. IDTV (or scan doubling) is an intermediate step
which creates a more solid picture image than regular broadcast video signals
by doubling the number of scan lines found in an NTSC, PAL, or SECAM video
signal.
HIGH RESOLUTION VIDEO SIGNALS - A TECHNICAL PRIMER
Most computer graphic signals are composed of the following
elements:
SYNC SIGNALS
The sync signal provides synchronization information that a
video or data display uses to properly display an image. Most displays create
an image by raster scanning as shown in Diagram 2.2. Horizontal lines are
drawn one at a time until a whole image (referred to as a frame) has been
displayed. Once the complete image has been drawn, the vertical sync signal
tells the display device to retrace back up to the top of the screen and start
drawing the next frame.
COLOR COMPONENTS
The primary Red, Green and Blue colors are used by display
devices to create all colors. As shown in the above diagram, combining RGB
colors of the same intensity creates 8 different colors (including black). By
changing the intensity of each primary color and combining them together, an
infinite amount of different colors can be generated.
Video Signal Types
Computer video cards output two types of video signals,
Analog or Digital. While all computer video
signals are originally created inside the computer as digital signals, many
types of video output cards use converters to change this to an analog signal.
Specific computers were designed with analog or digital video outputs depending
on the intended use of the computer. Early IBM PC video standards such as MDA,
CGA, EGA, and PGA were all digital signals while most current standards such as
VGA, XGA, MACII, and Sun SPARC employ analog video signals. Digital signals
are generally limited to 64 colors, whereas analog signals can produce millions
of colors.
ANALOG VIDEO SIGNALS
Analog video signals are defined as continuously variable
signals whose level can be adjusted very precisely. They are generally easy to
distribute, switch and transmit and are less susceptible to noise than digital
signals. Because of these properties, it is preferable to convert a video
signal to the analog form for proper distribution and transmission. In order
to insure accurate transmission of analog video signals, coaxial cables must be
used to make all display system connections. Using good quality coaxial cable,
analog signals can easily be transmitted from 50 to 75 feet with negligible
signal degradation. The exact length of cable run allowed will depend on the
scan frequency of the video signal and the bandwidth characteristics of the
coaxial cable.
DIGITAL VIDEO SIGNALS
Digital video signals (also called TTL) are a step variable
type of signal whose amplitude cannot be adjusted. By definition, a digital
signal can only be in one of two states, "on" or "off." On or
high is said to have a level of 1, and off or
low has a level of 0. The following example shows how digital
high and low states relate to various colors.
Each primary color can be on or off, creating a total combination of 8
different colors. While the example above shows 3 bit color (red, green, and
blue), more colors can be obtained by adding additional bits.
Digital signals are characterized as having high impedance and are susceptible
to interference from different electrical sources. Special twisted pair cables
should be used to transmit digital signals. Typically, if a digital signal
needs to be transmitted more than 6 to 10 feet, a special line buffer should be
used to maintain the integrity of the signal.
The most common format for high resolution video signals is RGBS, which stands
for Red, Green, Blue, and Composite Sync. Other sync formats exist and they
are listed in the following section. Another common format for high resolution
medical imaging signals is monochrome, where the video signal and sync signal
are combined together on a single line.
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