sbs_video.txt

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Introduction
This overview of video standards, methods, and system design factors
provides an historical perspective, starting with the advent of television half a
century ago, and continuing up to today’s latest emerging video standards.
Because some applications involve retrofitting new technology to existing
systems, standards encountered can be from the earliest mentioned to the
latest.
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Analog Video
In the Beginning: NTSC/ RS-170
The very earliest video standard was created in the late 1930s for television in the U.S., and
this standard still serves as the foundation for all video today. This method, which later
became standardized in EIA RS-170, involved sending video one line at a time, from left to
right from the top left corner of the image.
Altogether, 525 horizontal lines (of which 480 are active video content) were sent directly
from the camera, through the broadcast gear, onto the television’s CRT in millions of homes,
all synchronized. To capture motion well, these 525 lines were sent at a rate of 30 frames per
second (which is faster than standard motion picture rates of 24 frames/second).
Because CRT’s flicker badly when refreshed at only 30 frames per second, a method called
interlacing was used. Interlaced video causes less visible display flickering on a CRT monitor
than non-interlaced methods by alternating between drawing the even-numbered lines and
the odd-numbered lines of each picture. Each of these odd and even fields were sent at a
rate of 60 fields per second (resulting in 30 full frames per second), reducing flicker to
acceptable levels at normal TV viewing distances.
In contrast, a non-interlaced raster display draws every line of a picture, or frame, in
sequence from top to bottom. This takes a certain amount of time, during which time the
image on the CRT begins to decay, resulting in flicker.
An interlaced display reduces this flicker effect by drawing first all the even-numbered lines
(forming the even field), leaving spaces between them for all the odd-numbered lines (forming
the odd field) which it fills in afterwards to complete the frame. This results in the display
being refreshed from top to bottom twice as frequently as in the non-interlaced case.
The overall analog bandwidth required to send this picture with adequate fidelity was about
4.5 MHz, leading to a 6 MHz overall bandwidth (once audio and inter-channel spacing needs
were added) for each of today’s TV channels. This remains exactly the same way
monochrome signals are sent today, nearly 70 years later.
Analog Color: NTSC, RS-170A
In the 1950s, color television emerged, and it required a technique that was backward
compatible with existing monochrome television sets and stations. This was achieved by
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adding color information on a separate sub-carrier (signal) within each channel, which
monochrome television sets would not detect.
This method was called NTSC (National Television Standards Council), and a draft standard
called RS-170A was created based on this. Strangely, this standard was never finalized, even
though RS-170A is widely referenced, and simply taken to mean ‘NTSC’ standard.
European Standards: PAL, SECAM
Similar standards were also created in Europe: PAL in most of Europe and SECAM in
France. These standards were necessarily different because in each case, they were
designed to be synchronous with the power line frequencies: 60 Hz in the U.S., and 50 Hz in
Europe, to reduce the visual impact of power line noise in the picture.
As a result, PAL sends frames at 625 horizontal lines per frame, 50 frames per second, also
interlaced. SECAM is the same, but has a different method for encoding the color signal. In
all cases, the interlaced method is used.
Government Standards: RS-343, European STANAG Standards
Along the way, other standards came into play.
RS-343
In the 1960s, security applications required higher screen resolution than the
standard 525-lines provided by standard television, and in 1969 the new RS-343
standard provided higher resolutions, up to 1,023 lines. Today, the resolution most
widely associated with RS-343 is 875 lines, as with standard television. Although the
actual RS-343A standard specifies monochrome only, this has been extended to a de
facto color standard in the defense industry by providing three RS-343A lines: one for
each of R, G, and B. RS-343 and RS-343A again specify interlaced video.
STANAG 3350
The STANAG standards evolved in Europe as a means of standardizing more
rigorously what was already in common use. The following table shows which
STANAG standard applies to which conventional standard.
STANAG Standard Basic Method Based on
1 STANAG 3350 Class A 875 Line @ 60Frames/ Sec RS-343
2 STANAG 3350 Class B 625 Lines @ 50 Frames/ Sec PAL
3 STANAG 3350 Class C 525 Lines @ 60 Frames/ Sec NTSC RS-170A
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Composite vs. Component Video
All the standards mentioned above are in the category of ‘composite video’: as is necessary
in a television broadcast, all video information is sent on a single wire, usually coaxial cable,
or over the air. Composite video is sometimes abbreviated as CVBS – composite video
broadcast signal.
However, improved video quality can be achieved if multiple wires can be used, as is quite
possible for local, rather than broadcast systems, described next.
S-Video, RGB
In applications where the video is to be transmitted only a short distance, it is
economical to provide more than one wire to carry the video, and use higher
bandwidths, to provide improved video quality.
S-Video: The first method for improving quality is to separate brightness (also called
luminance) information, and color information into two separate wires. This is done in
S-Video, and results in higher color resolution since the luminance and color
information no longer have to share the same 4.5 MHz bandwidth: each can have a
separate 4.5 MHz of bandwidth. S-Video systems still specify interlaced format to be
television compatible.
RGB component video takes this a step further, and provides three wires, one for
each Red, Green, and Blue color ‘gun’ in a color CRT. This provides still better color
quality. RGB component video can involve either interlaced video, or in newer
systems ‘progressive scan’ video, where all lines on the screen are sent in order at
60 frames per second. This is used, for example, when DVD players are connected
to a nearby television set/monitor.
PC World: RGB, VGA
With the emergence of PCs in the 1980s, there was a strong need for higher resolutions than
broadcast television, and the interlaced method that works well for television viewing
distances did not work well for closer PC viewing distances.
Although some early standards addressed this (CGA, EGA), the standard that has stood the
test of time is VGA, and its descendants. Here is a list of today’s standard VGA resolutions:
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VGA Standards
Acronym Resolution
1 VGA 640 x 480
2 SVGA 800 x 600
3 XGA 1024 x 800
4 SXGA 1280 x 1024
5 UXGA 1600 x 1200
All VGA systems are progressive scan, and interlacing is not used. Unlike broadcast systems,
the frame rate is variable as agreed by the source (PC) and the monitor, and not fixed by the
standard. Many of today’s systems run at 75 FPS (Frames per Second) or higher, rather than
60 FPS. Due to characteristics of the human visual perception, flicker on displays is
noticeable at 60 Hz progressive, but is not noticeable to most at about 75 FPS and higher.
Analog VGA systems always send the video on three separate lines: R, G and B. As will be
seen later, VGA resolutions can also be sent digitally, rather than in this analog format.
PC World: RGB, VGA
In order to synchronize the video images, all the above methods employ a ‘start of frame’
‘vertical’ synchronization signal to tell the monitor/TV to jump to the start of the next field or
frame, and then a ‘start of line’ ‘horizontal’ synchronization signal to tell it to start the line.
In the single-line composite video methods, these are all embedded in the single wire using
different voltage levels and pulses.
For RGB applications, including the VGA standards, three methods are in use:
1. RGB V, H: Vertical and Horizontal Sync on separate wires, so 5 wires (actually
coax cables) altogether.
2. RGB Composite Synch: Vertical and Horizontal Sync both on a ‘Composite Sync’
line, so this involves 4 wires altogether.
3. RGB Synch on Green: Vertical and Horizontal Synch both on the ‘G’ Green video
signal, so 3 wires altogether.
Differential RGB
In some defense applications, rather than sending the RGB active video on three coaxial
lines, it is sent on 3 ‘differential pairs’, where the signal is the difference in voltage between
each half of the pair. This method eliminates the effects of any ground noise in the signal, by
canceling it out, and is useful when there can be ground voltage differences between the
send and receive ends.
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Digital Video
Differential RGB
During the 1990’s, much of the video broadcast world moved to digital rather than the above
analog techniques, because:
· Digital video does not gradually degrade as it is re-transmitted from system to
system as is inevitably true with analog. With digital, the end result is identical to
the initial transmission no matter how many stages in the system, for better enddelivered
picture quality, with no ‘snow’, ‘ghosts’, or other visual degradation.
· Digital video can be highly compressed, for reduced bandwidth (on cable TV and
satellite links) and reduced storage needs (on DVD’s) as described later.