Specifications

Video formats and interfaces
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Black Box Explains
Video formats and interfaces.
Today’s video formats fall into two broad categories: those designed
for broadcast video/television systems and those designed for computer
graphics.
The design requirements for these categories differ. Broadcast and
television video formats must limit the transmission bandwidth required
for the signal. Computer graphics formats, in contrast, are far less
restricted in bandwidth, but they must deliver a picture suitable for
viewing from very short distances.
We’ll explain many of today’s different video formats that fall into
these two categories.
Broadcast/television video formats
Composite video is very familiar to most of us. It’s the analogue
television signal before modulation
onto an RF carrier and is the standard
that connects most consumer video
equipment, including VCRs,
camcorders, security cameras and DVD
players.
As its name suggests, Composite video has the luminance (black
and white), chrominance (colour) and sync pulses combined in one
signal. When developed, Composite video was designed to work with
both colour and black-and-white TV signals. This backwards
compatibility ensured a smooth transition between the two formats
in the 1950s. Black-and-white TV sets were able to ignore the colour
component while newer sets separated it out and displayed it with
the luminance information.
Although this format solved
the problem of backward
compatibility at the time, by
today’s standards, Composite
video doesn’t project a very sharp
picture. Because all the video
components are transmitted together, they can interact with each other
and cause picture defects like dot crawl and colour smear.
Y/C video (also often called S-Video) was introduced in the 1980s
to overcome some of the shortfalls associated with Composite video. It’s
a less encoded video format. Colour (C) and luminance (Y) information
are transmitted separately to produce a sharper picture image on
the display device.
Most video equipment with an S-Video connector also has a
Composite video connector. When connecting devices together that
support both interfaces, it’s best to use the S-Video connector because
it will generally give you a sharper picture.
Component video (YCbCr) separates the signal to an even greater
extent than S-Video, further reducing the chance of interference and,
as a result, improving picture quality. Component video separates colour
information into two colour difference signals: B-Y (Blue minus
luminance, also called Cb or Pb) and R-Y (Red minus luminance, also
called Cr or Pr). These along with Y (luminance) result in a total of
three signals.
You can find Component video
on some DVD players and TV
receivers, displaying the very high
quality images permitted by DVDs
to their best advantage.
Broadcast video signal
standards
In addition to the video
formats discussed above, there
have also been more than a dozen
different broadcast standards in use at different times throughout the
world. Today, most countries use one of three standards. These standards
aren’t compatible, which means that when connecting video equipment,
you not only need them to support the same video format but also the
same broadcast signal standard.
Here’s a brief summary of the standards:
NTSC (National Television Systems Committee) standard systems
are used primarily in North and Central American countries, most South
America countries and Japan.
The technical format of NTSC is 480 viewable lines per frame with a
30 frames per second refresh rate. The 30 frames consist of 60 fields, the
timing of which is based on the 60-Hz electrical system used in these
countries. One field is one-half (every other line) of the interlaced frame.
Other countries use a 50-Hz electrical system, which means their
television systems are based on 50 fields per second rates.
PAL (Phase Alternation Line), developed in Germany, is the European
equivalent of NTSC and offers 576 viewable lines per frame. The refresh
rate is 25 frames per second based on 50 fields per second because it uses
the European 50-Hz electrical system. Compared to NTSC, PAL has a
greater number of lines. This adds detail to the picture, but PAL’s 50
fields per second rate (when compared with NTSC’s 60 fields per second
rate), means a greater chance of noticeable flicker.
SECAM (Séquential Couleur à Mémoire) is very similar to PAL with
the same number of lines and same frame rate. It was developed in
France and is also used in Russia, parts of Africa and Eastern Europe.
Despite the similarities of the two standards, SECAM is not compatible
with PAL because, unlike PAL, the chrominance is FM modulated.
HDTV (High-Definition Television), a very high quality digital
broadcast television standard, is the long-awaited, next-generation
solution to replace analogue TV formats like NTSC and PAL.
HDTV delivers much clearer, sharper images with 720 or 1080 lines of
resolution compared to the 625 lines of PAL, but requires the use of HD
compatible television sets and receivers.
Computer graphics video formats
Television video signals, as we have seen, are typically combined
together into a lower-bandwidth encoded signal like Composite video.
In contrast, computer graphics signals don’t have the same bandwidth
restrictions and, therefore, keep the red, green and blue colour signals
separate to allow higher-resolution pictures that are suitable for viewing
from short distances.
There are many different analogue graphics video formats, all based
on separate RGB signals but differing in the connector style used, how
the sync information is transmitted, and what resolutions and refresh
rates are supported. Care must be taken to select the correct display
hardware for a particular video interface because differing formats
are often incompatible and need active converters if they are to be
interconnected. Some display technologies, such as DVI, provide
digital video connectivity. This enables enhanced video quality but
requires the use of digital display devices to realise this.
Here’s an overview of computer graphic video formats, both old
and new:
VGA based formats
The VGA (Video Graphics Array) graphics card from IBM, introduced
in 1987, represented a huge improvement over EGA. With a horizontal
scan rate of 31.5 KHz (from 24.1 KHz), VGA supports resolutions up to
640 x 480 with 256 colours. The video signal is analogue RGB with
separate horizontal and vertical sync signals presented on an HD15
connector.
SVGA (Super Video Graphics Array), XGA (Extended Graphics Array)
and later formats have continued the drive to provide ever sharper
4
C Signal
3
Y Signal
1
Y Ground
2
C Ground
Y (luminance)
Y ground
R-Y (Cr or Pr)
R-Y ground
B-Y (Cb or Pb)
B-Y ground
S-Video (Y/C) (4-Pin mini DIN)
R
CA cable
The three Component video plugs.