Composite video is an analog video transmission that carries standard definition video typically
at 480i or 576i resolution. Video information is encoded on one channel, unlike the slightly
higher quality S-video and the even higher quality component video.
Composite video is usually in standard formats such as NTSC, PAL, and SECAM and is often
designated by the CVBS initialism, for color, video, blanking, and sync, or simply as video.
Signal components
A composite video signal combines on one wire the video information required to recreate
a color picture, as well as line and frame synchronization pulses. The color video signal
is a linear combination of the luminance of the picture, and a modulated subcarrier carries
the chrominance or color information, a combination of hue and saturation. Details of the encoding
process vary between the NTSC, PAL and SECAM systems.
The frequency spectrum of the modulated color signal overlaps that of the baseband signal,
and separation relies on the fact that frequency components of the baseband signal tend to
be near harmonics of the horizontal scanning rate, while the color carrier is selected
to be an odd multiple of half the horizontal scanning rate; this produces a modulated color
signal that consists mainly of harmonic frequencies that fall between the harmonics in the baseband
luma signal, rather than both being in separate continuous frequency bands alongside each
other in the frequency domain. In other words, the combination of luma and chroma is indeed
a frequency-division technique, but it is much more complex than typical frequency division
multiplexing systems like the one used to multiplex analog radio stations on both the
AM and FM bands. Colorburst is a composite analog video signal
generated by a video-signal generator used to genlock, keep the chrominance subcarrier
synchronized in television studios for color television.
Signal modulation Composite video can easily be directed to
any broadcast channel simply by modulating the proper RF carrier frequency with it. Most
home analog video equipment record a signal in composite format: LaserDiscs store a true
composite signal, while consumer videotape formats and lesser commercial and industrial
tape formats use modified composite signals. On playback, these devices often give the
user the option to output the baseband signal or to modulate it onto a VHF or UHF frequency
compatible with a TV tuner. The professional television production uncompressed digital
video videocassette format known as D-2 directly records and reproduces standard NTSC composite
video signals, using PCM encoding of the analog signal on the magnetic tape.
Standard connectors In home applications, the composite video
signal is typically connected using an RCA jack, normally yellow. It is often accompanied
with red and white connectors for right and left audio channels respectively. BNC connectors
and higher quality coaxial cable are often used in professional television studios and
post-production applications. BNC connectors were also used for composite video connections
on early home VCRs, often accompanied by either phono connectors or a 5-pin DIN connector
for audio. The BNC connector, in turn post dated the PL-259 connector which featured
on first generation VCRs. In Europe, SCART connections are often used
instead of RCA jacks, so where available, RGB is used instead of composite video with
computers, video game consoles, and DVD players. Video cables are 75 ohm impedance, low in
capacitance. Typical values run from 52 pF/m for an HDPE-foamed dielectric precision video
cable to 69 pF/m for a solid PE dielectric cable.
Modulators Some devices that connect to a TV, such as
VCRs, older video game consoles and home computers of the 1980s, output a composite signal. This
may then be converted to RF with an external box known as an RF modulator that generates
the proper carrier. Sometimes this modulator was built into the product and sometimes it
was an external unit powered by the computer or with an independent power supply. In the
USA, using an external RF modulator frees the manufacturer from obtaining FCC approval
for each variation of a device. Through the early-1980s, electronics that output a television
channel signal were required to meet the same shielding requirements as broadcast television
equipment, thus forcing manufacturers such as Apple to omit an RF modulator, and Texas
Instruments to have their RF modulator as an external unit, which they had certified
by the FCC without mentioning they were planning to sell it with a computer. In Europe, while
most countries used the same broadcast standard, there were different modulation standards,
and using an external modulator allowed manufacturers to make a single product and easily sell it
to different countries by changing the modulator. Video game consoles on the other hand were
less of an issue with FCC approval because the circuitry was inexpensive enough to allow
for channel 3/4 outputs. Modern day devices with analog outputs have
typically omitted channel 3 and 4 outputs in favor of composite and S-video outputs
as composite and S-video have become more common as inputs for TVs. In addition, many
TV sets sold these days no longer have analogue television tuners and cannot accept channel
3/4. But because composite video has a well-established market for both devices that convert it to
channel 3/4 outputs, as well as devices that convert things like VGA to composite, it has
offered opportunities to repurpose older composite monitors for newer devices.
Demodulation loss The process of modulating RF with the original
video signal, and then demodulating the original signal again in the TV, introduces several
losses. This conversion also typically adds noise or interference to the signal as well.
For these reasons, it is typically best to use composite connections instead of RF connections
if possible. Almost all modern video equipment has at least composite connectors, so this
typically is not a problem; however, older video equipment and some very low-end modern
televisions have only RF input; while RF modulators are no longer common, they are still available
to translate baseband signals for older equipment. However, just as the modulation and demodulation
of RF loses quality, the mixing of the various signals into the original composite signal
does the same, causing a checkerboard video artifact known as dot crawl. Dot crawl is
a defect that results from crosstalk due to the intermodulation of the chrominance and
luminance components of the signal. This is usually seen when chrominance is transmitted
with a high bandwidth, and its spectrum reaches into the band of the luminance frequencies.
This has led to a proliferation of systems such as S-Video and component video to maintain
the signals separately. Comb filters are also commonly used to separate signals, and eliminate
artifacts, from composite sources. Aspect ratio in composite signal
When used for connecting a video source to a video display that supports both 4:3 and
16:9 display formats, the PAL and NTSC television standards provide for signalling pulses that
will automatically switch the display from one format to the other. This is called Wide
Screen Signalling. Extensions to the composite video standard
Since TV screens hide the vertical blanking interval of a composite video signal and even
crop the edges of the picture, extensions have been implemented by taking advantage
of these unseen parts of the signal. Examples of these extensions include teletext, closed
captioning, digital information regarding the show title, transmitting a set of reference
colors that allows TV sets to automatically correct the hue mal-adjustments common with
the NTSC color encoding system, etc. Other extensions to the standard include S-video;
S-video is an extension to the standard because it uses parallel signal paths for luminance
and for chrominance, of which both of them can be connected to a composite video input
but with either monochrome, or uniform-luma color unless merging the signal paths with
a filter was done. See also
RCA connector Composite monitor
List of display interfaces NTSC color encoding
PAL color encoding S-video
References
External links Maxim - Apr 17, 2001 - Video Basics Tutorial
covering CVBS format structure. http:electronics.howstuffworks.com/tv9.htm