composite video signal (analog) displayed on a
DSO. 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 (Y) of the picture and a
chrominance subcarrier which carries the color information (C), a combination of
hue and
saturation. Details of the combining 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. The signals may be separated using a
comb filter. In other words, the combination of luma and chrominance 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. A gated and filtered signal derived from the color
subcarrier, called the
color burst, is added to the
horizontal blanking interval of each line (excluding lines in the
vertical sync interval) as a synchronizing signal and amplitude reference for the chrominance signals. In NTSC composite video, the burst signal is inverted in phase (180° out of phase) from the reference subcarrier. In PAL, the phase of the color subcarrier alternates on successive lines. In SECAM, no color burst is used since phase information is irrelevant.
Composite artifacts . Note the distinctive checkerboard pattern on the vertical edges between yellow and blue areas. The combining of component signals to form the 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 high bandwidth, and its spectrum reaches into the band of the luminance frequencies.
Comb filters are commonly used to separate signals and eliminate these artifacts from composite sources.
S-Video and
component video avoid this problem as they maintain the component signals physically separate.
Recording Most home
analog video equipment record a signal in (roughly) composite format:
LaserDiscs and
type C videotape for example store a true composite signal modulated, while consumer videotape formats (including
VHS and
Betamax) and commercial and industrial tape formats (including
U-matic) use modified composite signals
FM encoded (generally known as
color-under). The professional
D-2 videocassette format
digitally stores a sampled
analog composite video signal on
magnetic tape. With the advent of affordable, high-speed analog-to-digital converters, real-time composite-to-YUV sampled digital sampling has been possible since the 1980s, and raw waveform sampling and software decoding have been possible since the 2010s.
Extensions A number of so-called
extensions to the visible TV image can be transmitted using composite video. Since TV screens hide the
vertical blanking interval of a composite video signal, these take advantage of the unseen parts of the signal. Examples of extensions include
teletext,
closed captioning, information regarding the show title, a set of reference colors that allows TV sets to automatically correct NTSC hue maladjustments,
widescreen signaling (WSS) for switching between
4:3 and
16:9 display formats, etc. == Connectors and cable ==