A composite video signal is the sum of a luminance (brightness) signal
and a chrominance (color) signal. In video terminology they are referred
to as the Y and C signals respectively. The C signal is a specially
modulated combination of two other intermediate signals called I & Q,
which in turn were created from the original Red, Green, and Blue (RGB)
outputs of a video camera or film scanner.
Television signals can be transmitted over the air, or on cable systems,
by modulating an RF (Radio Frequency) carrier signal with audio and
composite video signals. The composite video and narrow bandwidth audio
signals are easily separated upon reception because they don't overlap
in the frequency spectrum. In order to minimize the bandwidth used by the
composite video signal, the frequency ranges of the Y & C signals are
designed to overlap. This permits more TV channels to be broadcast within
the total frequency spectrum available to broadcast television.
The problem created by overlapping the Y & C signal's frequency spectra
is that it is very hard to separate them completely. The original designers
of the U.S. NTSC television standard (in 1953) recognized this limitation,
but believed it was a good tradeoff to preserve frequency space and to
provide compatibility with the existing black and white TV system.
They very cleverly standardized on a system which interleaved the Y and
C frequencies where their spectra overlapped. The Y and C signal energy
clusters around specific frequencies related to the horizontal scan rate,
fitting together like the teeth of two combs. The signals alternate in
frequency between Y clusters and C clusters. This is illustrated by a small
section of the frequency distribution shown below.
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