The most fundamental job of a video decoder is to separate
the color from the black and white information for video composite
signals. This task has been achieved many ways since
the introduction of color television over 50 years ago. Many
different separation methods have been used through the
years. With the availability of new cost effective technologies,
the consumer has been seeing a gradual improvement in
picture quality and detail. Advances in display tube technology
and semiconductor processes have pushed the technological
envelope providing sharper, more robust video. But
separating the chrominance from the luminance information is
especially challenging due to the fact that the signals overlap
each other in the frequency spectrum. How do you separate
them, while minimizing display artifacts?
Composite Signal Construction
The composite video signal is constructed with 3 basic elements:
- Luminance Information from DC to 5.5MHz (B&W Detail)
- Chrominance Information modulated onto a carrier (at
3.58MHz or 4.43MHz)
- Synchronization Information (Horizontal and Vertical Sync)The three analog elements of a composite video signal carry
all the information necessary to display a two dimensional
picture on a cathode ray tube (CRT) television.
Luminance (a B&W World)
The luminance signal carries the black and white parts of the
picture. This component of the composite video signal
requires the most bandwidth (typically to 5MHz), and signal
integrity, to convey sharp and clear images. Edge information,
brightness, and contrast of the image are entirely contained
in the luminance portion of the signal. Until 1947 the
broadcast video signal was only black and white. To maintain
compatibility with the installed equipment of the time, color or
chrominance information was added to the luminance signal
to create the color composite signal as we know it today. Figure
1 shows the specified bandwidths for NTSC and PAL.