Philips 20PT643F Service Manual page 87

Comb Filter
Introduction
The video signal prepared for broadcast contains two major parts commingled, the
luminance (makes a black and white picture in full detail) and chrominance (coloration
with not quite all the detail). This method is used instead of red, green, and blue sub-
signals in order to get the best looking picture that can be transmitted in the limited
bandwidth of the broadcast channel.
Every TV receiver and VCR must contain a filter to separate the luminance and color (Y
and C) again. Less than perfect Y/C separators lose resolution -- horizontal, vertical, or
both. Also there are artifacts such as rainbow swirls where thin stripes should be, and
crawling dots where patches of different colors meet. The perfect Y/C separator does
not exist yet, although some 3D comb filters come close.
There are several methods for filtering:
No comb filter. The cheapest solution is to use simple filters (notch, low pass,
bandpass filters) that pass only the coarse and medium horizontal detail (lower 3
MHz or so) to the luminance circuits and pass the bulk of the color information
still commingled with the fine luminance detail (3 to 4.2 MHz) to the color circuits.
Two line ordinary filter. The improvements over 'no' comb filter are: revealing of
finer horizontal detail overall, and some reduction of rainbow swirls. Improvement
of fine detail is most prominent where details consist of upright dark and light
lines.
Three line ordinary filter. Improvement over the two-line filter consists of sharper
transition from one color to another at sharp horizontal color boundaries, and less
dot crawl.
Three line adaptive (a.k.a. 2D; dynamic) filter. This method adapts the mixing
according the line content of two fields. The big improvement that the 2D comb
filter brings, is the elimination (or near elimination) of dot crawl. This type of filter