Philips 20PT643F Service Manual page 89

Video Demodulation
The color decoder circuit detects whether the signal is a PAL, NTSC, or SECAM signal.
The result is made known to the auto system manager. The PAL/NTSC decoder has an
internal clock generator, which is stabilized to the required frequency by using the 12
MHz clock signal from the reference oscillator of the microcontroller / teletext decoder.
The base-band delay line is used to obtain a good suppression of cross color effects.
The Y signal and the delay line outputs U and V are applied to the luminance / chroma
signal processing part of the TV processor.
Luminance / Chrominance signal Processing
The output of the YUV separator is fed to the internal YUV switch, which switches
between the output of the YUV separator or the external YUV (for DVD or PIP) on pins
51-53. Pin 50 is the input for the insertion control signal called 'FBL-1'. When this signal
level becomes higher than 0.9 V (but less than 3 V), the RGB signals at pins 51, 52, and
53 are inserted into the picture by using the internal switches.
Also, some picture improvement features are implemented in this part:
Black stretch. This function corrects the black level of incoming signals, which
have a difference between the black level and the blanking level. The amount of
extension depends upon the difference between actual black level and the
darkest part of the incoming video signal level. It is detected by means of an
internal capacitor.
White stretch. This function adapts the transfer characteristic of the luminance
amplifier in a non-linear way depending on the average picture content of the
luminance signal. It operates in such a way that maximum stretching is obtained
when signals with a low video level are received. For bright pictures, stretching is
not active.
Dynamic skin tone correction. This circuit corrects (instantaneously and locally)
the hue of those colors, which are located in the area in the UV plane that