Colour difference demodulation
The weighted chrominance components U and V from the separator unit are applied to their individual colour difference demodulators to obtain B’- Y’ and R’- Y’. The third colour difference signal, G’- Y’, is obtained by a G’- Y’ matrix (Figure 14.6). For the B’- Y’ demodulator, the sub- carrier is shifted by 90° for the R’- Y’ demodulator, the sub-carrier is phase reversed every line through the PAL switch. Recall that the U and V components of the chrominance signal are the weighted colour difference signals, whereby
It follows that, before B’- Y’ and R’- Y’ are recovered, the weighted components must be deweighted. This is usually carried out at the demodulation stage by the inclusion of colour difference amplifiers which provide the B’- Y’ channel with more gain than the R’- Y’ channel.
In amplitude modulation, the information is contained in the change of the amplitude of the peak of the carrier. When the carrier is suppressed, the modulating information continues to reside in the changing amplitude of the modulated signal. To recover the original information, the amplitude of the modulated signal has to be detected when the carrier is at its peak. In order to do this, a synchronous detector is used. In essence, the colour difference synchronous demodulator is a switching device which detects the level of the incoming modulated U (or V) signal every time the regenerated sub-carrier is at its positive peak.