Down-conversion
The process for down-converting the chrominance signal is shown in Figure 21.32. The double-sideband signal is first separated from lumi- nance information in a high-pass filter, then its bandwidth is limited to around 1 MHz by a bandpass filter. This curtailment of the sidebands of the modulated signal has the effect of restricting the chroma signal detail to around 500 kHz, less than half that of the broadcast signal. The chroma signal now passes into a mixer where it beats with a stable locally gener- ated CW signal at around 5 MHz. This is the familiar heterodyne effect, and the mixer output contains components at two frequencies, those of the sum and difference of the two input signals. A low-pass filter selects the required colour-under signal, and rejects the spurious HF product.
The colour-under signal contains all the information that was present in the 4.43 MHz chroma signal except the finer colour detail; thus chroma amplitude and phase modulation are preserved, as well as the phase and ident characteristics of the burst, albeit based on a much lower sub-carrier frequency.
Up to now we have been quoting sub-carrier frequencies in round num- bers. Format specifications quote the colour-under frequency very pre- cisely, and taking VHS as an example, the local CW signal is at a frequency of 5.060571 MHz, which when beat against the 4.433619 MHz broadcast sub-carrier provides a colour-under centre frequency of 5.060571 – 4.433619 = 0.626952 MHz, or 626.952 kHz. Due to the broadcast sub-carrier modulation, sidebands extend for roughly 500 kHz on each side of this carrier.
The local CW signal at 5.06 MHz needs to be locked (i.e. have a fixed phase relationship) to the line syncs of the recorded signal and this is achieved by a phase-locked loop (PLL).