CHROMINANCE RECORDING
As Fig. 14.3 shows, VHS and Video 8 formats place the colourunder signal at around 700 kHz in the tape-frequency spectrum. Taking specifically the VHS system as example the recording arrangements are shown in simplified form in Fig. 14.6. Chroma signals are separated from the other components of the CVBS input signal in a composite filter based on 4.43 MHz. This filter has a bandwidth of about 1 MHz in order to limit each sideband of the chroma signal to approximately 500 kHz; in this way beating and interaction between taped luminance and chrominance components are avoided. A following a.c.c. stage regulates chroma amplitude by maintaining constant burst signal level in a sample-and-feedback circuit. For this purpose it contains a burst-gating circuit keyed by a suitably delayed line sync pulse. Here, too, is a colour killer to prevent the recording of ‘coloured noise’ on monochrome programmes.
Thus conditioned, the 4.43 MHz chroma signal enters a balanced modulator (mixer) where it is beat against a locally derived 5.06 MHz c.w. signal. The difference frequency – 630 kHz in round terms – is selected by a suitable low-pass filter and added to the luminance f.m. signal for application to the recording heads. In fact the colour- under frequency is precisely 626.952 kHz, derived from the 4.433619 MHz and 5.060571 MHz inputs to balanced modulator 2. The derivation and characteristics of this latter frequency is the key to successful elimination of the problems outlined earlier. It is produced by balanced modulator 1, and is itself the result of additive mixing of a 4.435571 MHz wave from a crystal oscillator; and a
625 kHz signal from the MN6061A IC. This chip is associated with a PLL wherein a 2.5 MHz oscilltor is locked to 160 times incoming line frequency by virtue of the ÷4 and ÷40 counters within the chip. Outputs from the ÷4 stage of the counter derive four separate phases of 625 kHz signal for use in the phase-select section of the IC. Dur- ing the tape-scanning period of video head A (as signalled by the PG pulse derived from a position-indicator on the head drum) the phase-select block passes 0° phase 625 kHz signal to balanced modulator 1. During the scanning period of head B, however, the phase-selector advances to the 90° position, effectively retarding the phase of the carrier of head B’s colour-under signal by 90° due to the action of subtractive balanced modulator 2. Head A sweeps next with a 0° subcarrier signal. The second sweep of head B sees the phase selector ‘step’ forward to render a recording phase of –180°; 0° for head A follows, then –270° for head B. Again head A sees the phase select block at 0°, and for the duration of the following head B sweep its phase, too, is stepped backwards to 0°. The step-back-phase action retards the chroma record phase by 90° per TV line for head B, then, while keeping head A chroma constant. It is important to understand that the 90° phase retard per line for head B is achieved by a phase advance in the PLL.
In this recording system, then, we have manipulated the colourunder signal in two ways: the frequency of the colour-under carrier has been tied firmly to recorded line sync frequency by the PLL, and a line-stepped phase-retard characteristic has been given to the colour signals recorded by video head B. The resulting pattern of colour signals on tape is indicated in Fig. 14.7, which is drawn in respect of the burst phases recorded on tape, The top row, for head A, is recorded without phase modification, so the burst phasors follow the standard PAL pattern over the eight TV lines shown. The phase- retard feature given to head B, however, results in a two-by-two line pattern in its recorded burst phasors. We shall return shortly to examine the lower lines in the diagram.