VIDEO 8 COLOUR PROCESSING
The Video 8 format uses another variation on the same theme in order to arrange for crosstalk signals to come off the tape in antiphase over a two-line period. Here the colour-under frequency is higher than in other formats at 732 kHz, corresponding to 471⁄8 h. This frequency is locked to incoming line sync pulses by means of a PLL running at 5.86 MHz (375 f h) whose output is divided by eight in a counter to give 732 kHz. During head A’s sweeps the phase of this colourunder carrier is advanced by 90° per line to give the phase pattern on tape shown in the top row of Fig. 14.10 – this simplified diagram takes no account of the swinging burst, though the crosstalk cancellation process to be described works in just the same fashion when the PAL burst is taken into account. Head B’s sweep records constant- phase chroma shown by the black arrows in diagram row two. Rows three and four show the phases of the main and crosstalk signals during replay by head A, respectively before and after the necessary phase correction, effected by introducing a phase retard of 90°/line. The fifth row of the diagram represents the 2 h delayed signal – row four moved two places to the right. Comparison of the signal phases in rows four and five shows the now-familiar pattern of signal reinforcement and crosstalk cancellation when applied to the circuit of Fig. 14.9. Although the ‘clean’ signal on row 6 is drawn for head A signals, crosstalk on B channel is cancelled in just the same way, as study of the upper rows will prove.
In other respects V8 chrominance circuitry follows the same broad principles as the system already discussed. To minimise the effect of recording noise the crucial burst signal is doubled in amplitude (+6 dB) before recording, and restored to normal level by a gated 6 dB attenuator during playback. This raises the burst level to that of a fully saturated colour on tape, and is called burst emphasis. It is used also in USA-standard NTSC VHS videorecorders.
Opportunity has been taken in V8 format to introduce two more refinements for the sake of better colour reproduction: a chroma emphasis network is provided in record, with corresponding de-emphasis in playback; this improves chroma S/N ratio, particularly for low-amplitude high-frequency components. Secondly the chroma crosstalk delay/matrix is attended by a correlation detector operat- ing on a feedback loop – it acts to prevent distortion of the chroma signal on horizontal colour transitions in the picture where the direct and delayed main chroma signals are markedly different.
Video 8 format has no provision for SECAM-coded colour signals – they are converted to and from standard PAL by a transcoder IC.