SPEED-CORRECTION SERVOS
For CAV discs the required motor speed is exactly 1500 r.p.m., as with a video head drum. For CLV discs the speed is variable, falling from an initial 1500 r.p.m. to around one-third that speed at programme end. In both cases the speed of the disc-drive motor must be closely and accurately controlled. Unlike a videocassette recorder the disc system is a replay-only one, so speed control need consist only of ensuring that off-disc line sync pulses come at intervals of 64 μs precisely. This is easily arranged in a servo identical in principle to those used for control of a videorecorder’s head drum during replay mode, employing speed and phase loops. Here the reference signal will be a stable crystal whose divided-to-f h output is phase-compared to off-disc line syncs to produce an error output drive to the disc motor.
Such a motor-servo system will ensure correct replay timing in the long term, but is unable to handle timing errors having a frequency higher than the 25 Hz disc rotation rate. For the correction of short- term speed errors a timebase corrector is used. Short-term variations in disc speed have the effect of bunching up or stretching out in time the off-disc signal to a point beyond the tolerance of a colour system which depends on the phase of the chroma subcarrier signal to convey hue information. To keep the timing error below 8 ns (the limit for good colour reproduction) a variable signal delay system is used, wherein the delay period is varied in equal and opposite characteristic to the off-disc timing variations to effectively iron them out. It can be done with either a CCD delay line (of the type we met in Chapter 7) whose clock frequency is governed by off-disc signals; or by (in later models) a digital field store (see Chapter 12) with a locally generated readout-address clock.