REFERENCE SOURCES
Since the angular position of the capstan shaft is not relevant in the same way as that of the head drum, there is no intrinsic requirement for a PG on the capstan motor or shaft, and a PG is seldom provided here. For optimum performance of the capstan servo, however, a dual-loop system is often used, with both speed and phase loops working from the capstan FG output. In its most basic form the capstan servo is purely a speed regulator, and so may be ‘paced’ by a stable reference crystal during record, playback or both. It is common practice to use the 4.43 MHz chroma crystal as reference, and count down its output frequency to a rate comparable with the FG frequency; alternatively a dedicated servo reference crystal may be found. Once locked to such a reference the tape speed is regulated with great accuracy, and any momentary speed variations due to changes in tape tension or other factors are rapidly nullified by the servo-feedback system. During record the capstan shaft speed is always locked to some local reference – on playback, capstan control may be more elaborate, as will be described shortly.
Coming to the drum servo, on record the invariable phasing refer- ence (which also sets the speed, of course) is incoming field sync, in order to comply with the one-complete-field-per-head-sweep requirement as already described. During playback there is no external reference frequency to lock to, hence no absolute phasing requirement, unless the videorecorder output is required to lock to another source of video signals for use with a vision mixer or effects console – rare in domestic situations, and difficult to implement with a machine not designed for this genlock feature. In a stand-alone situation, however, line, field (and audio) frequencies must conform closely to broadcast standards to ensure correct timebase locking, and to prevent spurious effects in the TV or monitor. In playback, then, the reference pulses which master picture timing come from a crystal reference, usually the same one as was used for capstan speed control during record. To ensure correct tracking during replay, however, one phase control loop is essential: the control track loop. In V8 format machines the ‘control track’ is read out as part of the video track signal itself; the ATF tone system has already been discussed. In VHS machines the control track is a separate longitudinally recorded series of pulses on tape, used during replay as a positioning reference for the video tracks.
VHS control track
At the time of recording the divided-by-two incoming field sync pulses trigger a flip-flop to provide a 25 Hz squarewave. It is used, as already discussed in this and the last chapter, for head-drum phasing and for head identification in the colour-under circuits. It is also passed to the stationary control-track head, which is part of the audio head assembly, placed about 6 cm ‘downstream’ of the video head- drum exit point. At each change of state of the head flip-flop waveform, i.e. at 40 ms intervals, a mark is made in the magnetic pat- tern of the control track. Because the head-drum to control-head spacing is standardised, each of these marks will be made on the tape in a fixed position in relation to the start points of alternate video tracks. During replay the same control-track head reads out the pulses. By establishing a fixed phase between these and the head- drum PG pulses a relationship is set up between video head positioning and the physical disposition of video tracks on tape – this is what is required for tracking control. On replay, then, a local reference crystal is used to establish correct speed for both head drum and capstan. The error voltage coming from the phase detector comparing control track and head-PG pulses can be applied to either the head-drum motor or the capstan motor. In either case it will establish and maintain a constant head path with respect to the pre-recorded video tape tracks; to make them coincide a suitable delay is inserted into the path of either the control track pulse or the head PG pulse. The delay period can be varied by the user tracking control over (typically) two track widths, corresponding to about 180° of head-drum rotation. This enables the user to set up correct tracking on any tape/ machine combination under any circumstances – except those of a faulty machine. The mid-way, ‘auto’ or default setting of the track- ing control corresponds to format standard and to a specific drum- to-control-head spacing. It is used during replay of the machine’s own recordings, and those recorded in other correctly set-up videorecorders.
ATF replay tracking
V8 format videorecorders have no separate control track as such. The need for tracking correction is indicated by imbalance in the two crosstalk-beat frequencies. As described in Chapter 13 and Fig. 13.7 an error voltage is produced from this, used to directly adjust capstan speed. In this way perfect and automatic tracking is assured, with no need for a tracking control.