HEAD-SWEEP PHASING
The use of two heads to record a continuous picture means that at some point a switch must be made between the two. To facilitate this the head-wrap is a little more than one half-turn around the drum – typically it is made 186°. The ‘spare’ 6° permits an overlap period during which both heads are scanning the tape. The changeover point will inevitably make a visible disturbance on the picture, and this can- not be allowed to appear on the screen. It must take place during or close to the field blanking period, and since continuity of the field sync pulse is essential for correct field scan timing in the TV or monitor, and the post-field-sync period is a crucial one for TV monitor stability and can contain important signals, the head changeover is arranged to take place at the extreme end of the active field period – typically seven lines (450 μs) before field syncs. The disturbance due to head-switching now takes place at the very bottom of the picture and will be hidden by the slight overscan for which TVs and moni- tors are usually adjusted.
Although the heads are not signal-switched during record it is essential that each head enters onto the tape at just the right moment in the field-scan period of the video waveform being recorded – each head-scan will then contain exactly one TV field of 312^ lines, and the field sync pulses will all be lined up along the bottom of the tape ribbon. The magnetic patterns corresponding to each line in turn will then be positioned end to end along the slanting track, as depicted in Fig. 13.8, which also shows the control track pulses referred to earlier. This correlation of tape tracks and TV lines and fields is achieved by phasing up a drum (hence head) position indicating pulse (tacho pulse) with the incoming field sync pulse in a phase-locked- loop. In fact this PLL is the head-drum servo, to be examined in detail in Chapter 15. The exact phase relationship between field sync pulse and head tacho pulse is adjustable in the record switch point preset, with which the seven-lines-before-field-sync condition is set up.