HEAD TRACKING
Although the tape guides and the lower-drum rabbet ensure that the head sweeps are in the right plane to align with the videotape tracks during replay (and to record standardised patterns on the tape during record) the earlier formats such as VHS do not incorporate any replay-head guidance system within the recorded track itself. For these systems a track-position indicator must also be recorded on tape. It takes the form of a marker pulse recorded on the control track by a stationary head downstream of the head-drum scanner. This control- pulse head is incorporated into the audio head assembly, and during record is fed with one pulse per 40 ms, marking the tape at alternate video tracks. The control pulse is derived from the incoming video signal itself, to which the head-drum speed and phase are also locked; this ensures correct phasing between video tracks and control-track pulses and establishes a fixed physical relationship on tape between the two. The control track pulses are used during replay to establish and hold the head-sweep paths in line with the video tracks on the tape.
At playback the head sweeps will be in the correct plane, but without tracking control there is no reason why they should be aligned with the pre-recorded tracks. As alternate tracks are recorded with offset azimuths, correct and noise-free replay can only be assured by means of a control loop based on the off-tape control-track pulses, referencing head-sweep positions to the actual tracks on tape. Video- recorders using this system have a tracking control with which tolerances can be taken up: the tracking control acts as a phasing control with which the ‘head-aiming path’ can be rocked about the nominally correct position. Optimum setting of the tracking control is when the head sweep is centred on the track and the reproduced picture shows minimum noise.