Depth-multiplex audio
This technique depends to some degree on the magnetic layer of the tape itself to discriminate between the video and audio signals. A separate pair of heads on the spinning drum is provided solely for the audio signal FM carriers. A typical layout of heads on the drum is shown in Figure 21.36. The two audio heads are mounted 180° apart and are arranged to ‘lead’ the video heads. The gaps cut in the audio heads have large azimuth angles: ±30°, sufficient to prevent cross-talk from adjacent tracks at the carrier frequencies (around 1–2 MHz) involved. The mounting height of the audio heads is set to place the hi-fi audio track in the centre-line of the corresponding video track. This track layout is shown in Figure 21.37, which also gives an idea of the relative audio track widths; the audio tracks are half the width of the vision tracks they accompany. The audio heads have a relatively wide gap resulting in a deep magnetic pattern penetrating some microns into the magnetic coating of the tape. Shortly following the audio head comes the regular video head, whose gap is in the region of 0.25 flm. Writing (for the most part) at higher frequencies, it creates shallower, shorter magnetic patterns which penetrate less than 1 flm into the magnetic surface. Thus, the recorded tape contains a ‘two- storey’ signal: a buried layer of long-wavelength audio patterns under a shallow top layer of video patterns.
During replay, the same heads operate on the same tracks as before. The video head picks off its track with little impairment, and with minimum cross-talk from sound tracks because of the large disparity (36° for VHS- SP) between the azimuth angle of recorded track and replay head. The audio head during replay is handicapped by the barrier presented by the ‘video layer’ on tape, but the resulting 12 dB or so of attenuation – thanks to the use of bandpass filters and the noise-immunity of the FM carrier system—does not prevent noise-free reproduction of the baseband audio signal as long as the tracking is reasonably correct. For hi-fi sound, the tracking performance is critical, especially in LP modes where narrow tracks tend to magnify any errors.
As with the video heads, switchover between the two rotating audio heads is carried out during the overlap period when both heads are momentarily in contact with the tape, one just leaving the wrap and one just entering. Because of the angular offset between video and audio heads on the drum (Figure 21.36), the head switchover point for the latter is set by a second, delayed head flip-flop square wave triggered from the drum tacho-pulse. The FM carrier signals to and from the audio heads on the drum require their own rotary transformers, which may be concentric with those for the video signals under the drum or mounted above.
The frequencies used for VHS stereo hi-fi in depth-multiplex systems are 1.4 MHz (L) and 1.8 MHz (R); the FM modulation sidebands extend for about 250 kHz on each side of the (unsuppressed) carriers. Each audio head deals with both FM carriers throughout its 20 ms sweep of the tape— during replay, the carriers are separately intercepted by bandpass filters for processing in their own playback channels.