AZIMUTH OFFSET
As the diagrams of Fig. 13.3 show, in all formats the video tracks are laid directly alongside each other, with no intervening guard band. When a track is being read out during playback the adjacent tracks give rise to spurious signals in the video head, even when it is correctly ‘tracked’ and exactly centred on the wanted track. These crosstalk signals can cause patterning and interference on the reproduced picture. Since adjacent tracks are scanned by different heads (i.e. the two heads’ tracks are interleaved) the problem of crosstalk can be reduced by making each video head insensitive to the tracks recorded by the other. It is done by the azimuth-offset technique.
Conventional (i.e. audio) tape recorders use heads whose magnetic gap is at right angles to the direction of travel of the tape; this may be regarded as ‘normal’ azimuth. If a tape recorded by a normal- azimuth head is replayed in a machine whose head-gap is not at right angles to the tape’s travel the signal developed in that replay head will be very deficient in h.f. response, and the greater the azimuth offset the more will high frequencies be attenuated; indeed a high- frequency tone on a test tape is used to set up the azimuth angle of stationary audio heads in video and audio tape machines. By cutting the gaps on the two video heads on the drum at opposite angles from normal, the magnetic patterns written into adjacent tracks will have offset azimuth characteristics as shown in Fig. 13.4: azimuth angles are generally about ±6°. Crosstalk will still take place at low frequencies but from 1 MHz upwards a significant and increasing immunity will be manifest. As we shall shortly see, the colour, and where applicable the ATF signals, occupy the vulnerable l.f. end of the tape frequency spectrum. Special circuitry eliminates the effects of crosstalk from the colour signal, while the ATF system depends on crosstalk for its operation. Each will be examined in due course.