AUDIO-ONLY RECORDING ON VIDEOTAPE
Whatever the type and format of a videorecorder, its automatic programme-timer facility and long running time (compared to audio Compact Cassettes) give it useful potential as an audio-only recorder, offering progressively higher levels of reproduction quality in Hi-Fi and PCM versions. There are several possibilities for using a video- recorder wholly for audio signals. With a conventional longitudinal- sound machine, an add-on ‘box’ is required to generate TV sync signals in order to keep the machine’s servos operating correctly to enable the recording of audio signals. Although this arrangement is very wasteful of tape area, and renders lower quality sound than an audio-cassette machine, it can sometimes be useful.
Many Hi-Fi/f.m.-sound equipped videorecorders have an audio- only facility. In this mode an internal timing generator provides TV-sync-like pulses to maintain servo lock during record, and to establish a control track for the same purpose during replay. With no video signal to present a mutual interference threat, the possibility of increasing audio-f.m. writing current (to reduce the effects of possible dropout and increase replay limiter margin) is there. Up to eight hours (VHS−LP mode, E240 tape) of high-quality sound recording and playback is thus possible.
PCM audio-only recording offers a high-standard domestic record- and-playback sound system. For use with conventional (i.e. VHS) videorecorders, a PCM processor can be used. In effect it is a codec, digitising audio signals and presenting them to the videorecorder in a form that it can recognise as a video signal. On replay the same pro- cessor decodes the video-like digital replay signal to render a very good quality audio output.
The Video 8 format, already designed to cater for a with-vision PCM sound system, has also the capability for high-quality audio-only use. The digital-compression system already described shows that only 30° of head sweep is required for one stereo sound programme. If the entire video signal area is used for PCM recording as shown in Fig. 17.9(c), a total of six segments, each occupying about 30° of head rotation, is possible. If these segments are used exclusively for audio, either six simultaneous tracks can be recorded and replayed (i.e. for sound-studio use and subsequent mixing); or the six segments can be used sequentially for a total (with a P5–90 tape in LP mode) eighteen hours of very high-quality sound. The arrangement of the six three-hour segments is shown in Fig. 17.9(d). Identification of the individual segments is made by a fifth ATF-like tone laid down during record.