Sound
An inter-carrier IF amplifier and detector follow the vision detector to pro- vide a sound signal for recording on a tape. After the detector, the sound recording processes follow audio cassette practice, with the audio signal being added to an HF bias source before recording on a longitudinal track on one edge of the tape as described earlier. The slow speed of the tape and narrow audio tape track do not make for ideal recording conditions, and noise reduction systems are commonly used to improve performance. Foremost among these is the Dolby system, which involves a form of non- linear and frequency-dependent pre-emphasis during the recording process, and complementary de-emphasis in replay.
Stereo machines use one of the hi-fi systems, VHS hi-fi, AFM-stereo or PCM. Virtually all hi-fi stereo VCRs have built-in NICAM decoders described in Chapter 17. Most pre-recorded cassettes offered for sale or rent also incorporate good stereo and surround sound tracks.
Bias oscillator
To provide a suitable a.c. recording bias for the ‘longitudinal’ audio signal and to generate an erasing signal to wipe out video tracks, a power oscil- lator is used. Usually consisting of a discrete transistor oscillator built round a feedback/driver transformer, it operates at about 60 kHz, and is used only in record mode. Besides the audio head, it powers a full-width erase head to wipe all signals, sound, picture and control track, off the tape on its journey towards the head drum during recording. As described in earlier chapters, no recording bias signal is required for the video heads as this function is performed for luminance and chrominance by the con- stant-level FM luminance recording signal.