Frequency-multiplex audio
An alternative approach, used in Video-8 VCRs, to helical sound record- ing is to use the video heads to lay down on the tape an FM audio sound- track. The baseband frequency response is limited to about 15 kHz. As Figure 21.26 earlier shows the audio FM carrier is based on 1.5 MHz and has a deviation-plus-side-band width of about 300 kHz. It is possible to insert this ‘packet’ between the outer skirts of the upper chrominance and ower luminance sidebands, permitting it to effectively become part of the video signal so far as the heads and tape are concerned. The FM audio sig- nal carrier is added to the FM luminance signal in the recording amplifier, and laid on tape at a low level – some 18 dB below that of the luminance carrier. This suppression of sound carrier level helps prevent mutual inter- ference between sound and vision channels, whose outer sidebands over- lap to some degree.
The original AFM recording system made provision only for mono- phonic sound. The advent of hi-fi stereo recording in the competing VHS format led to the adoption of a stereo AFM plan for Video-8 formats, using a second sound carrier at 1.7 MHz. Unlike VHS hi-fi, however, it is not possible here to simply use one carrier for each channel because compati- bility with mono AFM equipment must be maintained. The stereo-AFM system is illustrated in Figure 21.38. Incoming L and R audio signals are brought together in an adder, and the result halved in an attenuator to derive an (L + R)/2 signal for FM modulation and recording on tape at 1.5 MHz. This forms the compatible mono signal. The L and R record signals are also routed to a matrix in which they are subtracted to produce an L – R signal, which is now halved to render an (L – R)/2 signal. It is this which is processed and frequency modulated onto the 1.7 MHz carrier. During replay, the recovered L + R and L – R signals are added and subtracted in separate matrices to derive L and R signals. Readers who are familiar with VHF-FM analogue stereo sound broadcast/reception tech- niques will recognise this ‘stereo-difference’ technique – in radio broad- casts, the difference signal is conveyed in a sub-carrier at +38 kHz.
In replay mode, the separation of the various off-tape signals in Figure 21.26 is carried out by four separate filters: a low-pass type with cut-off around 180 kHz for ATF tones; a low-bandpass filter centred on 732 kHz for colour-under signals; a narrow bandpass one tuned to 1.5 MHz forinterception of the audio-FM carrier; and finally a high-pass (roll-on about 1.7 MHz) acceptor for the FM vision signal with its sidebands.