In 1966, The corporation's Research Department made a feasibility study of the use of pulse-code modulation (PCM) for transmitting television sound during the synchronising period of the video signal.
At the destination, the audio samples were converted back to analogue form and the video waveform restored to normal.
A pilot tone at half the sampling frequency was transmitted to enable the expander to track the gain adjustment applied by the compressor, even when the latter was limiting.
[1] Following successful trials with the BBC, in 1971 Pye TVT started to make and sell the S-i-S equipment under licence.
[5] A ruggedised version of the system was developed, which provided about 7 kHz audio bandwidth, for use over noisy or difficult microwave paths, such as those often encountered for outside broadcasts.