Sound-in-Syncs is a method of multiplexing sound and video signals into a channel designed to carry video, in which data representing the sound is inserted into the line synchronising pulse of an analogue TV waveform. This is used on point-to-point links within broadcasting networks, including studio-to-transmitter links. It is not used for broadcasts to the public.


The technique was first developed by the BBC in the late 1960s. In 1966, The corporation's Research Department made a feasibility study of the use of pulse-code modulation (PCM) for transmitting TV sound during the synchronising period of the video signal. This had several advantages: it removed the necessity for a separate sound link and offered improved sound quality and reliability.Pawley, E (1972). "BBC Engineering 1922-1972", pp. 506-7, 522. BBC. ISBN 0 563 12127 0.]


Sound-in-Syncs and its R&D engineers have won several awards, including:
* The Royal Television Society's Geoffrey Parr Award in 1972 [ [ BBC Research: RTS Awards] ]
* A Queen's Award for Enterprise in 1974 [ [ BBC Research: Queen's Awards] ]
* In 1999, a Technology & Engineering Emmy Award [ [ BBC Research: Emmy Award for Sound in Syncs] ]


Original mono S-i-S

In the original system, as applied to 625 line analogue TV, the audio signal was sampled twice during each television line and each sample converted to 10-bit PCM. Two such samples were inserted into the next line synchronising pulse. At the destination, the audio samples were converted back to analogue form and the video waveform restored to normal. Compandors operating on the signal before and after decoding enabled the required signal-to-noise ratio to be achieved. As the PCM noise was predominantly high-pitched, the compandor only needed to operate on the high frequencies. Also, the compandor only operated at high audio levels, so that modulation of the noise by the companding would be masked by the relatively loud high-frequency audio components. 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.

Following successful trials with the BBC, in 1971 Pye TVT started to make and sell the S-i-S equipment under licence. The largest quantities went to the BBC itself, to the EBU and to Canada. Smaller numbers went to other countries including South Africa, Australia and Japan.Holder, J.E., Spenceley, N.M. and Clementson, C.S. (1984), "A two channel sound in syncs transmission system", IBC 1984, IEE Conference Publication No. 240, p. 345]

Ruggedised SiS

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. [ [ Dalton, C.J. (1971), "A P.C.M. Sound-in-Syncs System for Outside Broadcasts", BBC Engineering, No. 86, April 1971, pp 18-28.] ]

tereo SiS

Later systems, developed in the 1980s, used 14-bit linear PCM samples, digitally companded into 10-bit samples by means of the NICAM-3 lossy compression algorithm. These were capable of carrying two audio channels and were known as stereo sound-in-syncs.

Notes and references

Further reading

* "Waveform Specification of the BBC Sound-in-Syncs Equipment", EBU Review, 121A, June 1970.
* Chorley, J.M. and Shorter, D.E.L. (1970), "P.C.M. Sound-in-Syncs: Operational Systems for Video Distribution and Contribution Networks", IEE Conference Publication, No. 69, 1970 International Broadcasting Convention
* Dalton, C.J. (1968), "The Distribution of Television Sound Signals by PCM Signals incorporated in the Vision Waveform", IEE International Broadcasting Convention, September 1968, Vol. 46, Part 1
* [ Sanders, J.R. (1967), "Pulse sound: a system of television sound broadcasting using pulses in the video waveform", BBC Engineering Monograph, No. 67, May 1967]
* [ Sanders, J.R. (1968) "Pulse-code modulation for high-quality sound signal distribution: Incorporation of the sound signal in the video signal." BBC Research Department report (PDF)]
* [ Shorter, D.E.L., Chew, J.R., Howarth, D. and Sanders, J.R. (1968), "Pulse code modulation for high-quality sound signal distribution", BBC Engineering Monograph, No. 75, December 1968]
* Shorter, D.E.L. (1969), "The Distribution of Television Sound by Pulse-code Modulation Signals incorporated in the Video Waveform", EBU Review, No. 113A, February 1969

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