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 television waveform. This is used on point-to-point links within broadcasting networks, including studio/transmitter links (STL). 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 television 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.[1]


Sound-in-Syncs and its R&D engineers have won several awards, including:


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 encoding 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.[1]

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.[5]

Ruggedised S-i-S

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.[6]

Stereo S-i-S

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


The ITV network used coders and encoders produced by RE of Denmark. The two variations of Sound-in-Syncs used by the BBC and ITV were not compatible. The terms DCSIS or DSIS was commonly used in ITV to describe dual channel Sound-in-Syncs. Very often material carried was dual mono and not stereo.

Notes and references

  1. ^ a b Pawley, E (1972). BBC Engineering 1922-1972, pp. 506-7, 522. BBC. ISBN 0-563-12127-0.
  2. ^ BBC Research: RTS Awards
  3. ^ BBC Research: Queen's Awards
  4. ^ BBC Research: Emmy Award for Sound in Syncs
  5. ^ 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
  6. ^ Dalton, C.J. (1971), A P.C.M. Sound-in-Syncs System for Outside Broadcasts, BBC Engineering, No. 86, April 1971, pp 18-28.

Further reading

2K resolution

2K resolution is a horizontal resolution of approximately 2,000 pixels on a display device or content. Digital Cinema Initiatives (DCI) defines 2K resolution standard as 2048×1080.In the movie projection industry, DCI is the dominant standard for 2K output.

441-line television system

441 lines, or 383i if named using modern standard, is an early electronic monochrome television system. It was used with 50 interlaced frames per second in France and Germany, where it was an improvement over the previous 180-line system. In the United States it was used by RCA with 60 frames per second from 1938 to 1941.


480i is a shorthand name for the video mode used for standard-definition analog or digital television in Caribbean, Myanmar, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay and Uruguay). The 480 identifies a vertical resolution of 480 lines, and the i identifies it as an interlaced resolution. The field rate, which is 60 Hz (or 59.94 Hz when used with NTSC color), is sometimes included when identifying the video mode, i.e. 480i60; another notation, endorsed by both the International Telecommunication Union in BT.601 and SMPTE in SMPTE 259M, includes the frame rate, as in 480i/30. The other common standard, used in the other parts of the world, is 576i.

In analogue contexts, this resolution is often called "525 lines". It is mandated by CCIR Systems M and J, which are usually paired with NTSC color - which led to the "NTSC" name being often inaccurately used to refer to this video mode. Other color encodings have also been used with System M, notably PAL-M in Brazil.


480p is the shorthand name for a family of video display resolutions. The p stands for progressive scan, i.e. non-interlaced. The 480 denotes a vertical resolution of 480 pixels, usually with a horizontal resolution of 640 pixels and 4:3 aspect ratio (480 × ​4⁄3 = 640) or a horizontal resolution of 854 or less (848 should be used for mod16 compatibility) pixels for an approximate 16:9 aspect ratio (480 × ​16⁄9 = 853.3). Since a pixel count must be a whole number, in Wide VGA displays it is generally rounded up to 854 to ensure inclusion of the entire image. The frames are displayed progressively as opposed to interlaced. 480p was used for many early Plasma televisions. Standard definition has always been a 4:3 aspect ratio with a pixel resolution of 640 × 480 pixels.


576p is the shorthand name for a video display resolution. The p stands for progressive scan, i.e. non-interlaced, the 576 for a vertical resolution of 576 pixels, usually with a horizontal resolution of 720 or 704 pixels. The frame rate can be given explicitly after the letter.

CCIR System A

CCIR System A was the 405 line analog broadcast television system broadcast in the UK and Ireland. CCIR service was discontinued in 1985.

CCIR System G

CCIR System G is an analog broadcast television system used in many countries. There are several systems in use and letter G is assigned for the European UHF system which is also used in the majority of Asian and African countries. (However some countries in Europe use different systems.)

CCIR System H

CCIR System H is an analog broadcast television system primarily used in Belgium, the Balkans and Malta on the UHF bands.

CCIR System I

CCIR System I is an analog broadcast television system. It was first used in the Republic of Ireland starting in 1962 as the 625-line broadcasting standard to be used on VHF Band I and Band III, sharing Band III with 405-line System A signals radiated in the north of the country. The UK started its own 625-line television service in 1964 also using System I, but on UHF only - the UK has never used VHF for 625-line television except for some cable relay distribution systems.

Since then, System I has been adopted for use by Hong Kong, Macau, the Falkland Islands and South Africa. The Republic of Ireland has (slowly) extended its use of System I onto the UHF bands.

As of late 2012, analog television is no longer transmitted in either the UK or the Republic of Ireland. South Africa expects to discontinue System I in 2013, and Hong Kong by 2020.

CCIR System M

CCIR (or FCC) System M, sometimes called 525 line, is the analog broadcast television system used in the United States since July 1, 1941, and also in most of the Americas and Caribbean, South Korea, and Taiwan. Japan uses System J, which is nearly identical to System M. The systems were given their letter designations in the ITU identification scheme adopted in Stockholm in 1961. Both System M and System J display 525 lines of video at 30 frames per second using 6 MHz spacing between channel numbers, and is used for both VHF and UHF channels.

Currently (as of 2015), System M and J is being replaced by digital broadcasting such as the Americas, Japan, South Korea, Taiwan and the Philippines.


Distortionmeter (or more precisely distortion factor meter) is an electronic measuring instrument which displays the amount of distortion added to the original signal by an electronic circuit.

Dolby AC-4

Dolby AC-4 is an audio compression technology developed by Dolby Laboratories. Dolby AC-4 bitstreams can contain audio channels and/or audio objects. Dolby AC-4 has been adopted by the DVB project and standardized by the ETSI.

Frequency offset

In radio engineering, a frequency offset is an intentional slight shift of broadcast radio frequency (RF), to reduce interference with other transmitters.

Ghost-canceling reference

Ghost-canceling reference (GCR) is a special sub-signal on a television channel that receivers can use to attenuate the ghosting effect of a television signal split into multiple paths between transmitter and receiver.

In the United States, the GCR signal is a chirp in frequency of the modulating signal from 0 Hz to 4.2 MHz, transmitted during the vertical blanking interval over one video line (line 19 in the U.S.), shifted in phase by 180° once per frame, with this pattern inverted every four lines. Television receivers generate their own local versions of this signal, and use the comparison between the local and remote signals to tune out any ghost images on the screen.

GCR was introduced after its recommendation in 1993 by the Advanced Television Systems Committee.


Near Instantaneous Companded Audio Multiplex (NICAM) is an early form of lossy compression for digital audio. It was originally developed in the early 1970s for point-to-point links within broadcasting networks. In the 1980s, broadcasters began to use NICAM compression for transmissions of stereo TV sound to the public.


PAL-M is the analog TV system used in Brazil since February 19, 1972. At that time, Brazil was the first South American country to broadcast in colour. Colour TV broadcast began on February 19, 1972, when the TV networks Globo and Bandeirantes transmitted the Caxias do Sul Grape Festival. Transition from black and white to colour was not complete until 1978. Two years later, in 1980, colour broadcast nationwide in Brazil was commonplace.

It is unique among analog TV systems in that it combines the 525-line 30 frames-per-second System M with the PAL colour encoding system (using very nearly the NTSC colour subcarrier frequency), unlike all other countries which pair PAL with 625-line systems and NTSC with 525-line systems.

Sound multiplex in broadcasting

In broadcasting, sound multiplex is a method to deliver alternative audio feeds on television and radio channels.

VIT signals

In television broadcasting, VIT signals (vertical interval test signals) are a group of test signals inserted in the composite video signal. These signals are used to weight the transmission characteristics of the system between the test generator and the output of the demodulator, where the system includes the microwave links, or TVROs as well as the TV transmitters and the transposers. There are both ATSC and EBU standards for VIT. (Because analogue television is being phased out globally, VIT standards are considered superseded.)


Zweikanalton ("two-channel sound") or A2 Stereo, is an analog television sound transmission system used in Germany, Austria, Australia, Switzerland, Netherlands and other countries that use or used PAL-B or PAL-G. South Korea formerly utilised its variant of this format in analogue television system until 31 December 2012. It relies on two separate FM carriers.

This offers a relatively high separation between the channels (compared to a subcarrier-based multiplexing system) and can thus be used for bilingual broadcasts as well as stereo. Unlike the competing NICAM standard, Zweikanalton is an analog system.

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