VIMCAS, standing for Vertical Interval Multiple Channel Audio System, is a

dual-channel In the fields of digital electronics and computer hardware, multi-channel memory architecture is a technology that increases the data transfer rate between the DRAM memory and the memory controller by adding more channels of communication betwee ...

Sound-in-Syncs 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 ...

mechanism for transmitting digitally encoded audio in a

composite video Composite video is an analog video signal format that carries standard-definition video (typically at 525 lines or 625 lines) as a single channel. Video information is encoded on one channel, unlike the higher-quality S-Video (two channe ...

analogue television signal. Invented by Australian company IRT in the 1980s, the basic concept of VIMCAS is to transmit two channels of

PCM Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the a ...

-encoded (i.e. digital) audio during the

vertical blanking interval In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next fram ...

of a composite video signal. The encoded audio was transmitted over 6 horizontal scan lines during that interval, the digitally encoded signal being placed onto a series of mid-grey pedestals, in much the same way that the colour

subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio bro ...

is placed on top of the monochrome signal. As with the colour subcarrier, there is 4.7kHz bandwidth, so six lines would provide 28kHz of bandwidth (actually slightly less, there being deliberate redundancy between the final packet of encoded audio on one line and the first packet of encoded audio on the next, in order to avoid signal corruption). This could be used as a pair of 14kHz channels for

stereo audio Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration ...

, or as separate channels to carry dual-language transmissions. In

outside broadcast Outside broadcasting (OB) is the electronic field production (EFP) of television or radio programmes (typically to cover television news and sports television events) from a mobile remote broadcast television studio. Professional video c ...

(OB) work, where VIMCAS was used from the OB site back to the studio, it could be used for separate audio channels where one would be effects (i.e. the ambient sound of a sports match) and the other would be the main audio (e.g. the voice of the commentator), or alternatively with the effects audio carried by VIMCAS and the main audio carried as NICAM 728. To fit into the available bandwidth, the audio signal would first be

companded In telecommunication and signal processing, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited dynamic range. The name is a portmanteau of the words compressing and expanding, ...

and limited before being sampled for PCM encoding. The encoded signal would be transmitted in the six scanlines in time compressed form, i.e. much faster than its actual speed. Decoding was simply the reverse process, with 100ms of audio (at a time) stored in the transmitted digital form into a digital memory and played out from that memory at original speed through a

digital-to-analogue converter In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function. There are several DAC archit ...

, with appropriate timing circuits to synchronize this playout with the accompanying video. A reduced version, using just one scan line instead of six and thus providing narrower bandwidth, was called VISCAS (Vertical Interval Single Channel Audio System), which was good enough for

talkback Talkback or talk back may refer to: * ''Talkback'' (album), a 1983 album by the Canadian band the Spoons * ''Talk Back'' (Kembe X album), 2016 *Talkback, an alternate name for Marvel Comics superhero Chase Stein *Talkback (recording), an audio ...

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