VIMCAS, standing for Vertical Interval Multiple Channel Audio System, is a dual-channel Sound-in-Syncs mechanism for transmitting digitally encoded audio in a composite video analogue television signal.
Invented by Australian company IRT in the 1980s, the basic concept of VIMCAS is to transmit two channels of PCM-encoded (i.e. digital) audio during the vertical blanking interval 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 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, or as separate channels to carry dual-language transmissions. In outside broadcast (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 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, 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 between the studio and the OB or foldback.
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog signal.
NTSC, named after the National Television System Committee, is the analog television color system that was introduced in North America in 1954 and stayed in use until digital conversion. It was one of three major analog color television standards, the others being PAL and SECAM.
Phase Alternating Line (PAL) is a colour encoding system for analogue television used in broadcast television systems in most countries broadcasting at 625-line / 50 field per second (576i). It was one of three major analogue colour television standards, the others being NTSC and SECAM.
Composite video is an analog video signal format that carries standard-definition video as a single channel. Video information is encoded on one channel, unlike the higher-quality S-video and the even higher-quality component video. In all of these video formats, audio is carried on a separate connection.
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 broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.
Serial digital interface (SDI) is a family of digital video interfaces first standardized by SMPTE in 1989. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video. A related standard, known as high-definition serial digital interface (HD-SDI), is standardized in SMPTE 292M; this provides a nominal data rate of 1.485 Gbit/s.
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.
576i is a standard-definition video mode originally used for terrestrial television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because of its close association with the colour encoding system, it is often referred to as simply PAL, PAL/SECAM or SECAM when compared to its 60 Hz NTSC-colour-encoded counterpart, 480i. In digital applications it is usually referred to as "576i"; in analogue contexts it is often called "625 lines", and the aspect ratio is usually 4:3 in analogue transmission and 16:9 in digital transmission.
A PCM adaptor is a device that encodes digital audio as video for recording on a videocassette recorder. The adapter also has the ability to decode a video signal back to digital audio for playback. This digital audio system was used for mastering early compact discs.
A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized audio channel, which is received by television receivers belonging to a public audience, which display the image on a screen. A television transmitter, together with the broadcast studio which originates the content, is called a television station. Television transmitters must be licensed by governments, and are restricted to a certain frequency channel and power level. They transmit on frequency channels in the VHF and UHF bands. Since radio waves of these frequencies travel by line of sight, they are limited by the horizon to reception distances of 40-60 miles depending on the height of transmitter station.
Television encryption, often referred to as "scrambling", is encryption used to control access to pay television services, usually cable or satellite television services.
Dolby Digital Plus, also known as Enhanced AC-3 is a digital audio compression scheme developed by Dolby Labs for transport and storage of multi-channel digital audio. It is a successor to Dolby Digital (AC-3), also developed by Dolby, and has a number of improvements including support for a wider range of data rates, increased channel count and multi-program support, and additional tools (algorithms) for representing compressed data and counteracting artifacts. While Dolby Digital (AC-3) supports up to five full-bandwidth audio channels at a maximum bitrate of 640 kbit/s, E-AC-3 supports up to 15 full-bandwidth audio channels at a maximum bitrate of 6.144 Mbit/s.
The 405-line monochrome analogue television broadcasting system was the first fully electronic television system to be used in regular broadcasting.
SMPTE 292 is a digital video transmission line standard published by the Society of Motion Picture and Television Engineers (SMPTE). This technical standard is usually referred to as HD-SDI; it is part of a family of standards that define a Serial Digital Interface based on a coaxial cable, intended to be used for transport of uncompressed digital video and audio in a television studio environment.
Ancillary data is data that has been added to given data and uses the same form of transport. Common examples are cover art images for media files or streams, or digital data added to radio or television broadcasts.
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.
CCIR System B was the 625-line analog broadcast television system which at its peak was the system used in most countries. It is being replaced across Western Europe, part of Asia and Africa by digital broadcasting.
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 amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.
CCIR System A was the 405-line analog broadcast television system broadcast in the UK and Ireland. System A service was discontinued in 1985.
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 and east 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.