NICAM (known also as NICAM 728, after the 728 kbit/s bitstream it is sent over), Near Instantaneous Companded Audio Multiplex, is a format for digital sound over television. Audio is encoded using 14 bit pulse-code modulation at a sampling rate of 32 kHz.
History of NICAM
NICAM was developed in the 80s by the BBC and was launched in the United Kingdom during 1991 (see 1991 in television) as their stereo TV audio transmission method. Some of the European countries like Denmark or France use it as their stereo TV audio transmission method, as do Hong Kong and New Zealand.
How NICAM works
In order to provide mono "compatibility", the NICAM signal is transmitted on a subcarrier alongside the vision carrier. This means that the FM or AM regular mono sound carrier is left alone for reception by monaural receivers.
A NICAM-based stereo-TV infrastructure can transmit a stereo TV programme as well as the mono "compatibility" sound at the same time, or can transmit two or three entirely different sound streams. This latter mode could be used to transmit audio in different languages, in a similar manner to what is done with international airline in-flight movies . In this mode, the user can select which sound stream to listen to when watching the content by operating a "sound-select" control on the receiver.
As far as NICAM-capable video cassette recorders are concerned, the common practice is to record the NICAM-derived stereo stream on the VHS Hi-Fi tracks while the mono compatibility signal is recorded on the linear track.
to be written - is this even necessary?
See also: A2
- A technical description of NICAM
- The BBC's information page on NICAM