NICAM

From Free net encyclopedia

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 on analogue television transmissions. Audio is encoded using 14 bit pulse-code modulation at a sampling rate of 32 kHz.

1 History of NICAM
2 How NICAM works
3 See also
4 External links

[edit]

History of NICAM

NICAM was developed in the 80s by the BBC. The first NICAM digital stereo programme was broadcast on the BBC in 1986, though programmes were not advertised as being broadcast in stereo on the BBC until some five years later, when the majority of the country's transmitters had been upgraded to broadcast NICAM, and a large number of BBC programmes were being made in stereo.

The BBC publically launched their NICAM stereo service in the United Kingdom during 1991 (see 1991 in television) though other UK broadcasters ITV and Channel Four advertised this capability some months earlier.

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. Many other countries use analog stereo instead, in much the same manner as stereo FM Radio.

[edit]

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 that used for in-flight movies on international flights. In this mode, the user can select which soundtrack to listen to when watching the content by operating a "sound-select" control on the receiver.

NICAM currently offers the following possibilities, autoselected by the inclusion of a 3-bit type field in the data-stream:

One digital stereo sound channel.
Two completely different digital mono sound channels.
One digital mono sound channel and a 352 kbit/s data channel.
One 704 kbit/s data channel.

The four other options could be implemented at a later date. Only the first two of the ones listed are known to be in general use however.

Other facts:

The upper frequency limit of a sound channel is 15 kHz due to anti-aliasing filters at the encoder.
The 14 bit original sound samples are companded digitally to 10 bits for transmission. Samples are divided into blocks of 16. If all the samples in a block are quiet, such that the most significant bits are all zeros, these bits can be discarded at no loss. On louder samples some of the least significant bits are truncated, with the hope that they will be inaudible. A 4-bit control signal for each block records which bits were discarded. Digital companding ensures that the encoding and decoding algorithms can track perfectly.

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.

[edit]