NICAM: Цифрлық аудионы қысу стандартының тарихы

Digital audio compression standard
a compression form for digital audio

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.

The idea was first described in 1964. In this, the 'ranging' was to be applied to the analogue signal before the analogue to digital converter (ADC) and after the digital to analogue converter (DAC). The application of this to broadcasting, in which the companding was to be done entirely digitally after the ADC and before the DAC, was described in a 1972 BBC Research Report.

NICAM's second role – transmission to the public – was developed in the 80s by the BBC. This variant was known as NICAM 728, after the 728 kbit/s bitstream it is sent over. It uses the same audio coding parameters as NICAM 3. The first NICAM digital stereo programme was the First Night of the 92nd edition of the Proms which was broadcast on BBC2 from the Crystal Palace transmitting station in London on 18 July 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 publicly launched their NICAM stereo service in the United Kingdom on Saturday 31 August 1991 (see 1991 in television) though other UK broadcasters ITV and Channel 4 advertised this capability some 18 months earlier. Channel 4 began tests much earlier in February 1989 via the Crystal Palace transmitter in London. It has been standardized as ETS EN 300 163.

In order to provide mono "compatibility", the NICAM signal is transmitted on a subcarrier alongside the sound 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. This is the spectrum of NICAM on the PAL system. On the SECAM L system, the NICAM sound carrier is at 5.85 MHz, before the AM sound carrier, and the video bandwidth is reduced from 6.5 MHz to 5.5 MHz. NICAM currently offers the following possibilities. The mode is automatically selected 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.

There are some latent issues involved with the processing of NICAM audio in the transmission chain. NICAM (unlike the Compact Disc standard) samples 14 bit audio at 32 kHz. The upper frequency limit of a NICAM sound channel is 15 kHz due to anti aliasing filters at the encoder. The original 14 bit PCM audio samples are companded digitally to 10 bits for transmission. NICAM audio samples are divided into blocks of 32. 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 3 bit control signal for each block records which bits were discarded. Digital companding (using a CCITT J.17 pre emphasis curve) ensures that the encoding and decoding algorithms can track perfectly.

NICAM sampling is not standard PCM sampling, as commonly employed with the Compact Disc or at the codec level in MP3, AAC or Ogg audio devices. NICAM sampling more closely resembles Adaptive Differential Pulse Code Modulation, or A law companding with an extended, rapidly modifiable dynamic range.

In order to strengthen parity protection for the sound samples, the parity bit is calculated on only the top six bits of each NICAM sample. Early BBC NICAM research showed that uncorrected errors in the least significant four bits were preferable to the reduced overall protection offered by parity protecting all ten bits.

VHS and Betamax home videocassette recorders (VCRs) initially only recorded the audio tracks by means of a fixed linear recording head, which was inadequate for recording NICAM audio; this significantly limited their sound quality. Many VCRs later included high quality stereo audio recording as an additional feature, in which the incoming high quality stereo audio source (typically FM radio or NICAM TV) was frequency modulated and then recorded, in addition to the usual audio and video VCR tracks, using the same high bandwidth helical scanning technique used for the video signal. Full size VCRs already made full use of the tape, so the high quality audio signal was recorded diagonally under the video signal, using additional helical scan heads and depth multiplexing. The mono audio track (and on some machines, a non NICAM, non Hi Fi stereo track) was also recorded on the linear track, as before, to ensure backwards compatibility of recordings made on Hi Fi machines when played on non Hi Fi VCRs. Such devices were often described as "HiFi audio", "Audio FM" / "AFM" (FM standing for "Frequency Modulation") and sometimes informally as "Nicam" VCRs (due to their use in recording the Nicam broadcast audio signal). They remained compatible with non HiFi VCR players since the standard audio track was also recorded, and were at times used as an alternative to audio cassette tapes due to their superior frequency range and flat frequency response.

While recording in video mode (compatible with DVD Video), most DVD recorders can only record one of the three channels (Digital I, Digital II, Analogue mono) allowed by the standard. Newer standard such as DVD VR allows recording all the digital channels (in both stereo and bilingual mode), whereas the mono channel will be lost.

Codecs for digital media on computers will often convert NICAM to another digital audio format to save drive space.