“NICAM”的意思、由来-开放百科全书

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.^[1] In the 1980s, broadcasters began to use NICAM compression for transmissions of stereo TV sound to the public.

History

Near-instantaneous companding

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).^[2] 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.^[3]

Point-to-point links

NICAM was originally intended to provide broadcasters with six high-quality audio channels within a total bandwidth of 2048 kbit/s. This figure was chosen to match the E1 primary multiplex rate, and systems using this rate could make use of the planned PDH national and international telecommunications networks.

Several similar systems had been developed in various countries, and in about 1977/78 the BBC Research Department conducted listening tests to evaluate them. The candidates were:

It was found that NICAM-2 provided the best sound quality, but reduced programme-modulated noise to an unnecessarily low level at the expense of bit rate. NICAM-3, which had been proposed during the test to address this, was selected as the winner.^[4]^[5]

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

Broadcasts to the public

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 broadcast on BBC2 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 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 months earlier. Channel 4 began tests much earlier in February 1989 via the Crystal Palace transmitter in London.

Nations using NICAM public broadcasts

Several European countries (as well as one Asian country) have implemented NICAM with the PAL and SECAM TV systems^[7]^[8]^[9]

Some other countries use Zweikanalton analogue stereo instead. Analogue stereo conversion thus begins.

How NICAM works

In order to provide mono "compatibility", the NICAM signal is transmitted on a subcarrier alongside the sound carrier. This means that the FM 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, a point below the FM 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.

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.

NICAM packet transmission

The NICAM packet (except for the header) is scrambled with a nine-bit pseudo-random bit-generator before transmission.

Making the NICAM bitstream look more like white noise is important because this reduces signal patterning on adjacent TV channels.

NICAM transmission issues

There are some latent issues involved with the processing of NICAM audio in the transmission chain.

NICAM carrier power

ITU (and CCITT) standards specify that the power level of the NICAM signal should be at -20 dB with respect to the power of the vision carrier.

note: if BWAnalyser is greater than R, the formula becomes L(NICAM) = L(Measured) + K

NICAM's unusual features

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.

Two's complement signing

±0 V has three binary representations

Parity checking limited to only 6 of 10 bits

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.

Recording of NICAM audio

VCR

VHS and Betamax home videocassette recorders ("VCR"s) initially only recorded the audio tracks using 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 exceptional bandwidth, frequency range, and extremely flat frequency response.

DVD

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.

Flash memory and computer multimedia

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