NICAM

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.

Contents

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.

It has been standardized as ETS EN 300 163.[6]

Nations using NICAM public broadcasts

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

Some Asia-Pacific nations/regions have implemented NICAM

Some other countries use Zweikanalton analogue stereo instead.

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 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 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 auto-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.

When measured with spectrum analyser the actual level of the carrier (L) can be calculated using the following formula:

L(NICAM) = L(Measured) + 10 log (R/BWAnalyser) + K

  1. L(NICAM) = actual level of the NICAM carrier [dBμV]
  2. L(Measured) = measured level of the NICAM carrier [dBμV]
  3. R = -3 dB bandwidth of the signal [kHz]
  4. BWAnalyser = bandwidth of the spectrum analyser [kHz]
  5. K = logarithmic form factor of the spectrum analyser ~2 dB

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

The two's complement method of signing the samples is used,[10] so that:

±0 V has three binary representations

Hidden services

NICAM's STL link capability

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

As far as NICAM-capable video cassette recorders are concerned, the common practice is to record the NICAM-derived stereo stream on the VHS or Betamax analogue Hi-Fi tracks while the mono compatibility signal is recorded on the linear track. The Hi-Fi tracks are written diagonally beneath the diagonal video tracks using depth multiplexing. The mono audio track (and on some machines, a non-NICAM, non-Hi-Fi stereo track) is recorded on the linear track. This ensures backwards-compatibility with recordings made on Hi-Fi machines with non-Hi-Fi video recorders.

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.

See also

References

  1. ^ Croll, M.G., Osborne, D.W. and Spicer, C.R. (1974), Digital sound signals: the present BBC distribution system and a proposal for bit-rate reduction by digital companding. IEE Conference publication No. 119, pp. 90–96
  2. ^ Bartlett, C.J.C. and Greszczuk, J. (1964), Companding in a p.c.m. system. Symposium on Transmission Aspects of Communication Networks, London, IEE 1964, pp. 183–186.
  3. ^ Osborne, D.W. (1972) Digital sound signals: further investigation of instantaneous and other rapid companding systems. BBC Research Dept. Report 1972/31.
  4. ^ Jones, A.H. (1978), Digital coding of audio signals for point-to-point transmission. IEE Conference Publication No. 166, pp. 25–28
  5. ^ Gilchrist, N.H.C. (1978), Digital sound signals: tests to compare the performance of five companding systems for high-quality sound signals. BBC Research Department Report 1978/26.
  6. ^ ETSI ETS EN 300 163, (previously: EBU T 3266)
  7. ^ "Broadcasting System Details". University of Surrey – Department of Electronic Engineering. http://www.ee.surrey.ac.uk/Contrib/WorldTV/broadcast.html. Retrieved 2007-08-30. 
  8. ^ Analogue TV technologies
  9. ^ World-Wide T.V. Standards
  10. ^ Steve Hosgood. ""All You Ever Wanted to Know About NICAM but were Afraid to Ask"". http://stoneship.org.uk/~steve/nicam.html. Retrieved 2007-08-30. 

Further reading

External links

Related websites or technical explanations