NICAM - History of NICAM, How NICAM works, NICAM's unusual features, VHS recording of NICAM audio

An acronym for near instantaneously companded [ie compressed and expanded] audio multiplex, a digital system used in television transmissions which provides high-quality stereophonic sound. First used by the BBC in 1986, it is now found in several other countries. A decoder attached to a television set enables the viewer to receive stereo sound along with any television programme which has recorded it.

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.

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.

Nations using NICAM

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

Belgium Denmark Finland France Hungary (Budapest only) Ireland Israel Luxembourg Norway Poland (only on transmitters bordering Lithuania) Portugal Spain Sweden

Sources

http://www.ee.surrey.ac.uk/Contrib/WorldTV/broadcast.html http://home.tiscalinet.ch/hahn/atvt.html http://www.videouniversity.com/standard.htm

Some Asia-Pacific nations have implemented NICAM

Hong Kong South Africa New Zealand Used by TVNZ, TV3 and C4.

How NICAM works

In order to provide mono "compatibility", the NICAM signal is transmitted on a subcarrier alongside the vision carrier.

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 an 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. Measuring the modulation level of the NICAM signal is difficult becuause the QPSK NICAM carrier waveform (unlike AM or FM modulated carrier waveforms) is not emitted at a discrete frequency.

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

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

L(NICAM) = actual level of the NICAM carrier [dBμV] L(Measured) = measured level of the NICAM carrier [dBμV] R = -3 dB bandwidth of the signal [kHz] BWAnalyzer = bandwidth of the spectrum analyzer [kHz] K = logarithmic form factor of the spectrum analyzer ~2 dB

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 Pulse Code Modulation.

Positive and Negitive full scale are not represented as you would expect with PCM

01111111111111 represents positive full-scale 10000000000000 represents negative full-scale

(+/-) 0v has three binary representations

00000000000001 represents 0V, with no +/- distinction this variation of 0V may be used to reduce DC patterns from emerging from transmission of silent material 00000000000000 represents 0V, with no +/- distinction 11111111111111 represents 0V, with no +/- distinction

Hidden services

NICAM's STL link capability

http://tallyho.bc.nu/~steve/nicam.html#SPARE

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.

VHS recording of NICAM audio

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.