NICAM

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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. It has been standardized as ETS EN 300 163[1].

Contents

[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.

[edit] Nations using NICAM

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

Sources

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

Some Asia-Pacific nations have implemented NICAM

Many other countries use Zweiton analog stereo instead. Zweiton works very similiarly to the global VHF FM stereo standard that is used in FM Radio broadcasting globally.

[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. The mode is auto-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.

[edit] NICAM packet transmission

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

  • The topology of this pseudo-random generator yields a bitstream with a repetition period of 511 bits.
  • The psudo-random generator's polynomial is: x^9 + x^4 + 1.
  • The psudo-random generator is initized with: 111111111.

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

  • The NICAM header is not subject to scrambling. This is necessary so as to aid in locking on to the NICAM data stream and resynchronisation of the data stream at the receiver.
  • At the start of each NICAM packet the pseudo-random bit generator's shift-register is reset to all-ones.

[edit] NICAM transmission issues

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 4-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.

[edit] 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.

  • The level of the FM mono sound carrier must be at least -13 dB.
  • 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

  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. BWAnalyzer = bandwidth of the spectrum analyzer [kHz]
  5. K = logarithmic form factor of the spectrum analyzer ~2 dB

[edit] 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.

[edit] Two's complement signing

The two's complement method of signing the samples is used,[2] so that:

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

[edit] ±0v has three binary representations

  • 00000000000001 represents 0V, with no +/- distinction. This may have originated as a method to reduce the emergence of DC patterns from transmission of silent material.
  • 00000000000000 represents 0V, with no +/- distinction
  • 11111111111111 represents 0V, with no +/- distinction

[edit] Hidden services

NICAM's STL link capability

[edit] 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.

[edit] 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.

[edit] References

  1. ^ ETSI ETS EN 300 163, (previously: EBU T 3266 )
  2. ^ http://tallyho.bc.nu/~steve/nicam.html

[edit] See also

[edit] External links

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