Editing NICAM (section)

==Operation==
In order to provide [[Monaural|mono]] "compatibility", the NICAM signal is transmitted on a [[subcarrier]] alongside the sound carrier. This means that the [[Frequency modulation|FM]] or [[Amplitude modulation|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 movie]]s 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.

The spectrum of NICAM on the PAL system differs from that on the SECAM L system where the NICAM sound carrier is at 5.85&nbsp;MHz, before the AM sound carrier, and the video bandwidth is reduced from 6.5&nbsp;MHz to 5.5&nbsp;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&nbsp;kbit/s data channel. 
* One 704&nbsp;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.

=== 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 pseudo-random generator's [[polynomial code|polynomial]] is: <math>x^9 + x^4 + 1.</math>
* The pseudo-random generator is initialized with: <math>111111111.</math>

Making the NICAM bitstream look more like [[white noise]] is important because this reduces signal patterning on 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.

=== NICAM transmission problems ===

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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&nbsp;kHz.
* The upper frequency limit of a NICAM sound channel is 15&nbsp;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 bit]]s are all zeros, these bits can be discarded at no loss.
* On louder samples some of the [[least significant bit]]s 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 carrier power ===
ITU (and CCITT) standards specify that the power level of the NICAM signal should be at -20&nbsp;dB with respect to the power of the vision carrier.
* The level of the FM mono sound carrier must be at least -13&nbsp;dB.
* Measuring the modulation level of the NICAM signal is difficult because the QPSK NICAM carrier waveform (unlike AM or FM modulated carrier waveforms) is not emitted at a discrete frequency.

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

# L(NICAM) = actual level of the NICAM carrier [dBμV]
# L(Measured) = measured level of the NICAM carrier [dBμV]
# R = -3&nbsp;dB bandwidth of the signal [kHz]
# BWAnalyser = bandwidth of the spectrum analyser [kHz]
# K = logarithmic form factor of the spectrum analyser ~2&nbsp;dB

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