Gamut of sum and difference modulationGamut of sum and difference modulation

Gamut of sum and difference modulation

18. As a result of applying modulation processing separately to the sum (M) and difference (S) signals, unnatural combinations of M and S signals become possible. That is, M and S signals may be generated in combinations which are not realisable in natural sound fields.

The maximum permissible phase deviation of the carrier is not more than 1.26 radians and the instantaneous carrier amplitude must not be reduced to zero with phase modulation simultaneously present. Hence the C‑QUAM modulator must apply limits to the level of difference signals (i.e. the S signal magnitude) during high instantaneous negative amplitude modulation (i.e. negative values of the M signal) or vice versa.

Figure A1.2 represents the gamut of M and S signals (normalised so that M lies between +1.25 and 1 and S lies between +1 and 1) that result in permissible levels of amplitude and phase modulation of the carrier.

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Figure A1.2: Gamut of C‑QUAM modulation

Note It will be observed that the limit on the S signal amplitude while the M signal is close to 1 is not a severe practical constraint. The long term average of normal stereo program material forms an ellipse on the above diagram with the major semi axis coinciding with the M axis. The ratio of major to minor axes of this ellipse is normally greater than 2:1.

Appendix 2 Emission Standard for the Australian Frequency Modulation Sound Broadcasting Service

The purpose of the emission standard for the Australian Frequency Modulation Sound Broadcasting Service is to define the parameters to be used for sound broadcasting in the frequency band 87.5 to 108 MHz. This standard supersedes the Emission Standard for the Australian Frequency Modulation Sound Broadcasting Service, published by the Department of Transport and Communications in July 1992. Provision is made for stereophonic as well as monophonic program transmission.

The standard for transmission of the main sound program follows international practice and is based on the pilot‑tone system specified in ITU‑R Recommendation BS.450.

The preferred standard for the transmission of an ancillary communications service (ACS) is that developed by the then Department of Communications and the Arts following field and laboratory tests. It is based on a 67 kHz sub‑carrier frequency. The scope of the ACS standard has been expanded to include specifications under which alternative ACS sub‑carrier frequencies may be used.

In general, tolerances are not specified as this standard is intended to define the nominal values for the transmitted signal.