Audio Scrambler and Sub-Audio Signalling Processor
CMX138A
7.4.3 Audio Compandor
The compandor is comprised of a compressor and an expandor. The compressor’s function is to reduce
the dynamic range of a given signal by attenuating larger amplitudes while amplifying smaller amplitudes.
The expandor’s function is to expand the dynamic range of a given signal by attenuating small amplitude
signals (e.g. noise) while amplifying large amplitude signals. The compressor is used prior to transmission
and the expandor is used in the receiver. Hence, using a compandor will enhance performance in a
communication system by transmitting a compressed signal, which is less likely to be corrupted by noise,
and then at the receiver expanding the compressed signal, which will push the noise picked up during
transmission down further.
The CMX138A uses a “syllabic compandor.” This type of compandor, as opposed to the instantaneous
compandor (e.g. µ/A-law PCM), responds to changes in the average envelope of the signal amplitude
according to a syllabic time constant τ. Typically, the steady state output for the compressor is
proportional to the square root of the input signal, i.e: for a 2 dB change in input signal, the output change
will be 1 dB. Generally for voice communication systems a compressor is expected to have an input
dynamic range of 60 dB, providing an output dynamic range of 30 dB. The expandor does the inverse.
Figure 11 Expandor Transient Response
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