In the encoder an audio input signal is firstly given an adaptive pre-emphasis characteristic by a pre-processing circuit (16) and is then converted to a digital signal for transmission by a delta-modulator (22). In order to minimise the digital bit rate the delta-modulator (22) is rendered adaptive by deriving a step-size control signal in a circuit (18) responsive to the slope of the input signal, and applying the control signal to vary the step-size of the delta-modulator (22). The bandwidth of the control signal is limited to say 50 Hz by the step-size circuit (18) itself or a subsequent low pass filter (24) in order to render the encoder insensitive to random step-size errors. The slow response of the control signal is compensated for by a delay circuit (20) preceding the delta-modulator (22). The corresponding decoder comprises an adaptive delta-modulator and a circuit complementary to the pre-processing circuit (16). In order to keep the decoder as simple as possible it has no step-size derivation circuit. Instead, the step-size control signal is transmitted in conjunction with the main digital signal by way of an analog to digital converter (26). In the encoder a complementary digital to analog converter is followed by another bandwidth limiting filter to reduce the effect of transmission bit errors. The slow response of the decoder control signal is compensated for by transmitting the step-size information from the encoder with a time advance relative to the main digital signal.
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