Instantaneously Companding Digital Signal Processors

We extend the technique of instantaneous companding (compressing and expanding) to digital signal processors (DSPs). Companding has been used for many years in non-dynamical channels, but the dynamical nature of DSPs causes output distortion in standard implementations of companding, where a compressor and expander are used at the input and output, respectively, without additionally modifying the DSP. In the proposed technique, we specify a method for combining input compression, output expansion, and application of nonlinear functions internal to the DSP, all in a manner transparent to the input-output characteristics of the DSP and its A/D interfaces, thus eliminating distortion in the final analog output. As a result, all the signals involved span most of the available bits, resulting in significant improvement in quantization errors and signal-to-noise-plus-distortion ratios over a large input range. The theory is supported by simulation and subjective listening tests.