Externally Linear Time Invariant Digital Signal Processors

We present a technique whereby internal signals in a DSP can be externally controlled without causing any output disturbances. While the resulting system is in general internally time-varying or internally nonlinear, the external input-output behavior is time-invariant and identical to that of the prototype linear time invariant (LTI) system. Our technique can be used to dynamically scale the input, output, and all internal states of a fixed-point DSP such that they always stay close to full scale, thus spanning most of the available bits and making possible a large signal-to-noise-plus-distortion ratio (SNDR) over a large (external) input range. We show that this application of our technique is an extension of companding (compressing/expanding) to DSPs. A hardware implementation of a companding DSP is presented. At low and medium signal levels, the output SNDR of our companding DSP is significantly larger than that of the corresponding classical DSP using the same number of bits.