Perception of slope-overload distortion in delta-modulated speech signals

An experiment was performed in which controlled amounts of slope-overload distortion were produced in specified sections of a test sentence. Preliminary trials showed that the commonly used measure "percent of time of slope overload" is not a good indicator of perceptual distortion in delta-modulated speech signals. A more appropriate parameter for specifying slope-overload distortion is the rms slope truncation, which is the rms value of the truncated portion of the time derivative of the signal beyond the maximum absolute slope of the delta modulator. For a 3 kHz low-pass filtered speech signal, the just-perceptible level of distortion for the most sensitive of three subjects was 1.8 quantization units/20 kHz sampling period where 1 quantization unit = 2^-11of the peak-to-peak range of the waveform. This figure varied slightly for different speech sounds, but the range of variation was substantially less than the differences between subjects.