A digital recording system has been designed which employs a new type of A/D converter in place of the 16-bit linear PCM converters currently in use. The conversion process described in this paper is called Companded Predictive Delta Modulation, and differs from adaptive delta modulation in that the input signal is dynamically adjusted by a voltage-controlled amplifier and then passed through a fixed (nonadaptive) delta modulator. Dither is added to provide a white noise floor and minimize low-level distortion. Instead of a simple integrator, the delta modulator uses a second-order feedback filter, which substantially increases its dynamic range. (If nonlinear stabilization techniques are employed, high-order filters may be used.) Since delta modulators have low sensitivity to bit errors, the error-correction circuitry can be simplified. A convolutional error-correction code was chosen for this application.
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