Intelligible voice communication through adaptive delta modulation at bit rates lower than 10 kbit/s

The paper describes an Adaptive Delta Modulation (ADM) system which has been developed to operate at bit rates of 9.6 kbit/s and lower. The step size is adapted by deriving a control signal from the output bit stream and modifying the feedback signal in the second loop of a two-loop ADM coder through an f.e.t. multiplier. The system has an s.n.r. of 16 dB or more over an input range of 20 dB for a sinusoidal input of 400 Hz at the bit rate of 9.6 kbit/s. Extensive subjective tests of speech processed at 9.6 and 7.2 kbit/s have been carried out using the category judgement method and the modified rhyme test. At 9.6 kbit/s a mean intelligibility score of 88% and at 7.2 kbit/s a score of 74% have been obtained.