Companded Delta Modulation for Telephony

An experimental delta modulation system that appears to satisfy Bell System telephone requirements in the telephone plant environment is described. A significant improvement in signal-to-noise ratio over a large dynamic range has been achieved with a modification that is akin to syllabic companding. It consists in adapting the modulator quantizing step size to the signal level over part of the dynamic range. The development was stimulated by the suitability of delta modulators for low-cost integrated circuits. The implementation is based on repeated use of two standard monolithic integrated circuits: an RTL logic gate and an operational amplifier. Several laboratory models have consistently satisfied Bell System transmission requirements with a bit rate of 96.5 kHz and a companding range of 26 dB. A series of subjective evaluation tests are being carried out at this time to assess listener reactions and to optimize system parameters accordingly. Indications are that a reduction of the bit rate will result from these tests. The delta modulation scheme is economically more attractive than pulse-code modulation (PCM) in telephone applications that require digital coding of the analog signal close to its source and with a minimum of common equipment.