Delta Modulation - SBS Application 24Kbps Delta Modulation

SBS Application 24Kbps Delta Modulation

Delta Modulation was used by Satellite Business Systems or SBS for its voice ports to provide long distance phone service to large domestic corporations with a significant inter-corporation communications need (such as IBM). This system was in service throughout the 1980s. The voice ports used digitally implemented 24kbit/s Delta Modulation with Voice Activity Compression or VAC and Echo Suppressors to control the half second echo path through the satellite. They performed formal listening tests to verify the 24kbit/s Delta Modulator achieved full voice quality with no discernable degradation as compared to a high quality phone line or the standard 64kbit/s µ-law Companded PCM. This provided an eight to three improvement in satellite channel capacity. IBM developed the Satellite Communications Controller and the voice port functions.

The original proposal in 1974 used a state-of-the-art 24kbit/s Delta Modulator with a single integrator and a Shindler Compander modified for gain error recovery. This proved to have less than full phone line speech quality. In 1977one engineer with two assistants in the IBM Research Triangle Park, NC laboratory was assigned to improve the quality.

The final implementation replaced the integrator with a Predictor implemented with a two pole complex pair low pass filter designed to approximate the long term average speech spectrum. The theory was that ideally the integrator should be a predictor designed to match the signal spectrum. A nearly perfect Shindler Compander replaced the modified version. It was found the modified compander resulted in a less than perfect step size at most signal levels and the fast gain error recovery increased the noise as determined by actual listening tests as compared to simple signal to noise measurements. The final compander achieved a very mild gain error recovery due to the natural truncation rounding error caused by twelve bit arithmetic.

The complete function of Delta Modulation, VAC and Echo Control for six ports was implemented in a single digital integrated circuit chip with twelve bit arithmetic. A single DAC was shared by all six ports providing voltage compare functions for the modulators and feeding sample and hold circuits for the demodulator outputs. A single card held the chip, DAC and all the analog circuits for the phone line interface including transformers.