Editing Delta modulation (section)

=== Satellite Business Systems 24 kbps delta modulation ===
{{More citations needed|section|date=January 2025}}{{Shorten|date=January 2025|fix=It just needs to be a brief overview of the system and tell how it is special or notable. Don't need too many technical details, which could be better presented by citing a technical paper which curious readers could then click on.|section}}
Delta modulation was used by [[Satellite Business Systems]] (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]]). Each traffic channel had a 32&nbsp;kbit/s bitrate.<ref>"SBS SYSTEM DESCRIPTION", W. H. Curry, Jr., Manager System Definition and Control Satellite Business Systems McLean, Virginia.https://commons.erau.edu/cgi/viewcontent.cgi?article=2543&context=space-congress-proceedings</ref> This system was in service throughout the 1980s. The voice ports used '''digitally implemented 24&nbsp;kbit/s delta modulation''' with Voice Activity Compression (VAC) and [[echo suppressor]]s to control the half second echo path through the satellite. They performed formal listening tests to verify the '''24&nbsp;kbit/s delta modulator''' achieved '''full voice quality''' with no discernible degradation as compared to a high quality phone line or the standard 64&nbsp;kbit/s [[μ-law]] [[Companding|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 24&nbsp;kbit/s delta modulator with a single integrator and a ''Shindler Compander''{{Definition needed|What is a "Shindler Compander"? Presumably it is a form of a [[companding]], but how is it special from other companders?|date=July 2023}} modified for gain error recovery. This proved to have less than full phone line speech quality. In 1977, one 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 [[digital-to-analog converter]] (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.