Genius!2: Robert Adams and the 18-bit A-to-D converter #249 December 2016
Mel Lambert writes: “My breakthrough moment came in the late-Eighties, when I realised that everybody else was heading in the wrong direction,” recalls IC designer Robert Adams, considered by many to be the father of one-bit delta-sigma converters. Such devices are based on oversampling by many times the target sampling frequency; they deliver higher precision and dramatically simplify the design of downstream decimating filters.
“I grew up in suburban New Jersey, and was an electronic tinkerer from an early age.” While working as director of research at dbx, Adams decided to take an orthogonal path to the development of PCM converter systems, and innovate the world’s first noise-shaped audio converter with better than 16-bit resolution.
“At that time,” he states, “existing delta-sigma converter chips were targeted at the telecommunications industry and satisfactory for voice applications, while high-resolution audio converters used traditional laser-trimmed, successive-approximation techniques that priced them out of the consumer-audio market.” Adams extended those one-bit chips that often produced spurious idle tones to four-bit operation “and added dither to eliminate the idle tones”. Adams’ second trick was to replace the switched-capacitor elements used in telecom chips, and “utilise instead continuous-time RC circuits that enhanced the circuit’s dynamic range”.
The end result, a 128-times oversampling delta-sigma converter with true 18-bit performance, was released by dbx in 1988 as a two-part chip set manufactured by NEC in Japan with digital filters from US manufacturer VLSI Technology Company, and used around that time on many Telarc recordings. As well as A-to-D and D-to-A converters, while at dbx Adams helped develop one of the first digital recorders.
Having joined Analog Devices in 1989, he continued to specialise in audio DSP theory and practice, together with DSP cores and algorithms, including asynchronous digital sample-rate conversion. “My first contribution at AD was to work on its first delta-sigma audio A-D converter, and how to predict the stability of one-bit high-order feedback systems.” In June 2016 Adams received the IEEE’s coveted Industrial Pioneer Award for his groundbreaking work on commercial delta-sigma converters.
Picture: Top: Robert Adams. Last: The fabled 128-times oversampling delta-sigma A-to-D converter from dbx, designed by Robert Adams to offer true 18-bit performance – an industry first.
Published earlier this year and sponsored by QSC Audio, Genius!2 is the second edition of Genius!, celebrating those clever people whose inventions have transformed the world of professional audio. The 30-page supplement is also available to read in a handy digital-edition form