Bob Adams is an Analog Devices fellow. He is known for his contributions to the invention of log-domain filters, the design of the first monolithic asynchronous sample-rate converters for digital audio, and the development of groundbreaking multi-bit sigma-delta converters. Yet he began as “one of those kids who from the earliest age was fiddling with electronics,” he said.
“The 1960s were the era of Heathkits, ham radio, and the space program. I was taking apart and fixing TVs and radios from an early age, and I wandered into ham radio in the sixth grade or so,” Adams said.
“So when it came time to go to college in 1972, I didn’t have to think very hard about what I was going to do. I arrived at Tufts University with a fairly good hobbyist background in the electronics of the day. Back then, Tufts had a pretty heavy lab component to the EE classes, and I liked that. I remember I got particularly excited about digital filters. That was pretty early days for them.”
Adams acknowledges that he got off to a shaky start with some of the more analytical aspects of electronic engineering, but by tackling them on his own time, he got a firm grasp on them.
“I have to admit that all that stuff about differential equations and LaPlace transforms didn’t jell for me until I left school and started to use it,” he said. “I guess I was dissatisfied that I was using this math that I didn’t really understand. So I went back to it and closed the loop, got my fundamentals more in order.”
The Real World
His engineering career has always had a focus on professional audio, perhaps because of another interest—he’s a talented amateur saxophone player.
“When I first got out of school, I worked for a small audio company called ADS. It made speakers and automotive hi-fi systems. I only worked for them for about a year before I went to a company called DBX, an early competitor to Dolby,” Adams said.
“They made noise-reduction systems and a lot of analog processing gear, largely based on novel log/antilog-based voltage-controlled amplifiers and RMS detectors. So, I got a good dose of analog signal processing,” he said.
“One of the early things that fired up my imagination—when I arrived, they were designing RMS detectors using this funny little diode circuit that no one really understood. It just seemed to work. I tackled that from a mathematical point of view and ultimately realized that it was actually a linear filter operating in the log domain,” he said.
“That was one of the earliest things that I got really excited about. I presented a paper about the technology at the AES conference, but the concept lay dormant for 15 years until it was rediscovered in the nineties, and log-domain filters became a hot topic with hundreds and hundreds of papers.”
But that wasn’t sigma-delta. Sigma-delta analog-to-digital technology was known, thanks to Bell Labs, and its use in telecommunications. But it wasn’t ready for high-end audio.
“As audio started to drift into the digital world,” Adams said, “the state of a-to-d and d-to-a conversion was pretty primitive. You could buy these big hybrid bricks from several companies, including Analog Devices, for 40 or 50 dollars each. They’d be manually laser trimmed to maybe a 16-bit level if you were lucky.”
Those were the converters used in some of the earliest digital recorders for audio professionals. At that time, he became interested in the whole topic of oversampled sigma-delta modulation.
“They were used extensively in telecom applications, but no one had ever tried to extend that technology to the professional audio level. I got pretty excited about trying to do that at DBX. I designed a digital recorder based on companded delta modulation and that product was sold into the studio market,” he said.
“That was back in the days when the only other digital recorder was one from Sony. It was a box that converted digital audio into a video signal and then recorded that signal on a video recorder. So I did a 1-bit sigma-delta version of that. It was moderately successful, though not necessarily high-volume. Then I tried to make a more general a-to-d converter based on the same concept with the addition of a decimation filter.”
Semiconductor Strategizing
That was where Adams began his education in semiconductor design and manufacturing.
“Because DBX did not have an integration capability, we teamed up with NEC in Japan. I went over there for sometimes weeks at a time and worked alongside their VLSI guys. I did the system-level design and the decimator design and they did most of the circuit-level analog work,” he said.