Interviews

High Definition Compatible Digital^® (HDCD^®), the proprietary process for improving the sound of 16-bit digital audio, has finally arrived. More than a dozen digital processors using the technology are on the market, and the professional encoder used to master HDCD discs is following closely behind.

Anyone who has followed my writings on HDCD (including the review of the Spectral SDR-2000 Pro processor in this issue) knows how enthusiastic I am about the sound quality offered by this new technology. I believe HDCD is a great breakthrough in digital audio sound quality, and one that brings unprecedented resolution and musicality to home playback—all on a standard compact disc.

Pacific Microsonics, the Berkeley, California firm established to develop and market HDCD, has been reluctant to release much technical information about how the process works. Moreover, they've kept a low profile during their nearly ten-year development effort. But now that digital processors incorporating HDCD decoding are a reality, I thought it time to find out more about HDCD and the individuals behind it. I visited Pacific Microsonics at their Berkeley laboratory/office and spoke with the three principles of the company: Michael Ritter, President and business manager; and the two inventors of HDCD, Keith Johnson and Michael "Pflash" Pflaumer. I began by asking Michael Ritter for a basic overview of High Definition Compatible Digital.

Michael Ritter: HDCD is a comprehensive process for greatly increasing the fidelity of a digital audio recording. HDCD is fully compatible with the existing standard for consumer playback, the compact disc, which is fundamentally a linear format with 16-bit resolution and a sampling frequency of 44.1kHz.

We had to make HDCD compatible with this standard. Yet the goal of HDCD was to achieve a vastly higher level of fidelity—a level of fidelity directly comparable to the finest recording technology available—ie, first-generation analog master tape or direct-to-disc by record lathe. To do this, the HDCD process had to be a conjugate system. By that, we mean a system where all aspects of the recording and the playback decoding had to be controlled as much as possible. For that reason, the HDCD process wraps around both the A/D conversion and D/A conversion.

At the same time, as a concomitant requirement for this overall level of fidelity, we had to be able to take the HDCD process and essentially cut it in half. That is, encode the recordings, but be able to play the recordings on any standard playback equipment and simultaneously hear not only no artifacting, but a substantial improvement in fidelity over what can be obtained with a commercially available A/D converter. We've been successful in achieving that.

We have a process that, when you encode with it and play it back on standard equipment, you have a better-fidelity recording than is available through any other current digital recording method. But when you encode and then play back through equipment with the decoder, and if the playback equipment is implemented to a state-of-the-art level of performance, then you have a record/playback fidelity that is arguably as good as—or maybe even somewhat better than—any other method currently available for recording and reproducing music.

Robert Harley: Before we talk more about HDCD, let's find out about your backgrounds. Keith, I understand that you built a tape recorder from scratch when you were 14 years old.

Keith Johnson: The recorder used vacuum tubes, pieces of a Sears Roebuck Silvertone machine, and heads that I built myself. It recorded three tracks, and made master tapes that were later released (footnote 1).

When people at Ampex heard about it, I became a summer student there but still went to regular school. Later, at Stanford, I became involved with instrumentation, transistors, and things of that nature. That was going to be my specialty. I liked music a great deal, so I was also taking pipe-organ lessons.

I liked recorded sound, but didn't like the tape noise that was happening at the time. I embarked on using photolithography to make very thin materials to produce tape heads that would operate at super-high frequencies. And with tricks of focusing magnetic fields, I could get rid of some of the noise, self-erasure, and other effects. The end result was a second three-track machine that Reference Recordings still uses to make its LP releases.

Out of that work I helped form a company called Gauss Electrophysics, usually known today as simply Gauss. It still makes high-speed tape duplicators, loudspeakers, and other things. I was involved in applying the head technology to duplication of cassettes.

Harley: Wasn't that the technology which made high-speed cassette duplication possible in the first place?

Johnson: It did make it possible. Because prior to that, the frequency losses were so great that one couldn't record high-level, high-frequency signals on the tape. At that point the cassette industry was kicked off in the way we know it now. Among other things were the use of high-speed endless-loop bins, automatic loading machines, and things of that nature. Things that are now considered day-to-day methodology were all pioneered at that time.

I also liked to make recordings, and worked with the Glendale Symphony in Royce Hall at UCLA. Those were exciting times—lots of experimentation with microphones and recording electronics.

Somewhere along the way I met Tam Henderson [in 1995 President of Reference Recordings]. I was recording a small classical ensemble at a party. He liked what he saw and heard, so we did several projects and introduced a recording called The Astounding Sound Show [RR-7]. That essentially put Reference Recordings on the map, and paved the way for more serious work on microphones, electronics, and the recording system we have now.

Ritter: One other thing you might mention is the work you did on optical discs.

Johnson: I did a lot of work with Paul Greg, who is essentially the granddaddy of the video disc. He's the guy who had the concept of embossing plastic and reading it back with a servo—the basics of the industry. I did one key patent in that system which everybody uses today.

Harley: You also designed and built the entire recording chain for Reference Recordings, and have engineered all their releases.

Johnson: That's true.

Harley: What's your background, Pflash?

Michael "Pflash" Pflaumer: I've been interested in electronics and music all my life. In high school, I was one of a few people to put a radio station on the air. We built all of the equipment, including the console, modulator, and transmitter. We got to play our record collections on it. And I was also very interested in ham radio, microwaves, and digital encoding schemes.

Around 1960, I built a digital PCM encoder and decoder. It was only 5-bit, but it was all built out of vacuum tubes, because vacuum tubes were cheap in 1960 and transistors were very expensive. It took several racks full of stuff to make a 5-bit D/A and A/D converter and transmit the signal over a microwave link. A friend and I did this as a research project, and experimented with different modulation schemes.

Later I got involved in television production for broadcast and was the chief engineer for a studio. We did a lot of music production, where we televised string quartets and things like that. I have some exposure as a recording engineer as well as being chief engineer at the facility.

About that time I also started getting interested in computers. When I was an electronics major in college, computer science was not a separate discipline—it was all folded into the electrical engineering department. In the early '70s, when the first integrated microprocessors came out, I bought an Intel chip set and built myself a microcomputer. I basically had to write my own software and operating system.

The next big step was getting involved with computer networking. A friend of mine had started a company to make a local-area network for CPM computers. He called on me when it was apparent the people he was working with were not able to actually make the thing work. I ended up designing network hardware and writing all the network software that would run on K-Pro computers. That was called the Web. It didn't sell more than a few thousand copies.

Out of that experience came the impetus to do a local area network called Tops, which was basically the first computer network to transparently interconnect machines with different operating systems. And by the time it actually reached the market in 1986, we supported the IBM PC and the Apple Macintosh, and very shortly thereafter some workstations running Unix. And from each machine, the rest of the network, which was a distributed file server, looked as if it were part of your machine. So if you were on a Macintosh, the file system of an IBM PC was still icons and folders. It all worked transparently, including name translations from long Macintosh-style names to IBM-style names to Unix-style names, and it had a very elaborate directory translation. It turned out to be a very successful product. It's actually still needed today, unfortunately. We sold the company to Sun Microsystems (footnote 2).

During the time I was doing Tops, Keith, Mike, and I got together to try to realize what we now call HDCD. Keith had come up with the seed ideas, and the core realization that something needed to be done about digital sound quality. The standard format, even when executed properly, doesn't contain enough information to be satisfying.

Ritter: I was involved with Keith in a project at that time. There were some people interested in funding a company to produce a line of high-quality recording studio equipment. The industry was suffering from equipment with hundreds and hundreds of cheap op-amps, lots of bells and whistles, and zero sound quality. That project didn't come to fruition, but that's why I was involved with Keith. Keith mentioned some ideas he had to me which were very interesting. I told Pflash a little bit about Keith's ideas, then brought the two of them together.

Pflaumer: Keith wanted to get people interested enough to put in money to develop his ideas, but he didn't want to tell anybody what he was really thinking about. He didn't want anybody to steal his idea and do it without him, and was very careful about what he revealed. I initially came along as kind of a technical expert for some of the potential investors Mike had brought in, to see whether I thought this was real or not.

After we had our meeting with Keith, the investors asked me whether I thought it was possible. I said "Yes," and told them what I would do if I were involved. Later, we figured we could trust each other and signed non-disclosures. Keith disclosed his ideas and I disclosed mine. Because we come from somewhat different backgrounds, it turned out that our ideas fit together very well. We both thought of a couple of things in common; he thought of things I hadn't thought of, and I thought of some things he hadn't considered. When you put them all together, it amounts to what years later has become HDCD. Keith said, "Let's become co-inventors and do it." Mike said, "I think I can get the money to do it." That's how it started.

We each have a little common knowledge of the whole territory. Keith is a superb analog designer and a superb recording engineer. I have a lot of strength on the digital side of things, computer algorithms, and digital signal processing algorithms. It all fit together.

Harley: What did you find lacking—musically and technically—in conventional digital that made you think the world needed a higher-quality method of encoding digital audio?

Johnson: In a nutshell, digital didn't give you the impression of "being there." Around the time we introduced Däfos [initially Reference Recordings RR-12, now Rykodisc RCD-10108], digital was just getting going. Early on, we rented a Soundstream machine and were appalled at how unlistenable the playback was. It produced a grating sound that made us not want to hear the music once it was recorded. We felt it was sheer craziness, and we made an outcry about it. One of the first outcries on my part was in Stereophile—the first Stereophile that had a full-color cover, incidentally (Vol.7 No.4, August 1984] (footnote 3).

It was painfully obvious that sub-order harmonic distortion and noises were getting in. It was the result of high-frequency things creating distortion components that were not harmonically related to the lower frequencies.

If I heard that kind of thing, I thought I should be able to measure it. I devised an eight-tone cluster test, a test signal composed of high-frequency tones strategically placed in frequency such that the difference frequencies between tones are multiples of each other's and are troublesome to the system's sampling rate. I figured this would probably make a mess. Indeed, on the early converters, it did. At best, the sub-order components were maybe 60–70dB down. At that time I didn't realize how bad that kind of performance was.

I proceeded to start tearing the system apart, looking at the digital systems and discovering what was going on. Why does a thing measure so well with continuous pure tones, yet the perception of music is so poor compared to the live microphone feed?

I built my own converter based on a Sony PCM-701ES, which at the time was a pretty much state-of-the-art A/D processor. It was properly dithered, the levels were well-defined and independent of program condition, it had a very good filter—things like that. It was used to make all the earlier Reference Recordings material.

You could hear that it was adding artifacts: timbral shifts—sounds of instruments that weren't right—and the lack of smaller "environmental"-type sounds that contribute to a sense of realism. In the recording sessions we'd use the analog tape for playback because it had more going for it. More entertainment. More of the artist. More of those things that count—things that got you involved in the program. And by that time, the quantization "gritchies"—the spray-can cymbals, the cardboard bass, and other kinds of things that are wrong with digital—were pretty well shaken out of the system.

Harley: And this was even on your custom-built digital recorder?

Johnson: Yes. You'd put music into the digital machine and it was like the bone stripped of flesh: not much there. The problem wasn't just distortion, it was a lack of information as well. It was clean and had a certain degree of transparency that was somewhat better than the analog tape, but it just wouldn't bring you into the music. The staging would never be wide enough, never deep enough, and you couldn't sense the front-to-back space in the room. There was almost always a difficulty in identifying instrumental timbres—soprano sax, alto sax, or clarinet would have problems. It was hard to identify how the instrument created the sound. You heard the piano sound, but couldn't easily identify how the sound came about: a hammer striking a string. Something was missing. That was the frustrating part of it.

I then set out to try to figure out what was going on. What is it that we hear that doesn't seem to show up in measurements? And what can be done about it? You go into the system and try to find out what's wrong, and then make a measurement to see if it does in fact create a problem. If there is a measurable problem, can you hear it?

It became obvious that the dynamic range of the CD wasn't even nearly adequate for a good recording. With an analog tape recorder, which might have a 70dB signal/noise ratio, you can gain an extra 10–15dB by pushing the tape into overload, and you could easily hear 15–20dB into the noise. That's more than 90dB of dynamic range. But with a digital system, anytime you had more than 40–50dB of dynamics in the program, you were in trouble. The granularity and loss of information at the lower levels were intolerable.

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Footnote 1: You can hear this tape machine and Keith Johnson's early work on The Red Norvo Quintet CD The Forward Look (Reference RR-8CD). The recording was made on December 31, 1957, but wasn't released until 1981.—Robert Harley

Footnote 2: Tops was enormously popular, winning many awards and becoming the standard multiplatform networking package in the computer industry.—Robert Harley

Footnote 3: Actually, the very first Stereophile to feature a four-color cover was Vol.3 No.12, Spring 1977. Vol.7 No.4, featuring Keith, was the second four-color cover.—John Atkinson