Works with existing CD players, may boost quality for other media, too

Digital audio recording has not changed significantly since the era of the Apple II computer. Efforts to improve the sound on compact discs and other digital formats have been stymied by late 1970s technical standards that limited the amount of information that could be captured and stored on digital recording media, and by the need to remain compatible with the millions of compact disc players already sold.

The digital recording of CDs still strikes many audiophiles, recording engineers and musicians as cold and artificial next to the best analog recording. Now a tiny Berkeley, CA-based company has found a way to breathe new life, and realism, into digital sound.

IMPROVED SOUND, BUT COMPATIBLE WITH EXISTING PLAYERS

Pacific Microsonics Inc. calls its solution High Definition Compatible Digital, or HDCD. As its name implies, the process offers improved sound while remaining compatible with existing compact disc or digital tape players. The technology is also applicable to film, video or multimedia.

Intelligent encoder. Like the Dolby System for analog recording, HDCD is an encode/decode process, meant to be used during both recording and playback. Pacific Microsonics designed most of the system’s “intelligence” into the encoder used during recording, so much of HDCD’s benefit can be realized on existing CD players. For optimal results, though, HDCD recordings should be heard on players incorporating the decoding circuitry.

Early reviews from the usually hypercritical audiophile press have been very favorable. “A Sonic Breakthrough in Digital Sound,” headlined The Absolute Sound, while Stereophile termed it “a wholesale leap forward.”

DIGITAL AUDIO WAS LIKE ‘THE EMPEROR’S NEW CLOTHES’

Keith Johnson, a recording engineer and audio equipment designer, co-invented HDCD with Michael “Pflash” Pflaumer, a digital electronics engineer best known for developing the Tops system that pioneered linking IBM PC compatibles and Macintoshes on the same network.

Digital audio was “a case of the emperor’s new clothes,” Johnson said.

“You measured it and the measurements came out perfect, but the measurements were only adequate for vacuum tubes.”

In other words, the types of distortion caused by digital devices are different from those of analog devices such as vacuum tubes. Johnson began his work five years ago by identifying distortions that were unique to digital, investigating what caused them and then how they could be eliminated.

Unique to digital. Several distortions unique to the digital process were primarily due to the limitations of the 16-bit, 44kHz sampling rate standard foisted on digital recording by Sony and Philips more than ten years ago (see Vol. 1, No. 7, p. 20).

They decided that digital audio would sample sound 44,100 times per second and store it in digital “words” 16 bits in length. In high frequencies, music extends to about 22,000 cycles a second.

The theory they used stated that a digital sample must be taken at least twice as often as the highest frequency to be captured, to avoid cycles occurring during gaps be-tween samples. So while 44,100 samples per second sounds high, it is in fact the minimum needed — and, many engineers say, barely adequate.

In addition, to avoid confusing the digital circuitry, a filter eliminates all signals above 22,000 cycles. Because these analog filters begin operating gradually far below that level, discontinuities result at lower frequencies, well within the audible range. High-frequency harmonics, the subtle aftertones, are lost. (This is why violins sound so metallic on some CDs, and also why CD sound lacks a sense of presence.)

Likewise, 16 bits fall short, even though capable of representing 65,000 different numbers. In analog-to-digital or digital-to-analog conversion, tiny moments in sound are frozen in time, and assigned a number. “If that number is off by as little as one in 100,000, you will hear it,” Johnson said. “That number turns out to be a lot greater than 16 bits.”

Low-volume passages use fewer than 16 bits, increasing distortion by using less data to describe those moments in time. (This is why digital sound actually increases distortion at low volume levels, the opposite of analog behavior.)

TODAY’S STANDARDS ARE ‘HOPELESSLY INADEQUATE’

Digital audio standards “are hopelessly inadequate for serious recording, so what we’ve had to do is essentially [what we call] process manipulation,” said Johnson. Johnson and Pflaumer are vague about details of the process; they’re still waiting for patents to issue.

What they’ll say is that they begin with a signal with a far higher sampling rate, employing 20 bits. This alone would make a better master recording, but one incompatible with existing players. To squeeze the signal into the existing CD audio format, they developed a proprietary algorithm and a high-speed circuit that analyzes the signal on the fly and stores only the data needed to encode a high-resolution recording. The circuit ignores non-essential data, freeing space within the 16-bit format.

Squeeze in more data. “We are altering the signal, but in ways that are not apparent,” Johnson said. “Through that alteration we are able to squeeze in a lot more information that is important and keep it compatible.”

“Compatible” means that most of the additional information is stored in such a way that it can be retrieved by any CD player, and will sound better than conventionally recorded CDs. But an HDCD decoder retrieves still more information, invisible to normal CD circuitry, providing a sound comparable to the best analog recordings.

Four CDs have been recorded with the prototype HDCD encoder by Reference Recordings, a San Francisco-based audiophile label with which Johnson is affiliated. Mainstream recordings by leading artists will be out by the end of the year.

A decoder will be on the market within 12 months, and Pacific Microsonics will license audio manufacturers to include it in their equipment. Ultimately the decoder will be reduced to a chip that could be included within a CD player, an interactive multimedia player, or even a PC sound board at very low cost.

Larry Fisher