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Dolby SR and the secret of good analog sound

technofile  by al fasoldt

Columns and commentaries in a life-long dance with technology

Simple gray rule

Dolby SR and the secret of good analog sound

By Al Fasoldt

Copyright © 1987, The Syracuse Newspapers

The most entertaining fights in the history of technology have been waged over side issues. Just such a debate has been going on for a couple of years between advocates of analog sound - the old-fashioned way of making records and tapes - and digital sound.

The outcome of this sideshow battle has never been in doubt: Digital sound will win and wipe analog sound off the playing field forever. Yet the analog partisans continue to fight on, insisting that their cause is just, and that they will achieve a partial victory even if they only call attention to the inadequacies of digital sound.

What is this battle all about? A reader calls attention to the issue with two questions. First, he asks if it is true that Dolby Laboratories, one of the leading companies in analog sound, has come up with a technique that makes analog better than digital.

Second, he says he has just learned that an analog record has a better frequency response - the range from low notes to high notes - than a digital compact disc. He wants to know how this could be.

The answers to both questions relate to little-known aspects of the way sound is perceived. Sound quality is not measurable the way picture quality is, to use just one example. For the most part, a picture is either sharp or it is not; it can readily be judged in its degree of faithfulness to an original scene.

But no such ready standards for sound accuracy exist, particularly in such a general area as faithfulness - which, after all, is another term for "high fidelity." This comes about because complex sounds such as music are heard differently from the way simple sounds such as test tones are heard.

When we listen to music, some sounds hide others, making them inaudible no matter how hard we try to hear them. This masking effect is used to good advantage in the universally used Dolby-B noise-reduction system.

Dolby-B, which is used in all hi-fi cassette recorders, manipulates the signal so that the hiss that is always present in tape recordings is masked by musical sounds. The point to remember is that the hiss is still there; audio engineers can measure it, but it can't be heard.

The latest Dolby system has been designed not for cassette recorders but for professional studio machines. However, it uses the same masking technique. Called Dolby SR (for Spectral Recording), the process shapes the signal in such a way that the overall fidelity of the recording is improved vastly.

Much of the Spectral Recording process relies on actual signal improvements, but the part that makes it seem better than digital recording is an amazingly complicated masking technique. This makes Dolby SR sound better than a digital recording in terms of its apparent noise level.

As for the second question, it is true that a good-quality phonograph record can possess a greater fidelity, in frequency response, especially, than a compact disc. Digital systems must include high-frequency filters in their circuitry to keep sonic "garbage" out of the overall sound, but analog records do not need such filters.

Of course, for most listeners the faults of phonograph records - pops, clicks, scraping noises and all the rest - more than make up for any advantage in frequency response. But those who can make use of their own psychological masking techniques, in which the pops and clicks are covered up by a determination to ignore them, can still enjoy the highest fidelity available today in a form that's been around for a century.

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