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Dolby SR, analog sound

technofile  by al fasoldt

Columns and commentaries in a life-long dance with technology
 

Simple gray rule


Dolby SR, analog sound and why digital isn't necessarily better
 

By Al Fasoldt

Copyright © 1990, 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 their cause is just and 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 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 measureable 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 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—it can be measured by audio engineers—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 one part that makes it seem better than digital recording is an amazingly complicated masking technique. This makes Dolby SR superior to a digital recording in terms of apparent noise level. (Note the word apparent, since what we are referring to here is an effect, not an across-the-board fact.)

As for the second question, it is true a good-quality phonograph record can possess a greater fidelity—in frequency response, especially—than a compact disc. Digital systems must incorporate filters that trim high-frequency response so they keep digital artifacts—a fancy term for digitally induced sonic "garbage"—out of the overall sound, but analog records do not need such filters.

Typically, a very good phonograph record can contain ultra-high frequencies ranging from 21,000 Hz to 25,000 Hz (cycles per second) or even higher, while a compact disc will not have any musical frequency above about 19,000 Hz.

Of course, for most listeners the faults of phonograph records—pops, clicks, scraping noises and all the rest—keep them out of contention when CDs are available.

But there are some super-audiophiles who will stick with top-quality LP records for a few years to come. Interestingly, Dolby's masking technique is used every time one of these LP-lovers plays a record.

By letting the music mask pops and clicks in the record, these audiophiles tune out the unwelcome noises. It is perhaps a psychological ploy at first, but after a while the masking becomes automatic.

Try it yourself the next time you are playing an LP. Give the effect a few minutes to take hold. You might find an unexpected delight in an old and seemingly outdated technology.


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