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The mini disc's sonic sleight-of-hand

technofile  by al fasoldt

Columns and commentaries in a life-long dance with technology

Simple gray rule

The mini disc's sonic sleight-of-hand

By Al Fasoldt

Copyright © 1993, The Syracuse Newspapers

If you've ever had a hard time picking up the details of conversation in a noisy restaurant, you'll understand why Sony did a little cheating in the design of its new mini disc.

The mini disc, no bigger than the lid to a soup can, wouldn't play for very long otherwise. If Sony had used the same digital technique as the compact disc, the mini disc would hold only a couple of songs—or, as somebody at Sony told me, "Beethoven's 2 ½ instead of his fifth." CDs play back basically all the sounds that are present at the microphones. That's the way old-fashioned tape recordings work, too, if you give an allowance for some of the lowest notes and some very high notes, too, which tend to get lost in regular recordings.

This takes up a lot of space. A CD that plays for 74 minutes uses up about 600 megabytes of storage space for the digital audio. If you put computer data on the CD instead, that would be enough space to store 300,000 pages of text.

Sony knew it needed a different technique when it set out to design a tiny disc that would play as long as a CD. So it did its own research into the way humans perceive sounds, adding to an already large amount of data from previous investigations.

What it came up with is an improved version an old technique called perceptive encoding. Other companies have used a minor form of perceptive encoding for years. The familiar Dolby B noise-reduction system, which gets the hiss out of cassette tapes, uses this kind of encoding.

The idea behind perceptive encoding is simple: You won't miss what you can't hear. If someone is shouting at the table next to you, you're not going to be able to catch the quiet conversation 10 feet away. And if you aren't looking in that direction, you won't even know it's taking place—so you won't miss it.

Dolby does this gently, pushing the hiss out of the way where you won't notice it—by hiding it under the loud portions of the music. When the music gets louder, the hiss does, too. But you can't hear it because the loud music literally confuses your ears.

Sony's mini-disc system is a lot more daring. When a mini-disc recording is made, sounds that are temporarily masked by other sounds are ignored, even if only for a fraction of a second. If they're likely to be noticeable, the mini disc's smart-recording circuit cuts them back into the mix.

If you took a mini-disc recorder to a concert, it would make the same kind of judgments that your ears make. It, too, would notice that the second violinist wasn't playing loud enough to be heard in the final movement of the symphony, so it would just leave out that violin's sounds.

This technique doesn't work perfectly, however. In the mini discs I've heard, I can sometimes detect some kind of semi-artificial intelligence—or some sort of artificial semi-intelligence, maybe—manipulating the sound in strange ways. This is a lot more noticeable in classical recordings than in rock discs, and is probably easiest to hear in a straightforward piano recording. Piano sound is hard to fake, and I think the mini disc shows its cheating heart on the keyboard.

But most of the time, mini-disc sound is no different from CD sound. Once you stop to think about, that's pretty amazing. After all, you aren't getting real high-fidelity in the standard sense, because the mini disc isn't faithful to the sounds at all. It's more like musical shorthand than high-class audio.

But it works, and that's amazing by itself. The fact that it works well is about as close to magic as you can come with two ears.

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