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Digital sound is just a numbers game

technofile  by al fasoldt

Columns and commentaries in a life-long dance with technology

Simple gray rule

How digital sound uses a numbers game to do its thing

By Al Fasoldt

Copyright © 1987, The Syracuse Newspapers

Digital audio is a numbers game. Considering all the fancy footwork that the electrons have to do inside a digital device, the actual principles themselves are amazingly simple: Voltages are turned into numbers.

The signals that are piped into a digital circuit arrive in a state of agitation. They vary from very weak to very strong, as measured by their voltage. In the case of audio equipment, voltage varies as a direct result of how loud the sound is.

All those little volts get their jolts for another reason, too. They wiggle from one extreme to another many times a second because of the pitches of the sounds they represent. They can do a relatively slow two-step at about 20 back-and-forth wiggles a second (for an extremely low-pitched sound, such as thunder), or get frenzied for all-night rock n' roll at about 16,000 swings a second (for the overtones of common sounds).

So the digital circuit has to make sense out of the two components of a regular signal, its strength and its frequency. It does its job with admirable single-mindedness. It treats these signals just like the Army treats recruits:

It gives each one a different number.

Bear in mind that it doesn't really matter what those numbers are, or what scheme is used to assign them. The only thing that is important is that the digital circuit has to use the same method in turning numbers into sound as it did when it turned sound into numbers in the first place.

Otherwise, Mozart could turn into Gordon Lightfoot - or, worse, into buzzes and crashes and burned-out woofers and tweeters. Fortunately, this happens rarely these days, with the advent of digitizing circuits on reliable silicon chips. What goes in is almost always the same as what comes out.

The reason numbers are used and not some other quantifying system has to do with a simple property of numbers that even first-graders know by heart. That property is the fact that numbers are always exact. The number 24 is not 23 and it is not 25, and it is not even "almost 24"or "just a little more than 24." The universe may grow very old, and all traces of our wanderings may be lost, but the number 24 will always be 24. No matter what has happened to the means by which a number has been carted about, it remains the same.

Look at it this way: Suppose I give you my phone number and you write it down on a piece of paper. You then give that paper to someone who paints the number on a sign, and someone who sees the sign enters the number into a computer database.

Tell me, which of those numbers was my number? The one I gave to you, the one you gave to someone else, or the one some stranger saw and copied down? The answer, of course, is that they are all my number. Or, to put it in an engineering context, my number didn't change just because it was being passed around or written in large letters or small figures. Numbers are inviolate.

And so all you have to do to turn analog sound into digital sound is give each varying volt a number, and maybe a second number, too, to indicate how much it was wiggling. Then you stick the numbers in any convenient storage place - a laser disc is fine, but tape works well, too.

Then when you are ready to turn the numbers into sound, you reverse the process. Except for the fact that we need billions of numbers to represent even a few minutes of sound, it's all quite simple.

Even people could be used as storage. The medium doesn't matter, since numbers are unaffected. A couple of billion is probably out of the question, but some day a few thousand of us could get together in the Carrier Dome and get our number assignments from a Digital Head Honcho, who would then tell us, on cue, to shout out our individual digits.

Somebody in the back of the room would then feed all these numbers into a digital circuit, and out would come a millisecond of sound. Maybe Bach. Or a car horn honking.

In light of the nature of the experiment, I suppose the car horn would be just what we'd get.