But What Makes It Digital?
I was recently talking with a non-technical acquaintance
about the various digital media that have revolutionized home-entertainment electronics in
recent years. As a music and movie fan, he was well aware of the improvements over earlier
systems, "But what makes it digital?" he asked, "and why is that
better?"
Most people are aware of digital audio and video, and
vaguely understand that they use computer technology somehow, but beyond that,
"digital" is not much more than a buzzword. So here's a brief primer.
Digits are the basic building blocks of arithmetic systems:
437 is a number; 4, 3 and 7 are the digits that make it up. The word comes from the Latin
for "finger," because that's what a lot of people count on and, because we have
ten, our numbering system uses ten digits, 0 to 9. The system is said to have a
"base" of 10.
While it's natural enough that we use ten digits, it's not
necessarily ideal. Experts say that a base of 12 would be better, because more of the
individual digits would divide evenly into the base number. But even in our ten-digit
system, there are lots of vestiges of alternative ways of numbering. Every time you look
at a clock or buy a dozen eggs, you're counting by twelves; to talk of "scores"
-- threescore years and ten, meaning seventy -- is an old-fashioned way of counting by
twenties in English, and the French routinely count in twenties from the number sixty
upwards.
Larger bases mean you have to memorize more separate
digits, but you can use fewer of them to express larger numbers. A ten-digit system needs
only three to describe a thousand different quantities (beginning at zero), while a
two-digit system needs ten to express roughly the same amount.
That's hardly practical for humans calculating on paper,
but such a base does have its advantages. The digits in a two-digit system can be written
as 0 and 1, and this readily translates into all sorts of bipolar sets -- on/off,
transparent/opaque, reflective/non-reflective, magnetized/unmagnetized -- which can be
handled by electronic devices. They can easily react to absolute on and off, but would
have much more difficulty dealing with the degrees of "on-ness" that would be
involved in a ten-digit system.
Those 0s and 1s are called "binary digits,"
contracted in computer-speak to "bits," and they are invariably what is meant
when you refer to a medium as digital.
But it's just a numbering system, albeit a machine-friendly
one, so what does that have to do with music or video?
Audio and video signals start out as voltages in a wire
continuously varying between zero and whatever maximum the system can handle -- degrees of
on-ness again. In an old-style analog system, throughout the recording and reproducing
chain this signal is converted from one state to another a number of times -- electrical
to magnetic, stylus excursion to electricity, and so forth -- but it must keep the same
waveform, and each of the devices must be able to handle that without adding or
subtracting anything. Things invariably do affect the signal on the way through, however,
and the final reproducer can't distinguish the additions from the program material, and
reproduces them as distortion.
In a digital system, the signal also starts off as a
continuously varying voltage, but early in the chain a series of measurements are made of
those voltages, and the resulting numbers are what's recorded, as a series of pulses and
gaps that represent strings of binary 1s and 0s. There's an enormous amount of data
involved in this, so any digital medium must have a lot of bandwidth, but once it has
that, nothing else matters very much.
The playback device uses these numbers to build an output
waveform which, although newly created, is identical to the original. The player only
needs enough signal to detect the numbers; anything else added along the way -- noise,
level variations, even speed irregularities -- have no effect on the numbers themselves
and are ignored.
As an analogy, imagine a piece of paper with a few lines of
Shakespeare written on it. The paper probably started out clean, but it may have picked up
smudges, coffee stains, rips and marginal doodles along the way. It might have been
handwritten or typed, in large letters or small. None of that matters if you can still
read the words, which contain the full weight of Shakespearean eloquence whatever might
have befallen the physical piece of paper along the way.
The words themselves are the code, like those 1s and 0s in
a digital recording, and as long as you can still see them, you can always copy them out,
creating a new document that is verbally identical to the original.
The poet's art consists of the words themselves, rather
than their physical manifestation. But if that paper had contained a Picasso sketch and
had been similarly damaged, you might be able to repair it to some extent, but there is no
way you could use the drawing as code to create a new original. Like an analog recording,
that work of art would be stuck with whatever had damaged it along the way.
...Ian G. Masters
ian@mastersonaudio.com
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