Audio advice: Analog to Digital Conversion

All about turning your audio into little ones and zeroes.

It seems the "D" word pops up everywhere these days–we’ve got
digital television, digital pagers, digital satellites, even digital cameras.
If you remember back a few years, however, the first place digital technology
really gained a foothold was in digital audio. This is because converting
sound or music to digital data didn’t require massive amounts of processing
power or storage, and the benefits were numerous.

The same holds true today, making digital audio a great place to get started
in the realm of digital video production.

An Analogy

Before we explore how digital signals work, it’s important to understand
analog signals. In simple terms, an analog signal is an analogy of a sound–the
signal’s voltage does its best to mimic every tiny nuance in a sound. If
a sound gets a tiny bit louder, the voltage swings by a small amount. If
a sound causes a large increase in air pressure, the analog signal creates
a corresponding large change in voltage.

The problem with this system comes when real-world factors step in. Anything
that makes even the tiniest change in the analog signal is distortion, and
directly affects the audible sound. A tiny spike of noise from a fan turning
on embeds itself in the signal, audible as a distorted click that mars your
sound. Random noise, an unavoidable facet of electronics, adds hiss to the
signal every time you record or process it. Everywhere it goes, forces conspire
to corrupt the analog signal’s delicate representation of the original sound.
If that’s not bad enough, the effects of noise and distortion on the audio
signal are cumulative; they just keep getting worse the more you copy or
manipulate the signal.

A digital signal can thumb its nose at these forces because it doesn’t carry
a direct representation of the original sound. Instead of a delicate changing
voltage, a digital recording carries numerical measurements of the voltage.
These numbers, when boiled down to a binary stream of ones and zeroes, are
virtually impervious to corruption by outside forces (see figure 1). When
the digital signal gets to the other end of the cable, the receiving device
reconstructs a new audio signal from the digital data.

How It Works

The device responsible for changing an analog signal into a series of numbers
is the analog-to-digital converter (or A/D converter). It measures (or "samples")
the strength of the changing voltage at regular intervals, generating a
steady stream of numbers. Two parameters directly affect the quality of
the resulting audio: sample rate and bit depth.

The converter’s sample rate dictates how often it measures the signal to
generate a new value. The more frequently the converter measures the signal,
the more accurate the resulting data. Sample rate corresponds directly to
frequency response–the highest frequency a digital system will capture
is exactly one-half the sample rate. To capture the full audio spectrum
up to around 20,000 cycles (or 20kHz), a sample rate of 44.1kHz is common.
Higher sample rates make for increased treble response and a more "hi-fi"
sound. Low sample rates sound duller and darker.

Bit depth affects how many bits the converter uses for each numerical measurement
of the signal. More bits equal a more accurate measurement, which explains
why 16-bit CD audio sounds so much better than an 8-bit multimedia sound
file. A low bit depth is like forcing the converter to measure the sound
with a yardstick marked only in inches. A higher bit depth allows the converter
much greater accuracy (a yardstick marked in 1/8th-inch increments, for
example).

It may help to picture the interaction of sample rate and bit depth as a
grid used to measure an audio signal. A higher sample rate corresponds to
better accuracy on the horizontal axis; greater bit depth corresponds to
vertical resolution (see figure 2).

When you work with digital audio, there’s a tradeoff: the higher the audio
quality, the more storage space required to store the data. When sample
rates and bit depths are high, audio files can be quite large. Switch from
8-bit audio to 16-bit, and file sizes will double. Likewise, jumping from
a 22kHz sample rate to 44kHz causes file sizes to double as well.

The DV videotape format gives us an interesting case study in audio quality
versus file size. DV has a set amount of storage space set apart for digital
audio, with two different ways to allocate it. One mode divides the bit
pool into four tracks of 12-bit audio sampled at a rate of 32kHz. The other
mode gives you two tracks of 16-bit audio at 48kHz. Do a little math, and
you see that the numbers add up to the same amount of total audio data (see
figure 3).

So What?

All this fancy chat about bits and samples is great for learning the theory,
but what does digital audio really mean to the videographer?

  • Digital audio spells the end of the tape hiss that builds up each time
    you re-record (or edit) analog tape.
  • When an analog tape transport varies in speed slightly (and they all do),
    the audible result is flutter, warble and fluctuating pitch. Because digital
    audio perks along at a precise rate, these speed-related distortions don’t
    occur–ever.
  • When copying an audio signal digitally, the theoretical end result is a
    perfect clone of the original data. Even when converting a digital signal
    to analog for copying, however, you’ll get much better sound than with
    analog tape.
  • For all its benefits, digital audio requires some extra diligence on your
    part. Here are some things to watch for when working with digital audio:
  • Analog tape distorts gently when levels sneak above the maximum level.
    Digital signals, on the other hand, usually clip severely the instant levels
    get too high. Pay careful attention to keep signals below maximum when
    recording digitally.
  • Whereas quiet signals sink into a quagmire of noise in analog recording,
    digital signals can get gritty and harsh-sounding if recorded too low.
    This is especially true when recording at 8 or 12 bits. Keep digital record
    levels as high as you can without clipping.
  • High-quality digital audio takes more space to store, but it’s almost always
    a worthwhile tradeoff. Before you begin recording at a low sample rate
    or bit depth to conserve drive space, consider two simple facts. First,
    once you throw away audio quality, you can only get it back by returning
    to the source and re-recording. Second, your audio quality only goes downhill
    between source tape and final dub–it never gets better. Err on the side
    of quality whenever possible, and invest in a good backup system to keep
    your hard drive space freed up.

Now that you’ve gained all this knowledge about digital audio, what good
is it? Well, the next time you find yourself capturing video clips for nonlinear
editing, you may be faced with a choice or two about the bit depth or sampling
rate of your accompanying audio. As mentioned above, even some of today’s
DV camcorders offer a choice between two types of digital audio (12-bit
and 16-bit); we hope that the knowledge you’ve gleaned from these pages
will help you to make the most informed choices for your productions.

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