Recording - December 2014 - 38

I'd like to introduce you to three effects that everyone knows
can adversely affect your audio... and then explain how what
"everyone knows" is largely untrue.
Dither, jitter, and bad AC power-the monsters under the bed
Dither is an important final step in the production process when
preparing a 24-bit mixdown file for CD. If you don't use dither,
the frequency spectrum and soundstage will be compromised,
with soft passages and reverb tails sounding grainy and distorted... according to conventional wisdom. But what's the reality?
In practice, dither does reduce quantization distortion... from
too soft to hear to even softer than that! I never argue against
using dither, because the improvement is real and can be measured. Most audio editor programs include an option to dither,
and there's no reason not to use it. Since music passes through
many pieces of gear between the studio microphones and the
listener's loudspeakers or earbuds, it's important to minimize
quality loss at every step, even if the degradation added by
each individual device is inaudible. Dither never hurts, and it
usually helps a tiny bit... so it's worth using.

Another frequent audio whipping boy is inadequate or "dirty"
AC power, which is accused of "blurred imaging" and "flabby
bass," among other claims. The fact is, AC power problems
severe enough to have an audible affect are rare in most parts
of the civilized world.
There are power problems that are easily heard as clicks and
pops, for example when a refrigerator turns on or off, or as
buzzing caused by solid-state light dimmers. Both of these noises are caused when short-duration impulses are sent back into
the power line by the offending device. When a power spike
has an extremely short duration, with correspondingly fast rise
and fall times, it contains high frequencies that can leak through
power transformers. Such impulses can also radiate through the
air as radio waves to be received by the pickups in an electric
guitar or bass.
These voltage spikes are very large, though-generally hundreds
of volts! Compare that to the usual amount of noise riding on AC
power lines, which is typically measured in millivolts, or thousandths
of a volt. The power supplies in audio gear routinely filter out such
low-level noise.

However, suggesting that people experiment with different dither
types by ear to select the one that sounds best is like chasing unicorns. If you have to crank the playback volume unnaturally loud to
hear the effect of dither on a reverb tail, what's the point? Further,
claims that dither affects the frequency spectrum and soundstage of
your mix are easy to disprove with standard measurements.
Jitter is another common audio bogeyman-a periodic timing
error similar to wow and flutter, but at a much higher frequency.
Jitter due to inferior digital clocking is often accused of harming
localization, and causing instruments to lose fullness and focus,
resulting in generally bad sound. But the amount of jitter in modern digital converters is literally 1000 times less than the flutter
of even the finest analog tape recorder! It's true that studios having multiple digital sources can benefit from an external master
clock, but that's related entirely to synchronization issues.
Jitter manifests as noise 100+ dB below the music. This is softer than the background "hiss" of a CD, which I've never heard
at normal listening volumes. Likewise, the effects of dither are 90
dB below peak level. Nobody can hear artifacts that soft, especially when they're masked by the music that's playing. When
these things are compared in a proper level-matched blind test,
all of a sudden what had been "an obvious difference that even
my mother can hear" becomes impossible to identify.

One task of an AC power source is to supply a reasonably constant voltage that is then converted by the power supply inside your
audio gear to the lower DC voltage required by the solid state circuits in preamps, equalizers, and other devices. AC power must
also provide enough current, especially for power amplifiers that
will deliver several amperes to a loudspeaker load. A 100 watt
power amp driving an 8 Ohm speaker provides about 3.5 amps,
and that current comes from the AC mains.
If an AC power source is unable to provide sufficient voltage or current, the result is distortion, but AC power rarely varies more than a few
volts except during summer brownouts, and all audio gear routinely
handles such normal variations. A standard 16 gauge power cord can
deliver sufficient current for any audio device in your rack. If a highpower amp needs more current, certainly the vendor will provide a suitable power cord. The idea that "power problems" can create a subtle
lack of fullness and loss of musical detail or image width is, again, easily disproved with basic audio measurements and proper blind tests.
The notion that any of these factors affects sound quality per
se is pure fiction, yet these myths have been repeated so often
that they're accepted as fact by many people. Why do people
sometimes swear they hear a difference with low-jitter clocks, or
various flavors of dither, or after replacing one perfectly competent AC power cord with another?

RECORDING December 2014

Recording - December 2014

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