Recording - December 2015 - 65

Many signal processors, both hardware and software, make constructive
use of harmonic distortion, adding mostly
second and a little third harmonic content
to an otherwise pristine signal chain. But
we also love the transparency of gear
with THD well under .005%-it's a matter
of taste, on a project-by-project basis.
Digital distortion
Distortion in a digital signal path is a
different can of worms. Early digital systems got a bad rap mostly due to nonlinearity of the A/D converters.
7
Today's oversampling converters
have brought distortion and noise
in digital systems to an extremely
low level. What distortion remains
tends to be IMD resulting from
clock jitter, and noise unrelated to
the signal that finds its way to the
output-usually through inadequate shielding, a poorly regulated power supply, or grounding
problems.
Nobody likes the sound of digital distortion, and digital audio devices get
cleaner with each generation as designers learn how to deal with the problems.
In low-power devices such as phones,
tablets, and some handheld recorders,
the worst offender is the analog output
stage, but in the new "high resolution"

playback devices, much care has been
taken with the analog part of the design.
Lossy data compression (MP3 and
such) introduces distortion of an entirely
different nature. Rather than frequencies
coming out that didn't go in, certain harmonics and other low-level signals in the
original source that the data compression
algorithm expects to be acoustically
masked by other sounds are simply
removed. With less data, the data file
that represents the audio is smaller after
compression than the original. Alarmists

will tell you that, because the compressed
file contains only 20% as many bytes as
the original, "you've lost 80% of the
music." That's baloney, but it's about as
far as specifications go in this little corner
of the digital audio world.
One might argue that in this day of hypercompression and other forms of distortion in

the interest of achieving maximum reproduced loudness, any discussion of distortion
is simply academic. The amount of distortion in the recorded source far exceeds the
distortion of a decent playback system.
However, the recent enthusiasm for "HighRez Audio" has brought new playback
devices into the market that can take advantage of high sample rates and lossless compression algorithms, and with higher quality
analog circuitry. There's hope.
The takeaway
Don't get wrapped up in comparing THD numbers on the spec
sheet when you're choosing a
piece of gear. Try to determine
what goes into that THD number
when comparing specifications.
Musically related harmonics are
relatively harmless, but non-musical distortion products usually
result in unpleasant sound. It's best
to start with the lowest distortion
you can afford, and then add a little spice here and there. Use your ears
and your good taste, as always.
Mike Rivers (rivers@recordingmag.com)
is a studio and live sound engineer, gear
design guru, and educator. Learn more
(lots more!) at mikeriversaudio.wordpress
.com.

http://mikeriversaudio.wordpress.com http://mikeriversaudio.wordpress.com http://www.RoswellProAudio.com http://www.realtraps.com

Table of Contents for the Digital Edition of Recording - December 2015