GUITARTECH
Distortion: Ride the Wave
Most of us love good distortion. Saying that
sounds weird, but distorting a signal in a
musically useful way is an incredibly complex
process. The simplest signal waveform, a sine
wave, can help illustrate how it works, although
a guitar produces a tone much more like a
sawtooth wave than a sine wave. The trick is to
clip the waveforms; here’s how it happens.
simple harmonic distortion, which generates distortion related to the fundamental
frequency and adds character to instrument sound, and intermodulation distortion
(IMD), which produces non-musically related
frequencies at multiples of the sum and differences of two frequencies, considered to
sound harsh to the ear.
to one degree or another in a number of
pedals. Having the signal completely full-wave rectified eliminates the fundamental
frequency. But if we half-wave rectify a sine
wave, the result still has a lot of the fundamental in it, in addition to a noticeable
octave sound. We can also partial full-wave
rectify a signal, which makes for an even
more noticeable octave sound.
A sine wave is the purest tone possible, just
a single frequency. No musical instrument
produces a really pure sine wave, although
tuning forks get close. The
sine wave is smoothly
rounded everywhere, with
no sudden changes in
direction.
Ultimately, the name of the game is to produce harmonic distortion, which makes musically good sounding notes, and to minimize
Here’s the big trick: filtering of the signal both
before and after the distortion usually has at
least as much, if not more,
to do with how the distor-
tion sounds than the actual
method of distortion.
If we take that same sine
wave and electronically
clip off the tops and bot-
toms at a set clipping
level, a crazy thing hap-
pens. Just snipping off
those tips has a big effect
on the sound. A pure sine
wave is somewhat generic
sounding. Distorting the
sine wave by clipping
introduces a whole bunch
of new sounds called
“partials” or “harmonics”.
The sound is much more interesting to listen
to, as long as we don’t get too carried away
with it.
A sine wave is the purest
tone possible; think of the
sound you hear when a
person runs a wet finger
around the rim of a glass
IMD, which makes unmusical sounds and
harsh buzzes. Unfortunately, no matter how
hard we try, we can never get all harmonic
distortion and no IMD.
A combo amp with a
speaker or two and an
open back implements a
multi-pole low-pass filter all
by itself. This is at least one
of the reasons that mic’ing
an amp is preferable to
running a distortion signal
directly into a PA mixer.
The speaker cabinet’s
low-pass action smooths
off those offending treble
shrieks. An electronic multi-pole low-pass filter is the
essence of all the cabinet
simulators you hear these days.
In music, there are fundamentals and partials.
A fundamental is the basic tone of any given
note – for instance, the frequency A-440 is
the standard for musical tuning, which is a
sine wave that has a frequency of 440 Hz
(Hz is short for Hertz, and stands for one
cycle per second). We recognize that a tone
of A-880 is one octave higher than A-440.
Musicians refer to a musical note that has
both A-440 and A-880 mixed as having a
fundamental and the first partial. A frequency
of three times the basic note frequency is
also referred to as the third partial of the
basic note. Four times the fundamental is
the fourth partial and so on. Since harmonic
is another name for a partial, when you hear
the term “total harmonic distortion” it means
whatever is left after taking out the fundamental sine wave in some waveform.
One way to keep mostly harmonic distortion
is to not have sharp corners on the waves.
Tubes help with this process. Tubes generally
produce very softly rounded, squashed tops
when done on square waves. A pure sine
wave is almost devoid of information, having no wiggles or corners in that smoothly
rounded top. However, if we took a square
wave and massively amplified it, or a triangle
wave, or a wave with spikes all over its top
and did the same amplify/clip operation on
it, we’d get exactly the same result as with
the sine wave with massive clipping.
Filtering before the distortion is also interesting. Hendrix used a wah pedal before his Fuzz
Face. The band-pass effect makes the signal
bigger at the frequencies boosted by the
wah, so those frequencies get distorted most.
You get an interesting change in distortion
depending on what notes you hit. This cuts a
lot of the harsh sounding IMD, too.
If you are studying science in high school or
physics in college and your teacher doesn’t
like your long hair and your iPod, bring in a
Tube Screamer and present this little distortion lesson to your class. Everyone will love
you for it.
So there are really two kinds of distortion:
There are other distortion tricks besides
clipping a signal. To get an octave sound, a
process called full-wave rectification is used.
What happens here is the negative half-wave
of an alternating current is converted into a
positive half-wave. This trick has been used
Rick Wheeler
Rick Wheeler currently works as Larry Carlton’s guitar
tech and front of house engineer. He is also an accom-
plished jazz guitarist, vocalist, and educator.
You can contact Rick at rickwheeler@hughes.net
