[music-dsp] emulating Distortion Vs. Overdrive

Sergio R. Caprile yogurthu at arnet.com.ar
Mon Aug 19 18:55:19 EDT 2002

I'm afraid I can't be of any help for the new industrial and modern sounds you
are searching for, I'm an old sea wolf and don't even like "grunge"..
But as a guitarist and 36 year old, let me tell you somethoing about overdrive
and distortion, from an analog engineer point of view.

Back in the days of the first tube guitar amplifiers, it was a pain the arse
to get a nice and decent sustain sound, so the guys started experimenting and
someone noticed that when they cranked up the volume they could get a nice and
lasting sound, so the next step was to connect two amps one to the other and
"overdrive" the second amp's input. The overdrive sound was born by overdriving
the tube stages, and its sound is generally warm, produced by the non-linear
response of the tubes, that tends to compress peaks instead of clipping them.
This is often associated with soft-clipping, but it is more a non-linear
transfer function without much clipping at all. Some commercial overdrive
circuits are just a plain linear amplifier, the effect is produced inside of the
guitar amp, not the effect box. Di Marzio's super distortion pickup has a clean
sound, although its output is two or three times a regular pickup's output
level, enough to drive the second or third stage into distortion. Look for a
thread on 12AX7 (famous guitar preamp tube) and you'll get source code for a
nice traditional overdrive sound. Plot the curve and you'll see a tube transfer
function when overdriven.

Some years later, the guys got tougher and started looking for a harder sound,
so they wanted more overdrive and pushed the hardware to their limits. When an
amp can't amplify anymore, it clips the signal. Abrupt clipping generates hard
edges and so high order harmonics. Fortunately, guitar amplifiers and
particularly speakers are low pass filters, so most of the horrible sound is
filtered out (horrible in case you don't specifically want it, in which case
you remove the LPF and connect your fuzz box straight through to the hi-fi...)
You can distort by saturating an amp (very high gain) or constructing it
specifically to saturate or adding some components to clip at low level, like a
couple of diodes, as most commercial distortion boxes have. The logarithmic
nature of the diode transfer function rounds up the edges of the signal.

Not many overdrive/distortion boxes have buit-in compressors. Most guitarists
add it on a separate box. Not many guitarists love compression (I don't), as it
removes part of your expression, although I have to reckon that it does magic
to the sustain whe properly used.
Usually, there is an EQ AFTER the distortion process, as the distorted signal
is more rich in harmonics and more easily colored. Sometimes metal guitarists
like adding a MID boost BEFORE distortion. Other guitarists like removing some
BASS BEFORE the distortion, and sometimes they use a "treble booster" which is
nothing but an amp with an HPF to amplify only treble sounds. Like Brian May,
they usually remove the excess treble at the amp.
Try this:
HPF (bass remove) -> BPF (mid boost) -> distort -> EQ (color) -> LPF (treble

Overdrivers are usually for a bluesy sound, and fanatics like to cascade many
tube stages with just a bit of overdrive to each one. This way they get more
sustain with less distorion, due to the tube compression.
A typical overdrive sound you can hear is when the guy plays mostly clean and
when he picks hard the notes are distorted at the attack, and then return
smoothly to a clean sound. Try listening to Larry Carlton (although he uses a
volume pedal to control the overdrive to his amp)

Hope it helps. Try visiting Harmony Central for more explanations. I have an
effects page myself but didn't post distortion data, thought there is enough in
the www of it.

 AMIGA | Sergio R. Caprile, Bs.As., Argentina |  DSP - Music  |
   //  |   mailto: yogurthu at arnet.com.ar      |  Electronics  |
 \X/   |   http://www.geocities.com/scaprile  | Tai Chi Chuan |

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