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Distortion Article


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7/9/1999 11:35 PM
Jon Blackstone
Distortion Article
Hi all.  
 
 
 
I've just put up on the web a very brief article explaining the basics of guitar distortion, a subject that I know is near and dear to many readers of this BBS. I'd like to hear any comments people might have - correct inaccuracies, or tell me if you find it interesting/informative.  
 
 
 
The article features a Java applet that demonstrates the relationship between harmonics and waveforms.  
 
 
 
The URL is http://www.mindspring.com/~j.blackstone/dist101.htm  
 
 
 
It's part of a site promoting the pedals I manufacture.  
 
 
 
Thanks,  
 
Jon Blackstone
 
7/13/1999 0:38 AM
Lee

It was interesting reading and I had a ball playing with that Java app!
 
7/13/1999 3:00 AM
Ed Rembold

Dear Jon,  
 
Thanks for article, Very interesting.  
 
You know what would be neat? How about putting up a wave form of your pedals' output, at a set frequency?  
 
Now That would Really be interesting.  
 
Thanks, Ed R.
 
7/14/1999 9:15 PM
Jon Blackstone

Actually, I've found that the sine wave response waveform doesn't tell me very much about a pedal or amp. That's partly why I put in that paragraph about there being many different waveforms with the same harmonic content (due to the ear's insensitivity to phase of harmonics). Spectrum plots are better, but they can also be hard to find meaning in. While doing spectrum analysis on a bunch of pedals recently, I observed that the best sounding distortions go back and forth between emphasizing odd and even harmonics over the range of amplitudes that corresponds to a guitar signal. It's almost more interesting to listen to this than to look at it on a spectrum analyzer. You feed a sine wave into the distortion device and, as you vary the level, you hear it morph from a clarinet-like sound (odd harmonics) to an insect-like sound (even harmonics).  
 
 
 
No transitions = static, unsatisfying sound/feel? Any takers on this theory?  
 
 
 
Jon Blackstone
 
7/14/1999 9:27 PM
Jim S.

I wonder if what you've experienced relates to asymmetrical clipping. In other words, at lower amplitudes, only the bottom or top of the waveform gets clipped; at higher amplitudes, both get clipped, but one more severely than the other. At still higher signal levels, both bottom and top get heavily clipped. Perhaps this may have something to do with the varying mix of odd and even harmonics?
 
7/14/1999 10:01 PM
R.G.

Yes, it does.
 
7/14/1999 10:01 PM
R.G.

This notion has been around for several years; the most direct espousal I can think of was advanced a couple of years ago by a fellow who was Carvin's lead engineer. He had an article on the topic in EE Times. He called it "duty cycle modulation", with the positive/negative duty cycle of the clipped waveform varying with signal intensity. Carvin didn't patent, and the circuit is public domain now.  
 
 
 
Here's a quote from the "Distortion 101" section of the Guitar Effects FAQ at GEO (http://www.geofex.com, written in early 1995:  
 
===========================================================  
 
Based on an article in Electrical Engineering Times, the actual distortion method may not be as important as the pre- and post-clipping filtering, and the amount of offset in the duty cycle of the resulting clipped waveform and the variation in the duty cycle with input signal level. This article (by Brian Murphy, interviewing John Murphy, who used to be chief engineer at Carvin; EET, October 3, 1994) outlines Murphy's ideas on the degree to which he can simulate tube distortion in solid state designs. He holds that if you filter properly, soft clip, and arrange for the duty cycle of the clipped signal to vary with varying signal levels, you get something very close to tube distortion. This is the basis of the "tube emulation" circuitry in Carvin's SX series of amps.  
 
===========================================================  
 
Murphy's stock circuits don't ring my bells much, but they do have a spectrum that wavers from asymmetrical to symmetrical with signal level, and it's an interesting sound if your ear is sensitive to that.  
 
 
 
I've been messing with variable-offset clipping pedals based on Murphy's approach and others for about the last three years, driving the variable offset with a number of things, including LFO's and signal envelope. Getting the right mix of clipping, filtering, and amount of offset to the clipping can be very tricky. Most people that do this don't recognize what's happening and just tinker components until their ear tells them they like it.  
 
 
 
The fellow who puts up the LXH2 page theorizes that amps change from the asymmetrical limiting in the preamp stages to the more symmetrical limiting in the output stage as they get driven harder, and used a transition from asymetrical to symmetrical clipping in his distortion preamp simulators. That's been around for a couple of years.  
 
 
 
You might want to read the Guitar Effects FAQ at GEO if you haven't already.
 

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