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Author Topic: Spectral effects of rectification
Digital-
Larry

Posts: 192
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Post Spectral effects of rectification
on: August 19, 2012, 07:44
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I found a cool DIY fuzz project (simple but deadly fuzz) at this cool website, don't know if anyone's heard of it:

Simple But Deadly Fuzz(Tonefiend DIY Club Project #2)

Now you guys might be getting the drift that I am a total circuit geek and that is true. I have a degree in electrical engineering and I am not really trying to pass on the sum total of that in a few posts. But I am fascinated by electric/electronic music circuits, especially when they violate all aspects of "proper" design and make cool sounds anyway!

I started looking at rectifier circuits and what they do to your spectrum. I'm just having my morning tea here and will add some drawings later to make this easier to grasp, so put on your thinking caps in the meantime.

I have recorded my voice counting from one to six.

Closeup of the waveform:

Image

A half-wave rectifier will pass either the top half or lower half of your signal.

Image

A Sine wave clipped in half has the fundamental frequency, a strong second harmonic, decreasing even order harmonics and NO odd order harmonics other than the fundamental (which isn't really a harmonic).

Here is the same clip of my voice, with a half wave rectifier applied.

If you run the half-sine wave through a lot of gain and clip it off, you've now created a square wave that has NO even order harmonics.

A typical high-gain overdrive such as a couple of diodes in the feedback loop of an op-amp (which are paralleled by a resistor) will create square waves when hit with a strong signal, but as your signal decays, less and less of the waveform will be clipped and so the distortion (and upper harmonics) die away in a "natural" fashion.

A rectifier circuit like the SBD fuzz on the other hand, has very little linear range and tends not to track your signal dynamics very much, if at all. You either have enough signal to cross the switching threshold, or you don't. Hence the gating behavior of the SBD fuzz.

A full wave rectifier puts out the absolute value of the input signal. A sine wave put through a full wave rectifier looks like someone folded it in half.

The resulting output spectrum has NO fundamental and is ALL even harmonics. Hey you just made a frequency doubler! A "Green Ringer" is at its core a full wave rectifier. This makes all sorts of bizarre noises with single notes and a complete mess of chords. If you hit a note and slide up, you'll hear frequency components going DOWN. We are NOT in Kansas anymore!

Here is my voice with full-wave rectification applied.

Image

Notice that "four" now sounds like "four-eh" and "five" sounds a bit like "foive". Does it sound like my voice was pitch shifted up an octave? Well, not really. This is one instance where extrapolating the effect on a simple sine wave to a complex waveform does not work. The waveforms shown at SoundCloud.com are bizarre - they show a symmetrical waveform even though it clearly wasn't!

Does example this sound like my voice would going through a Green Ringer? Probably not. As Joe noted in his main blog article on octave fuzzes, DSP octavers don't have the same character as analog pedals. These examples were created using the "expression evaluator" in Goldwave (sorry, Windows only). As such, they are very clean and exact. I'm going to continue to investigate this and try to figure out why.

Now, guitar signals are not simple sine waves and so this analysis is oversimplified. Nevertheless I hope it gives you some insight into why these circuits sound the way they do.

joe
Administrator
Posts: 224
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Post Re: Spectral effects of rectification
on: August 19, 2012, 11:46
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Larry, I share you fascination with the rectifier stuff, and the Green Ringer (itself sort of a ripoff of the Ampeg Scrambler, another cool octave/fuzz/rectifier thingie). Definitely a great circuit to breadboard and fool around with. I've come up with some really phenomenal variations on that circuit that I'm sitting on until I figure out the best way to exploit the gullible and make a fortune market them. 🙂

bear

Posts: 153
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Post Re: Spectral effects of rectification
on: August 19, 2012, 12:59
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Visual Sound's Angry Fuzz is supposed to be a Green Ringer derivative and is pretty danged hip. A lot of old circuits continue to be worth revisiting.

joe
Administrator
Posts: 224
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Post Re: Spectral effects of rectification
on: August 30, 2012, 10:13
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Quote from bear on August 19, 2012, 12:59
Visual Sound's Angry Fuzz is supposed to be a Green Ringer derivative and is pretty danged hip. A lot of old circuits continue to be worth revisiting.

I'll emend that slightly — a lot of old circuits are worth revising. Not that they weren't great, but in many cases, they can be improved, or at least adapted to better suit modern gear and playing styles.

I've got Green Ringer on the brain, for reasons hinted at here. it does really cool harmonic stuff, but it's SO absurdly under-powered. But when you add some beef in front of it, well, stand back!

mwseniff

Posts: 149
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Post Re: Spectral effects of rectification
on: August 31, 2012, 09:50
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Quote from joe on August 30, 2012, 10:13

Quote from bear on August 19, 2012, 12:59
Visual Sound's Angry Fuzz is supposed to be a Green Ringer derivative and is pretty danged hip. A lot of old circuits continue to be worth revisiting.

I'll emend that slightly — a lot of old circuits are worth revising. Not that they weren't great, but in many cases, they can be improved, or at least adapted to better suit modern gear and playing styles.

I've got Green Ringer on the brain, for reasons hinted at here. it does really cool harmonic stuff, but it's SO absurdly under-powered. But when you add some beef in front of it, well, stand back!

There was a lot of stuff designed for music by folks that were electronicists but not engineers, as such there was some less than ideal electronic standards of design. I've updated a lot of fx pedals for customers and myself when appropriate to do so. True bypass switching is a good example tho' it was seen as a holy grail by many but not always appropriate. My biggest pet peeve is the polarity of power supply plugs with the negative being center pin. If you use a power supply with muliple plugs to power your pedal the barrel of the plug being positive can short the supply if the barrel touches a pedal case or metal plug shield. Had I been there when that standard was created there would have been a different outcome.

Digital-
Larry

Posts: 192
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Post Re: Spectral effects of rectification
on: September 4, 2012, 10:34
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I have added some sound samples to the original post demonstrating the effect of half wave and full wave rectification on a short voice clip.

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