Clipping question

[Chris60601]

What makes a clipping option asymmetric or compressed?
Would asymmetric be just one diode where compressed would be 2?

Cheers
Chris

[induction]

Asymmetric clipping means that the clipping threshold (the signal voltage where clipping occurs) has a different value on the negative swing than on the positive swing. This can be achieved by using an asymmetric arrangement of diodes for positive and negative clipping. Two identical diodes in antiparallel will give symmetric clipping, pretty much anything else will be asymmetric. For example: two 1N914's on one side and one 1N914 on the other, or one 1N914 against 1 LED, or Si against Ge, or whatever. In hard clipping arrangements, lower forward voltage diodes give more dirt, and higher forward voltage gives more volume. I like asymmetric hard clipping because sonically you get the perceived dirt of the lower Vf and the perceived volume of the higher Vf (or pretty close).

Compression and asymmetry are different concepts. Compression means reducing the volume of parts of a signal that exceed some threshold, usually in conjunction with boosting the result. This makes the volume more consistent. The sonic effect of strong compression is that when you play harder, the signal doesn't get any louder, and small changes in pick attack from one note to the next don't have as much effect. Clipping is a form of compression (technically it would be called hard limiting), whether its symmetric or asymmetric. Technically speaking, symmetric clipping is no more compressive than asymmetric clipping. (Let's say you have an 1N914 and a 1N34a as a clipping pair. Everything else being equal, you would get more compression than a pair of 1N914's, and less compression than a pair of 1N34a's.)

But you can make the case that symmetric clipping gives the sonic effect of more compression because asymmetry gives the dirt characteristics of the lower Vf and the volume characteristicsof the higher Vf. This means that the asymmetric arrangement would give more volume variation than an equivalently distorted symmetric arrangement. In other words, with compression, what you are really compressing is the distribution of the volume levels in a signal. The asymmetric arrangement adds distortion , but leaves the unclipped volume distribution more intact.

[Frank_NH]

Excellent response by induction.  For more information and insight on distortion I highly recommend this essay at Geofex:

Distortion 101

"But you can make the case that symmetric clipping gives the sonic effect of more compression because asymmetry gives the dirt characteristics of the lower Vf and the volume characteristicsof the higher Vf. This means that the asymmetric arrangement would give more volume variation than an equivalently distorted symmetric arrangement. In other words, with compression, what you are really compressing is the distribution of the volume levels in a signal. The asymmetric arrangement adds distortion , but leaves the unclipped volume distribution more intact."

This is a good way of thinking about signal clipping using diodes or other means.  Strong clipping removes the harmonic content of the original signal as your waveform starts looking more like a square wave.  The good thing about mild symmetric clipping or asymmetric clipping is that more of wave form stays intact.  In the case of asymmetric clipping, one half of your wave will be mildly clipped while the other looks like a square wave, and so retains more sonic character.  I definitely like this better personally.

Here's more on compression and limiting in pedals:

Compression and Limiting in Distortion Boxes

One more thought - it's easy to analyze signals assuming they are constant amplitude pure tones (sine waves).  But guitar signals are anything but.  For example, the initial note attack can be high amplitude and therefore clipped, but as the note dies away, it transitions into a low amplitude unclipped signal.  And the note itself will have strong, non-uniform harmonics, some of which will be clipped and others while will remain unclipped.   And that's just a single note - when you have two or more notes ringing together, the waveform becomes very complicated.  That's why we spend so much time tweaking our overdrives!   

[Chris60601]

Ah - thank you both!
While for now it's clear as mud (LOL) it'll be a big help as I digest the info.


Cheers
Chris

[rocket88]

Lol. It takes time, and experimenting I think to get some of this stuff, especially playing with tonestacks. Btw, I'll have my comp to send you stuff Sunday, so don't think I forgot about you Chris.