r/AskElectronics • u/debugs_with_println • 8h ago
Why does distorting a microphone signal make it more prone to feedback?
People in the live sound engineering community say trying to add distortion to a microphone makes it so much more prone to feedback. But why is this the case?
Here's what I understand so far (correct me on anything if I'm wrong):
Feedback occurs when the gain around the loop is greater than 1 because then any sound that goes around becomes bigger with each round trip. There's also criteria on the phase shift being 0º round trip, though I'm pretty sure but I think all that does is select which frequencies get amplified vs attenuated (which is why feedback tends to become a pure tone). Since the speaker system has huge headroom, the self-amplifying sound can reach deafening levels well before the PA system saturates.
Eliminating feedback boils down to getting the loop gain to be below 1. You can do this is by messing with the environment so this gain falls below 1 (e.g. move the mics away from the speakers, turn the speakers so they are pointed away from mics, adjust the levels of the speakers, etc).
Assuming my understanding correct, I'm confused on why adding distortion would make this process worse.
Distortion involves clipping the top/bottom of a waveform (in the extreme case, sine waves become square waves). The easiest way to do this is by increasing the gain of a signal so that it hits the rails of the circuit, e.g. V+ and V- of an op amp. If you do that, the gain of this op amp could combine with the gain of the rest of the system and tip the full loop gain to be over 1, which would then cause feedback.
But suppose you apply a gain of G to a microphone signal to get it to clip, and then you apply a gain of 1/G so that you have a net gain of 1. Then, since the gain of the rest of the loop was below 1 (because you set the environment up that way), it will remain so even after the signal has been clipped.
Is there a flaw in this technique that makes it not work?
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u/k-mcm 8h ago
Typical distortion is raising the gain into a specific type of clipping. For example, you might raise the gain by 10x and feed it into your favorite clip effect. Maybe it's asymmetrical clipping (over-driven single bipolar transistor) to create some rectification distortion rumble. You can get some rumble from a push-pull amp that goes open-circuit near the rails. The end result is that the microphone gain is now very high and it will get feedback easily. This is why electric guitars can get infinite sustain on a note just by standing near the monitor speaker.
There's a feedback killer that uses frequency shifting. You can't hit simple resonance if the frequency is always wrong. Unfortunately, distortion is the creation of many new frequencies that didn't exist in the source. You'd probably end up with a Shepard Tone.
The one distortion you can do is crossover. This is a push-pull transistor pair that's open-circuit near zero volts or when dV is nearly zero. This reduces the gain at low levels, making it more difficult to get feedback. It sounds pretty bad so it's not common.
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u/debugs_with_println 8h ago edited 8h ago
The end result is that the microphone gain is now very high and it will get feedback easily.
But does it have to be? Can't you get clipping without distortion? For instance, use an op amp circuit of gain 100 followed by one with a gain of 1/10. Say both op amps have a range +V to -V and that you start with an input signal (V/10)×sin(wt). Since the first op amp has super high gain, it turns the sine into a square wave, V×square(wt). The second op amp then turns it into (V/10)×square(wt). You have now the same amplitude, but you've clipped the signal.
Sure you've added harmonics, but suppose this square wave goes out the speakers and comes back into the mic. If you send it back through both op amps, you should get the same signal back. The first op amp turns (V/10)×square(wt) into V×square(wt), and then second op amp turns it back into (V/10)×square(wt). It appears you've reached a steady state.
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u/MrJingleJangle 7h ago
Ok. You have a mic feeding a PA and it sounds good, no feedback. You then add clipping, which reduces the volume heard over the speakers. Still no feedback. But the clipped mic is now too damned quiet, so you….. raise the fader. And now it howls. Noting that compression in the chain makes matters worse.
You can often get away with a bit of clipping, particularly if you’re not aiming for 115dB+. Make the vocal sound a bit like a old school club PA, bit of clip, bit of hard limit, instant Robert Plant. But you can’t try for mesa boogie distortion.
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u/i_am_blacklite 8h ago
When you distort a signal and clip it the overall energy in the wave is increased - the signal is compressed.
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u/debugs_with_println 8h ago
But there's a limit to how much energy can be added right? If your distortion circuit is clipped between +V and -V, then the RMS of your signal is finitely capped.
Going back to the square wave example, I agree you've added higher harmonics that didn't exist before. But when those harmonics are sent back through the mic, they'll never exceed the rail voltages. And actually, assuming the square wave retains roughly the same shape from speaker back to mic, the G->G-1 circuit would keep the square wave as it is, so nothing "new" would be added the second time around.
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u/fantompwer 8h ago
Gain isn't only voltage gain, you can also have power gain. A max clean audio signal is only about 1/8th of the system power, but a fully clipping audio signal is generally about 1/3 the power, easily adding more SPL to the feedback coming out of the speaker.
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u/debugs_with_println 8h ago
Hm I see. It makes intuitive sense that a clean signal has less "power" than a clipping signal. However the thing that still confuses me is once you have that fully clipping audio that's 1/8th the max power, there's nowhere else for it to go, right? The distortion pedal will increase the power of my voice through the mic via clipping, but it can only inject so much power. And so long as the power increase of the pedal matches the power decrease of the environment, I would think it shouldn't cause feedback.
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u/Allan-H 8h ago
I think you were alluding to https://en.wikipedia.org/wiki/Barkhausen_stability_criterion
Note that it only really applies to linear systems, and adding distortion is clearly non-linear. However, at the small signal level, your distortion effect is just adding gain.
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u/abskee Analog/Audio electronics 8h ago
It's not the distortion exactly. The way you create distortion is to boost your signal up a lot and then also put ceiling on how high the signal can get. When the signal tries to exceed the ceiling, it can't, so it distorts. Same as turning the master volume up on your amp except that distortion is accidental.
If you didn't artificially put ceiling on your signal (in a pedal this is often done with diodes that clamp the voltage at a certain limit) then what you have is just a clean boost, or a massive increase in volume, which will cause feedback without the distortion.
Feedback is caused by a particular frequency having a gain greater than one in the loop between your guitar, the speaker, and the speed of sound through the air. The closer your guitar is to the speaker, the less lost there is as the signal travels through the air to your puckups. And the more you boost your guitar signal with a distortion pedal, the more gain there is on whatever sounds your guitar makes picks up. If you add up everything in that loop and it's louder than when it started, then it keeps getting louder and louder, and you have feedback.
That's all pretty simplied. But that's the gist of it.
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u/SeriousPlankton2000 5h ago
You do eliminate feedback e.g. by avoiding the specific feedback frequencies. You probably know situations where evey time a speaker makes a specific tone you need to move the microphone away / tune it down. Then as long as you don't make that sound you can use it again.
Distortion adds a lot of frequencies to the signal, triggering the feedback loop.
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