Phase and feedback control

[Chuck H]

A while back there was a thread where a guy wanted to build a phase inversion device to align parallel pedal boards. This evolved into something else and I mentioned it there at the time, but no one stepped up and I lack the experience and expertise to implement such an idea. So, for the more knowledgeable and technically inclined, here's what I think is a nifty idea...

Acoustic feedback is partially phase dependent. Shouldn't it be possible to make a device that could select a frequency or frequencies and alter the phase or volume independently? Because if you could it might be a very useful stage tool. Rather than simple notch filtering for feedback control one could perhaps cut less, adjust the phase of the offending frequency and keep some of it in the mix if they wish. We've all had to compromise stage mix EQ for the sake of feedback control. Or at the very least had to suffer through a show with an unavoidably bad stage mix. This could be a very useful and gratifying tool. It could also translate into an effect for guitar. As it is we guitar players have to turn this way and that, changing stage positions and otherwise are always trying to compensate for different environments to achieve our sustain and feedback desires. Carlos Santana is notorious for his sustain partly because he experiments with stage locations for different effects and marks those spots with tape before each show. I've watched guitarists struggle badly, obviously lost and unable to get their guitar and amp to behave acoustically on a strange stage. It's a heart breaker. Maybe the same principals could be applied to this situation too.

I know this isn't new technology, but perhaps a new application. If it's already been done just ignore my ramblings.

[Mick Bailey]

The Peavey Feedback Ferret does a pretty neat job of identifying problem frequencies with its digital filters, and reducing feedback without affecting adjacent frequencies. It doesn't seem to do anything with phase, though. It can be a challenge to set up if you want feedback, or have a sustained keyboard note in a song.

I wonder how the overall sound would change if the phase shifted as well as the level, due to cancellation/reinforcement effects?

[Chuck H]

I wonder how the overall sound would change if the phase shifted as well as the level, due to cancellation/reinforcement effects?

I had the same consideration. I figure such a device for stage filtering would need to work narrow bands like existing filters. Waaayy broader bands for a guitar effect.

[soundguruman]

In a PA system when all the speakers and horns are phase aligned, it doubles the volume, with no increase in amp power.
So, phase alignment can be quite a big help. In fact it's huge.
Signals out of phase will cancel, in phase will ring or sustain...

But normally signals are aligned with digital delays.
On the JBL 6 way electronic crossover (for example), each output has adjustable delay. So alignment is very easy if you know how.
We take 2 identical microphones, plug them into a dual trace scope, and line up a sine wave(s) in phase.
And we go thru subs, lows, mids, highs, and line them all up, using those 2 microphones to compare the phase alignment, of each speaker component.

[Chuck H]

Right. Got that. Knew that. What I'm talking about would be to skew the phase of an offending frequency for a specific signal to be amplified. As in "Gee! My microphone feeds back acoustically when I turn this way, but not when I turn that way!?!" If you want to be turned "this way" instead of "that way" the phase of the offending frequency could be altered to help compensate without the need to remove that frequency entirely from the amplified signal with no affect on any other phase relationships on stage. Or, conversely: Gee! My guitar doesn't get the sustain I need for that part in this room unless I turn my ass to the audience while standing on my head!?!" It may be possible to tune the phase of the guitar and amps relationship to the room so that "standing on my head" isn't necessary.

[Alan0354]

I did a lot of Santana type of feedback in my days, I did not find it hard, it's quite easy for me to sustain notes. Just loud, muff the unwanted strings. More technique than the equipments.

That said, I don't think you can do any sort of electronic filtering without changing the sound. here is a patent by Steve Shattil that do noise cancellation BUT it is also used as electronic violin. The idea is to drive a signal through an EM wave transmitter close to the pickup with signal, that is either in phase to the guitar signal produce positive feedback for sustaining OR out of phase signal of the hum to neutralize the EM noise from the power source. That is close to what you are talking about to produce positive feedback.

https://www.google.com/patents/US620...ed=0CDcQ6AEwAA

The pdf verion can be downloaded in the link above. Here is the pdf viewer with diagram.

https://docs.google.com/viewer?url=p.../US6208135.pdf