Using A Frequency Shifter to control Feedback


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I was wondering if anyone is using a frequencey shifter to control feedback in live audio situations. If so, do they work well with music or do they only work well with speech.

For those of you who do not know what I am talking about just a quick explanation:

Frequencey Shifters do exactly what the name says. The audio output from the mixing desk is shifted up or down by a few hertz eg 0.5 - 5hz. What this means is that the frequencies put out through the FOH speakers is slightly different then the input. This means you can't get into a feedback loop as the input and output signals don't add to each other.

This sounds like an interesting concept.

However, instead of feedback, wouldn't you get an increasing pitch runaway?

For example. input pitch is around 1000 hz.
output is 1005
but then the aforementioned mics pick that up, and increase it to 1010, then to 1015, etc etc... Would it just slowly roll up the frequency? I wonder if it will only go up a few hz, then down a few hz, to level it off?
I'll be sticking with my graphs. This just seems messy. After you do this enough, the tune of the music will be off, or the person talking will sound different.
I would imagine it would roll up jump jet suggested. If the feedback were happening at the resonant frequency for the room, or one of its harmonics it might actually help it, however if it moved the feedback into the resonant frequency or a harmonic watch out because it could get louder that way. It is an interesting theory though. In order to make it sound correct should it be 5Hz over the entire spectrum, or should it be weighted to the octaves? An example of that would be a 2Hz difference between 20 and 40Hz, then a 4 Hz difference between 40 and 80Hz? I have never heard of frequency shifting the whole spectrum, the only frequency shifting I have heard of is phasers, flangers, and chorus.
Then don't forget monitor vs house in that intance as well. Can't use it on mons, then the artists would have a hell of a time staying in tune. But if used at FOH and not mon. world there would be some horrible tones resonating around the room.
I cant really really see any benift of using a frequency shifter over a good EQing job. At my college we always EQ stuff basicly to the point that we can wave the mic infront of the speaker without feedback. We narrow the frequency down enough that it's not that noticeable in the end that we are missing the frequencies we cut out. Does anyone have a real idea of a benifit of the frequency shift technique?
Well, frequency holes wouldn't be there from EQing it out. Play a 20Hz to 20kHz sweep in that room and it would have some dead spots most likely. Play the same sweep with a phase-shifted setup and the result would not be the same frequencys, but more consistent. I would say EQing is a better method though.
Thanks for everyones replies. I had thought of building a unit out of an old magazine article but might not worry about it for now.

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