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From: Spehro Pefhany
Subject: Re: Acoustic Feedback reduction
X-Newsreader: Forte Agent 1.92/32.572
Date: Sat, 11 Jan 2003 03:55:12 GMT
NNTP-Posting-Date: Fri, 10 Jan 2003 22:55:12 EST
On Sat, 11 Jan 2003 14:05:54 +1100, the renowned "Phil Allison"
>"Spehro Pefhany" wrote in message
>> Many years ago in good old _Wireless World_ (long before "wireless"
>> acquired its present meaning(s)) there was a design published for a
>> device used to allow higher volume levels in PA systems before
>> feedback occurred (with a given microphone). IIRC, they used
>> one or two multipliers in a circuit that shifted the audio pitch by
>> around 10Hz.
>> Has this sort of thing been integrated into a single chip?
> ** No. The cheapest and best way to frequency shift an audio signal is
It *could* be a special-purpose analog chip, of course.
> I designed a high quality + 5 Hz Audio Frequency Shifter which was
>published in Electronics Australia magazine. It used a number of TL074
>op-amps and a pair of AD633 multipliers to do the job.
Sounds challenging. I looked at a couple of sites with information on
the theory of this:
So, your design consists of a quadrature sine wave oscillator, two
multipliers and a Hilbert transformer (implemented as a filter), and a
high-pass filter to get rid of the 5Hz?
> Such a device is ONLY useful in reverberant rooms (most rooms are)
>where standing waves are the major cause of acoustic feedback. Where the
>acoustic path is short and direct there will be little or no benefit -
>except if you perfer a warble to a squeal.
Thanks, Phil. This is probably the most important bit of information!
Out of curiosity, any ballpark numbers on how much louder it can be
with the frequency shifter in place, in a typical reverberant room?
ISTR the increase was not a lot of dB in the WW article.
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