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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 04:31:20 GMT
NNTP-Posting-Date: Fri, 10 Jan 2003 23:31:20 EST
On Sat, 11 Jan 2003 15:15:08 +1100, the renowned "Phil Allison"
> It *could* be a special-purpose analog chip, of course.
** It must be a well kept secret then.
Fortunately for us, everything that *can* be done has not yet been
> ** Not quite. The Hilbert transformer ( two networks of all pass filters
>designed to be 90 degress out of phase with eachother over the audio band
>+/- 2 degress) - yes, a 4.5 Hz sine ( phase shift ) oscillator followed by
>a single all pass filter adjusted to give 90 degress shift, then two AD 633
>four quadrant multipliers and a summing amp. Plus balanced in / out and on-
>board PSU regulators and bridge/filters so only a small tranny was needed
> The PCB and all parts were sold as a kit here for only $A 85.
That sounds quite reasonable, especially at current exchange rates.
>> > Such a device is ONLY useful in reverberant rooms (most rooms are)
>> >where standing waves are the major cause of acoustic feedback. Where
>> >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?
> ** Used in a Church the result was about a 8 - 9 dB increase in the
>feedback threshold - even if the pastor was using a radio mic and wandering
> A big plus is no feedback squealing at all but instead a slight
>warble is heard in the sound when the limit is reached and hence gives a
>warning to the operator to back off the gain a bit or have the person with
>the mic move away from the speakers.
> Also if you want to use a graphic eq as well they do not conflict -
>just set the eq first and then switch the shifter to "on" from bypass.
Thank you for the information.
"it's the network..." "The Journey is the reward"
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