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Subject: Re: Acoustic Feedback reduction
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Date: Sat, 11 Jan 2003 06:31:32 GMT
NNTP-Posting-Date: Sat, 11 Jan 2003 07:31:32 MET
Spehro Pefhany wrote:
> 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
> done. ;-)
>> ** 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 externally.
> Ah, thanks.
>> 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 the acoustic path is short and direct there will
>>>> be little or no benefit - except if you perfer a warble to a
>>> Thanks, Phil. This is probably the most important bit of
>>> 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 about.
>> 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.
> Best regards,
> Spehro Pefhany
Philips sells these ICs for hands-free phones, they should be able to do the
electronic hardware designer
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