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From: "Phil Allison"
Subject: Re: Acoustic Feedback reduction
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Sat, 11 Jan 2003 15:15:08 +1100
NNTP-Posting-Date: Sat, 11 Jan 2003 15:04:40 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Spehro Pefhany" wrote in message
> On Sat, 11 Jan 2003 14:05:54 +1100, the renowned "Phil Allison"
> > ** No. The cheapest and best way to frequency shift an audio signal
> >still analogue.
> It *could* be a special-purpose analog chip, of course.
** It must be a well kept secret then.
> > I designed a high quality + 5 Hz Audio Frequency Shifter which
> >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?
** 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.
> > 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.
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