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From: "Phil Allison"
References: <96dU9.21919$jM5.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
Subject: Re: Acoustic Feedback reduction
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Mon, 13 Jan 2003 19:00:41 +1100
NNTP-Posting-Date: Mon, 13 Jan 2003 18:50:00 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Don Pearce" wrote in message
> >> >>
> >> >> First - I am not looking for a fight.
> >> >
> >> >
> >> > ** But I am gonna give you one Mr Expert.
> >> >
> >> >>
> >> >> But - the only difference between pitch shifting and frequency
> >> >> shifting is that in pitch shifting, each frequency is shifted by a
> >> >> certain percentage of itself, while in frequency shifting each
> >> >> frequency is shifted by a given number of Hz.
> >> >
> >> >
> >> > ** THAT is one HELL of a difference.
> >> >
> >> Not in the context - both do indeed necessitate a frequency shift.
> > ** Bull, it is massive in context. Stick to RF Don. One cannot use
> >pitch shift in a PA system to reduce feedback.
> >> >> The two are different in one important regard - in pitch shifting
> >> >> harmonic relationships are preserved and the result remains musical.
> >> >> In frequency shifting this does not happen, and if the shift is
> >> >> carried too far, the result is very unmusical.
> >> >
> >> >
> >> > ** A few Hz upshift has no detectable impact on most ears.
> >> >
> >> Depends on the frequency. At low frequency a few Hz shift could
> >> represent a semitone - decidedly detectable.
> > ** Not on music programme. Only on tones and only if quickly A-B
> Nonsense. How about a 5Hz shift - that is a few Hz, no? Do that to a
> guitar, and you have shifted the E to an F (82 to 87Hz). Do you really
> suppose that if a guitarist plays an E, and the audience hears an F,
> nobody will notice? Crap!
** I am NOT supposing anything, that is what *you* are doing Don. I
have experience with shifters and you plainly do not. It is pig arrogant of
you to so rudely contradict. Your straw man thought example above is very
The fundamental is NOT used by the ear to judge pitch but the upper
harmonics are. The percentage change in those is very small and no pitch
shift is obvious.
If you have NOT heard the effect of a shifter on music programme you
>> >> Shame you decided to start out by being rude. Especially as in the
> >> rest of the post you haven't really contradicted anything I said -
> >> merely expanded on it.
> > ** I contradicted the whole stupid lot.
> You really are very rude, aren't you?
** But not in your Pommy smug smartarse league one bit.
> > There are a few areas of specific error in your
> >> last section, though. The main one is your confusion between
> >> reverberation and standing waves. They are not the same thing and a
> >> space can be highly reverberant with no standing waves whatever.
> > ** Complete bullshit. What planet do you come from ?
> Planet Earth - and you? I suggest you go and learn what standing waves
> are, and what reverberation is. Then go and listen to reverberation is
> somewhere like a cave - where you will find no standing waves
** On what planet do they install PA systems in caves Don ??
The Planet of the Apes??
> > You have clearly never seen the *slow* sweep response of an
> >auditorium - it is full of sharp peaks and dips. What do you suspect
> >causes them? Men from Mars ?
> Of course I have - that is caused by standing waves, not reverberation.
** That is tortured logic. The causes of both are one and the the same.
> > Ever done a slow response sweep of a digital reverb unit - guess
> >same peaks and dips - guess why ???
> Because you get a comb filtering effect from summing signals at
> different delays - there are no standing waves in a digital box,
> obviously. You need some more theory here too.
** More tortured false logic. Delays ( echos) are just what the walls of
a room produce.
> >And> of course your arbitrary limits of 4 to 20Hz have no meaning.
> > ** Only to a moronic jerk like you Don. These are the lowest
> >standing waves that can be supported in auditoria and rooms where a PA
> >be used. All other standing waves are derived from these by integer
> Really? Can you explain why 3.5Hz, or 21Hz are out of the question?
> I'm sure there are more moronic jerks out here who don't quite see
** More tortured logic. See:
M R Schroeder and K H Kuttruff in, "On Frequency Response Curves, in
Rooms, Comparison of Experimental, Theoretical and Monte Carlo Results for
the Average Frequency Spacing between Maxima", Journal of Acoustics Society
of America Vol 34, No 1. p 76 ( 1962) They derived a formula for the
spacing of major maxima as 8 / Tr where Tr is the reverberation time in
> >> And of course there are resonant structures that affect howl frequency
> >> besides the room.
> > ** The room is the one with the really high Q resonances - several
> >thousand of them with Qs in the thousands at high frequencies.
> Qs in the high thousands? You understand you are in Quartz crystal
> territory with these numbers... No, 'fraid not. There are no
> structures in rooms with these kinds of Q. You need to go and study
> again - unless of course you can cite a source to confirm your
> extraordinary claim.
** More tortured logic from Don the dickhead. The room structures are
not resonating, how stupid even to suggest it. Sucessive response peaks
are typically 10 Hz apart and have -3dB bandwidths of around 2 Hz . At 5
Khz this gives a Q of 2500.
> >The loudspeakers and microphones are a mass of
> >> resonant peaks themselves. Being able to shift the feedback tone off
> >> the top of one of these peaks can give you an extra few dBs of margin
> >> in the onset of feedback.
> > ** No, the peaks in them are too broad. It makes practically no
> >difference - plus they are not evenly spaced by a fixed number of Hz like
> >a room.
> And so what if they are not evenly spaced
** Shifting will not work - end of story.
- you only need one peak to produce a feedback howl, and that peak does not
need to be a narrow one. It just needs a range of phases so that at some
point there is 360 degrees net, and a loop gain of greater than 1 - hey
** And this proves what ? Absolutely nothing in relation to room
resonance induced feedback or that a small shift will work in any other
> And that is all you are getting from me. You are simply too rude and
> unpleasant to deal with.
** You are complete smartarse Don - you proved that a before with the
audio dBm issue that was all due to your dumb errors.
Same again here. You are an audio ignoramous.
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