Re: [Audacity-devel] Better EQ curves
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From: Martyn S. <mar...@go...> - 2010-03-24 01:48:47
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On 16/03/2010 22:35, Richard Ash wrote: > On Mon, 2010-03-15 at 00:48 +0000, Martyn Shaw wrote: >> This doesn't seem like a good way forward (modifying the current FIR >> algorithms to give good/accurate phase response). If we want to get >> something-like the correct phase response as well as amplitude >> response then we should be looking at using IIR filters shouldn't we? > My courses never used the FIR/IIR jargon so I'm relying on wikipedia, Apologies for that jargon. Wikipedia seems to be doing you alright though! > but yes, all real RIAA/IEC etc preamps use IIR analogue filters, that's > why they have non-flat phase responses. The same would be true of almost > any analogue tone control system, varying from mild shifts on hifi-type > controls to quite large ones on the high-Q filters found in audio > production consoles and effects pedals. > > At the core of this is the fact that there are an infinite number of > watys to design a filter with a given frequency response curve. Even > allowing for the fact there are fewer good ones, there will always be > more than one possible interpretation on any interface that only > provides a frequency response input. > > Providing a phase and frequency response input removes all ambiguities - > the two curves _completely_ define the filter's response, any filter > that implements the two curves correctly will have no measurable > difference from any other (That's an over-simplification, but anything > that is outside it isn't really EQ, certainly in hardware). This is why > I was suggesting adding one extra set of parameters (hidden by default > I'm sure) as a simple way to provide almost any EQ function that could > ever be desired. It's true, but not that simple. You can't have an arbitrary amplitude response with an arbitrary phase response and still have a Real signal (versus a Complex one) (I think). And there certainly wouldn't be many users that would need that. The extra complexity would be a big step. >> We haven't got those at the moment in EQ but could make it if we >> wanted. We could have an interface that's lets us set those break points. >> >> I'm no 'golden ear' but I don't believe that people can hear a >> difference in phase, and I have tried this a number of times. But you >> can hear a change in phase of one component to a constructed waveform. I mean, if one components phase is changed, I can hear it changing, but once it stays still I can no longer hear the difference. I used to have access to a device that could do this but it's hard to simulate in software. >> I'm sure that we could get into 'heated debate' here about phase, >> but let's not. > > Do you mean by this you can't hear an overall phase shift, but you can a > frequency-dependent one? That would be a reasonable statement to me as > well. > > The usualy illustration of this is filtering the high harmonics of a > square (or nearly square) wave with a filter with linear phase response > and with the RC-type response - one has ripples on it's waveform where > as the other looks rounded off, and this is claimed to be audible. Can you hear a difference between the first and second half of http://dl.dropbox.com/u/1327769/phaseTest.wav ? I can't, but the waveforms are clearly different, although the spectrograms are the same. That's what makes me say that the (fixed in time) phase of the frequency response isn't very important. TTFN Martyn > Richard > > |