[music-dsp] minimum samples for lowpass IIR

[Jon Watte]

Wooah! Hold on there! If you clear the filter between blocks, it 
will not do the right thing. It will introduce potentially terrible 
artifacts. I thought you were talking about the start-up case. You 
cannot take very small blocks of signal and filter them all 
individually and expect to get a coherent whole out of it. The result 
will be as if you filtered each block in the presence of a vacuum, 
and then sliced out the first 32 response frames (including initial 
ringing) and strung them together. Not at all similar to filtering a 
continuous signal.

If you want to filter a continuous signal, you need to preserve the 
state between blocks. Else it would be like filtering a discontiguous 
signal :-) For an IIR, it's un-clear how much state you need to 
preserve -- after long gaps, it's probably fine to clear it to 0. For 
a FIR, you know exactly how much history to save -- it's the amount 
used by your convolution.

Cheers,

			/ h+


> -----Original Message-----
> From: owner-music-dsp at shoko.calarts.edu
> [mailto:owner-music-dsp at shoko.calarts.edu]On Behalf Of Marc Poirier
> Sent: Friday, December 07, 2001 1:12 PM
> To: music-dsp at shoko.calarts.edu
> Subject: RE: [music-dsp] minimum samples for lowpass IIR
> 
> 
> >An IIR filter is "fully effective" from the first sample on, if
> >you just initialize all the delay memory with zeros. Whatever
> >"fully effective" actually means is somewhat under-specified,
> >though :-)
> 
> You know, doing its thing...  :)
> 
> Okay, I did some tests & found that the filtering does indeed sound 
> fine applied to when the history is wiped out & then filters small 
> chunks of samples (like 32), but the continuous filter (the one that 
> doesn't wipe out the history ever) is louder.  So I guess all that's 
> in order is some make-up gain.  Which makes sense since you're 
> starting off adding a bunch of zeroes together rather than larger 
> numbers.
> 
> Thanks again,
> Marc
> 
> -- 
>     [ Destroy FX - http://www.smartelectronix.com/~destroyfx ]
> 
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