Subject: Re: [linux-audio-dev] jamin and FFT filtering
From: Steve Harris (S.W.Harris_AT_ecs.soton.ac.uk)
Date: Wed Aug 11 2004 - 11:32:11 EEST
On Tue, Aug 10, 2004 at 12:28:40 +0200, Alfons Adriaensen wrote:
> You don't need sharp discontinuities in the frequency curve in order to
> have these pre-ringing or pre-echo effects. Consider the following freq
> response:
>
> G = 1 + 0.0116 * cos (2 * PI * f / 100 Hz)
>
> This is a flat response plus a 'ripple' with an amplitude of 0.1 dB and
> a period of 100 Hz. For all practical purposes, this a a flat response
> that should have no audible effect at all.
>
> Now the inverse FFT of this is an impulse response consisting of a
> single impulse at the centre, and two smaller impulses 10 ms before
> and after it.
>
> On other words, there will be a pre-echo 10 ms before the signal, at
> a level of approx. -42 dB. This is *very* audible.
I just checked jamin, and there is some ripple around the impulse (as you
would expect), but it peaks at around -56dB, and I couldn't find traces of
a pre echo visibly, or audibly when amplified. There is what looks like
post echo though (at around -66dB, 4.5ms after), which is odd, I would
have expected the response to be symmetrical?
Theres a screenshot from audacity with 50-odd dB's of gain applied:
http://www.ecs.soton.ac.uk/~swh/jamin-impulse.png
and a gzipped float WAV file of the output, when given an impulse:
http://www.ecs.soton.ac.uk/~swh/jamin-impulse.wav.gz
The wave which makes up the ripple looks like a fs/2 sinewave.
- Steve
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