Subject: Re: [linux-audio-dev] jamin and FFT filtering
From: Alfons Adriaensen (fons.adriaensen_AT_alcatel.be)
Date: Wed Aug 11 2004 - 13:24:36 EEST
On Wed, Aug 11, 2004 at 09:32:11AM +0100, Steve Harris wrote:
> 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?
Yes, that's odd.
> The wave which makes up the ripple looks like a fs/2 sinewave.
It's not clear to me what exactly you did to obtain these results.
I assume it's of the form
1. input --> FFT --> complex x(f)
2. y(f) = x(f) * r(f), with r(f) a real symmterical function
3. y(f) --> IFFT ---> real output
Note that for symmetry, r(f) must be of the form
1 + a * cos (b * f), not 1 + a * sin (b * f)
Using a sine 'ripple' would only show up in the imaginary component
the output, which is probably not even computed.
If the ripple is at "fs/2", it would be removed anyway by any windowing
that is apllied before overlapping parts are added together.
Could you try a cosine ripple at "fs/4" or "fs/8" ?
BTW, when do you apply windowing : before step 1, after step 3,
or both ? What is the FFT size ?
-- FA
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