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 - 14:04:16 EEST
On Wed, Aug 11, 2004 at 12:24:36 +0200, Alfons Adriaensen wrote:
> 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.
Its a full JAMin running with a flat freq response. I was testing it to see
how bad the pre-echo was.
> 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)
In this example r(f) = 1, its just a flat response.
> Using a sine 'ripple' would only show up in the imaginary component
> the output, which is probably not even computed.
Yes, its a half-complex FFT.
> 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" ?
Applied where?
> BTW, when do you apply windowing : before step 1, after step 3,
> or both ? What is the FFT size ?
Before 1, the FFT is 2048 bins, 8x overlap.
- Steve
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