[linux-audio-dev] jamin and FFT filtering

[Alfons Adriaensen]

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