Subject: Re: [linux-audio-dev] jamin and FFT filtering
From: Alfons Adriaensen (fons.adriaensen_AT_alcatel.be)
Date: Tue Aug 10 2004 - 13:28:40 EEST
On Tue, Aug 10, 2004 at 10:53:39AM +0100, Steve Harris wrote:
> Yep, we have heard this on some material where you have sharp diconnects
> in the gain of adjacent bins, particularly in the low frequencies, it can
> be moderated by increasing the overlap, but I dont really understand why.
> The effect is not often very noticable though.
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.
-- FA
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