10 Aug
2004
10 Aug
'04
8:44 a.m.
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