11 Aug
2004
11 Aug
'04
6:47 a.m.
On Tue, Aug 10, 2004 at 12:28:40 +0200, Alfons Adriaensen wrote:
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.
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?
Theres a screenshot from audacity with 50-odd dB's of gain applied:
http://www.ecs.soton.ac.uk/~swh/jamin-impulse.png
and a gzipped float WAV file of the output, when given an impulse:
http://www.ecs.soton.ac.uk/~swh/jamin-impulse.wav.gz
The wave which makes up the ripple looks like a fs/2 sinewave.
- Steve
