room impulse response and deconvolution

[Martin Czech]

Again room responses.

One of the problems is that I can not apply a unit impulse
to the room. Balloons look more like differentiated unit pulses,
and together with microphone far worse then that. So
the recorded response is somehow convoluted with an unpleasant function.

One can try to deconvolute this, but the problem was, that:

1. time domain deconvolution is impossible for fixed sequence length
2. seems to not converge for infinite sequence length
3. frequency domain deconvolution operates circular, ie.
   expects the deconvolution filter only to work with circular
    convolution, which is not what we need.
    
After running arround in the woods for a couple of hours I thought
that linear convolution yields the same result as circular, if
only the deconvolution filter is repeated again and again,
of course, because this will mimic circularity for a while.

Ok, experimental software rewritten, cat copies 3 times the 
deconvolution filter sequence, convoluted with the balloon free
air response, voila, two nice sharp unit pulses in the middle,
and of course some trash at the beginning and the end.
But a silver lining. As you might have guessed, the next step
is proper windowing, as always if abrupt sequnence ends have to
be fixed. 

First experiments with a simple trapezoid window give encouraging results,
a sharp unit impulse of magnitude 4000, and some garbage arround,
but smaller then 50. 

The next steps should improve the window, and also verify that the garbage
can not be heard because of masking by the big unit pulse.

So things get better.

The poor man's method (balloon) seems still to be reasonable for 
acoustic measurements. 

And for electronic measurements (electronic reverbs) I could
try repeated pulses (signal enlarges by N^2, but noise only by N),
so S/N can be improved (and is critical if it is hissing broad band
noise).

m.c.