[linux-audio-user] Making a room sound smaller

Phil Mendelsohn phil at rephil.org
Tue Feb 28 10:26:49 EST 2006

> I have a recording from a lecture. It was recorded in a large
> auditorium and of course it show in the recording. I'd like to process
> the recording to get a more intimate sound. It there any way of
> reducing that ballroom echo a bit? If it can be done with a  ladspa
> plugin so much the better.

Hi, Alex,

You can try various filtering things, but all are subject to a certain
amount of failure.

I don't know about plugin modules, but I can tell you in general, the way
you'd fix this is using "Deconvolution."  The idea is simple -- it's
basically "unfiltering," but there are a couple of problems.

1:  You have to have some idea what the room does to the signal. 
(Possibly some info can be taken from your recording.)

2:  It's really sensitive to the numbers you get from the first step -- so
even in theory (i.e., on paper) it doesn't work all that well.  In
practice it's pretty hard to make work -- this is really "black magic"
DSP.  [though there's no such thing.]

So, having said all that, you will probably get *better* results by using
noise gates with a fairly high "off" level, i.e., not hard-gating, and
possibly by using a key input that is bandpass filtered at about 2500-3000
Hz (tune by ear to the speaker's voice -- and filter out lows and highs.)

I'm pretty sure the last will be possible using LADSPA tools -- though I
can't say exactly which you're looking for!

If it's any help in the future, I've found that most speakers (if this
were at University, not public speaking for $$) will let you park your
recorder closer to the stage -- the best answer of all is to keep the
reverb out of the recording to begin with.

It'll never sound like a voice over recorded in a studio, but maybe
something here is helpful.

Phil Mendelsohn

Dept. of Mathematics, 342 Machray Hall
U. of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
Office:  446 Machray Hall, 204-474-6470
http://www.rephil.org/   phil at rephil dot org

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