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Re: Stereo Parabolic recordings

Subject: Re: Stereo Parabolic recordings
From: Lang Elliott <>
Date: Fri, 11 Jun 2004 14:46:11 -0400
Re: Stereo Parbolic recording

I've been doing lots of recording using my custom stereo Telinga MKH20
setup. Go to the address below to listen to a recording I made last weekend.
It features a Rose-breasted Grosbeak plus the sounds of some other animals,
big critters of some sort. Maybe someone in this group can tell me what
creature so rudely interrupted my grosbeak recording. Be sure to listen
using headphones so that you can fully appreciate the marvelous stereo
effect:

http://www.naturesound.com/mp3/rbgroby.mp3

Studying the waveforms of my various stereo parabolic recordings on the
computer, I've discovered the following important fact. Unless the target
bird is exactly focused, with the bird's song being identical amplitude in
both channels, there is often a resultant phase difference between channels.
This occurs even when the target bird is "slightly" off-axis.

This can cause major problems when one desires a monaural recording and sums
the two channels to obtain it. Phase cancellations then reduce the amplitude
of the target bird's song, and may also result in other anomalies. For this
reason, it is not a good idea to mix the two channels to monaural. It is
better to choose the channel where the target bird is the loudest and use it
as your monaural source.

This negates my earlier conclusion that the stereo parabolic setup produces
a monaural recording superior to that of a monaural parabola. This is true
in theory, but only if the target bird is perfectly focused and there is no
phase variation between the two channels. In actuality, it turns out that
this is rarely the case, unless the target bird is in direct view and
focusing becomes foolproof. When the bird is not seen and one focuses the
parabola "by ear", it seems to be the norm that it will end up slightly off
axis, with phase differences evident.

It follows that if one chooses the loudest channel in a stereo parabolic
recording to produce a monaural version, then this will produce a result
that is somewhat inferior to that produced by a monaural parabolic setup. I
haven't tested this in a controlled situation, but I expect that using only
one channel will result in a loss of signal-to-noise of 3-5 db or so. Maybe
one of you tech-heads can tell me what would be expected in theory.

I expect that the Telinga Stereo DAT mike exhibits the same behavior as my
custom setup. A quick preliminary test revealed that this is indeed the case
(I noticed similar phase differences between channels).

In spite of this potential "problem", I am totally sold on stereo parabolic
recording. If you listen to the mp3 I've posted, using headphones, then
you'll hear why. The listening experience is far more pleasurable that one
has with mono. Off axis sounds come from their proper directions and the
sense of spaciousness is superb. Listening at home becomes a real joy, and
monitoring while recording in the field is much more exciting.

Lang



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