At 01:14 AM 2/14/2003 +0100, you wrote:
>Stereo depends mainly on 3 things, - amplitude, time and phase differencie=
s.
>
>Two omni mics close to one-another make a monomic. Both are pressure sensi=
ng
>and both will sense "the same" pressure. You will neither get amplitude,
>time, nor phase differencies, except at very high frequencies.
>To get stereo out of two omnis, close to one-another, you have to create a
>difference between them. Our ears have a head in between. It gives both
>amplitude, time and phase differencies.
>
>Take a Telinga with a stereo DATmic and remove the dish. What do you have?=
?
>Answer: You have a binaural stereo microphone. You can use it for nature
>sounds, but you can't pick out a certain individual with it, unless you ge=
t
>very close to it.
>
>Now put the parabol on. What do you have?
>Answer: You have the same binaural stereo mic, but you have also added a
>mono signal, reflected by the dish! Now you can do stereo recordings + pic=
k
>out an individual.
Klas:
I disagree. When you compare separating sounds for two mics with and
without a parabola, it is exactly like comparing two film grains of silver=
halide with and without a lens in front of it. BIG difference, a resolved=
image versus nothing. You cannot explain about it by restricting your
discussion to amplitude, time and phase differences, unless you use the
actual math expressions - way beyond me! Ideally, all path lengths from
sound source to mic that are reflected by the parabola are identical in
length, so sound waves that have spread out and weakened to cover the whole=
parabolic surface area now re-converge onto your mic surface. Having
travelled identical distances they are all in phase, and amplitudes sum.
In the camera case, all the light from the left side of the pond hits grain=
#1 because the lens is focusing it to there and ONLY to there. All the
light from the right side of the pond hits grain # 2 for the exact same
reason. Waves do focus. With no lens you have nearly identical wave
fronts arriving at both mics. Producing stereo -- getting sound to arrive=
differently at two mics -- without the parabola is totally unlike the
parabola lens reflector case we are discussing.
With no lens you have to approach stereo in a totally different manner, as=
our own ears do. But what produces stereo with no lens is, I claim,
totally irrelevant to the case WITH the lens. With a parabola, you let it=
just do its job, and put the bright spot or globe of sound on each mic.
I did this on and off for thirty years with little Sony ECM $30 mics and
got wonderful cassette-quality stereo. Cornell LNS, from Dave Wickstrom
and Jim Gulledge onward, repeatedly tried to talk me out of it, but I
ignored them. I hypothesized they were afraid that if their movie
contacts ever found out I had a library of stereo recordings it would be
bye-bye LNS birdie.
The reflected focused sound arriving at a mic near the focal point behaves=
utterly differently from free-field sound. Tiny differences in mic
placement make huge differences in amplitude, time and phase. This is
never true in free field sound from a distant bird. With the parabola, move=
a single mono mic left one inch and you must now move the parabola right
some 2 to 5 degrees to hear the sound again. THAT is how you get stereo
from a parabola. There is no apriori reason I have ever been able to think=
of that it should sound much like binaural stereo, but it rather does. I
liked it especially because in the field aiming the parabola was a lot
quicker and more intuitive.
my best regards,
Marty Michener
MIST Software Associates PO Box 269, Hollis, NH 03049
EnjoyBirds.com - Software that migrates with you. http://www.EnjoyBirds=
.com
________________________________________________________________________
________________________________________________________________________
|