"greggery peccary" .@. wrote in message ...
hey out there, i'm in an audio production class and the instructor avoids a
direct answer to this question: is there something in the nature of a
slightly out of phase recording (such as that created by a difference in mic
distance) that can make something sound "warmer" like analog? i know all
about the "if it sounds good do it" philosophy, but i'm wondering to what
extent do engineers delve into phase corrction with their software? thanks
-greg

There is certainly a different sound when using a coincident pair of
mics vs. a spaced pair of mics, even if the exact same pair of mics is
used in the same room on the same acoustic source. The difference is
due to TOA (time of arrival) differences, but is this phase difference
is not constant with all frequencies or all parts of the source.
Imagine that a flute sitting to the front left of a spaced microphone
array, playing a melody. The sound from that flute arrives at the left
microphone first, then the one on the right. So there is a TOA
difference between the two outputs of the mic. However, the *phase*
difference varies with frequency, since phase is a sine *angle* issue.
In other words, the higher frequencies are more "out of phase" than
the lower frequencies (i.e. a greater angle of phase displacement).

For this situation, you actually could adjust the TOA between the two
signals by delaying the signal from the left microphone to match the
signal from the one on the right.

Now, where the problem comes in is that usually, there are musicians
all the way across the front of the array, so some musicians are to
the left, some in the middle, and some to the right. All producing a
wide range of frequencies, and with a huge number of TOA differences
(some signals even arrive at both mics at the same time - those
signals from sources directly between the two mics). And this does not
include the reflections, which come to the mic array from all angles.

So no, it is not possible to correct the TOA differences of a spaced
pair, across the entire frequency range, to bring this pair of mics
"in phase".

But, it is just these TOA differences that give the listener a greater
sense of "space" than if a coincident pair is used. Perhaps some
people characterize this as "warmth". However, I would guess that it
is more about the type of microphones typically used for the two types
of mic placement. For coincident pairs, usually directional mics are
used (cardioid is the most common choice for XY, for instance) and
omni mics are most often chosen for spaced arrays. The thing here is
that directional mics have proximity effect, and the low end response
is dependant on distance from the source. Most people equate this to
"boosting the bass when the source is close to the mic" but what many
people forget is that the opposite is also true: greater distance from
the source *reduces* the bass response. Most mics are measured at 1
meter, and even at this distance, you can see the majority of
directional mics show reduced LF response. So imagine what the LF
response is at typical acoustic orchestra recordings - 2, 3, 5 meters,
perhaps.

Omni mics do not exhibit proximity effect, and thus are not dependant
on distance for the LF response. If the mic shows flat at 30Hz at 1
meter, it will be flat at 30Hz at 10 meters. It is THIS factor I thing
probably contributes more to the subjective "warmth" of recordings
done with spaced pairs.

One further issue... then I'll stop. Coincident pairs use "intensity"
(differences in volume between left and right, depending on the
direction of the source) to generate a stereo signal. Spaced pairs
rely on TOA, as mention above. Despite subwoofer manufacturers' claims
that low-frequency energy does not give a sense of direction, one very
nice factor of spaced pair recordings with omni is that they give you
*stereo bass*. The combination of this, with the extended and natural
LF response of omnis is a great effect when done correctly.

All this being said, spaced pair recordings do not usually sum well to
mono...

OK, I'm done.

Karl Winkler
Lectrosonics, Inc.
http://www.lectrosonics.com