Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: Steve_Graham@ub.cc.umich.edu
Newsgroups: rec.audio.high-end
Subject: 3 mikes/3 tracks
Message-ID: <7243@uwm.edu>
Date: 26 Oct 90 13:01:19 GMT
Sender: news@uwm.edu
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Approved: tjk@csd4.csd.uwm.edu

Writing about Mercury Living Presence recordings, William Spencer
asked:
 
 > Would someone like to explain exactly how 3 mikes are used for stereo? 
 > What kind of mikes? Spread? Etc. 
 
So...  There are many ways to produce a stereophonic recording, but they
generally fall into one or the other of two classes: Spaced or Coincident. 
 
With coincident systems a pair of directional mikes (that is, mikes which
produce a stronger signal for sounds coming from some directions than
others) are mounted as close together as is physically possible.  They are
angled away from each other, and the stereophonic effect is produced
entirely by differences in loudness: sounds from the left are picked up more
strongly in the mike that points left, and vice versa.  This "single point"
pickup gives very precise left-right localization, but to some ears (mine,
for instance) lacks depth.
 
With spaced mikes a new element is introduced: time of arrival.  Usually
omnidirection mikes (which are in theory equaly sensitive to sounds coming
from all directions) are used.  Since the mikes are spaced apart, some
instruments will be heard louder in one mike, and some in the other; but
more importantly, sounds from the left will reach the left mike first, and
sounds from the right will reach the right mike first.  Our hearing system
uses these time of arrival differences to establish location; a sound heard
first in the left ear will be heard as coming from the left.  So with spaced
mikes much of the stereophony comes from timing information.
 
One problem which arises is that if the mikes are spaced only a short
distance apart then virtually all of the stereo effect is from timing, as
both mikes will "hear" all the instruments with almost equal loudness, and
because of the way speakers work in a room this may lead to some aural
confusion.  If the mikes can be moved farther apart the left-right
separation will be more apparent, and at the same time it may be possible to
locate the mikes for greater string presence, for instance.  On the other
hand if the mikes are spaced more than two or three feet apart the imaging
will be rather unstable:  Sounds which are only a little off center will
appear to be all the way to the side, giving rise to the infamous "hole in
the middle" effect.
 
Some of these problems could be overcome by using directional microphones;
the ORTF technique, which uses a closely-spaced directional pair of mikes,
combines (to a degree) the advantages of spaced and coincident mike
techniques.  However, along with other factors, it must be remembered that
in the 50's omnidirectional mikes gave the highest fidelity pickup, overall. 
And so to fill in the "hole" and give a more balanced stereo presentation of
the orchestra a third mike was added, in between the left and right mikes. 
(The distance between left and right mikes can vary widely, depending on the
engineer or producer's preferences.)
 
This classic three-mike system is still in use today by Telarc, though they
mix the three mikes directly to two tracks, and sometimes mix in
supplemental mikes as well.   The technique can provide good orchestral
presence, spaciousness and depth, and an excellent sense of the hall in
which the performance takes place.  If balanced with the necessary care, the
instruments seem to be spread fairly evenly between the speakers, rather
than just at left, right, and center. 
 
 
In the early days of stereo triple-track tape-decks were used in the USA for
a number of reasons. 
 
There were those who proposed actually using three channels to reproduce
stereo: Left, Right and Center; the idea being that images from the center
would be more stable, but sounds from the left and right would remain fully
separated.  (Some people still think this is a good idea.)
 
At the same time, record companies were pretty quick to realize that given
three tracks they could put, say, stereo orchestra on two tracks and use the
third for the singer, and then would be able to fine-tune the balance
between the singer and the orchestra after the musicians had gone home, or
even redo the singer's track altogether, without having to have the
orchestra hang around (and get paid).  Alternatively, various microphones
could be fed to the three tracks to enable the balances within the orchestra
to be varied later on.  RCA used the tracks in this way, for instance.
 
Another way to use the three-tracks is to set up simultaneous two-channel
stereo and single-channel mono balances on the same tape, so that only one
editing session would be required for both stereo and mono releases.
 
I believe Mercury used the three tracks for a combination of reasons:  They
hung three microphones in the "classic" spaced array (left-center-right),
feeding each mike to one track on the tape.  If I'm not mistaken they used
only the center mike for the mono records, but used all three for stereo.  I
think they were hoping (initially at least) that real three-channel stereo
would become viable in the home.  But in the meantime the three tracks were
mixed down to two in such a way as would give similar results to
three-channel stereo, over the two available channels; that is, the left and
right mikes fed the left and right channels, and the center mike was mixed
equally into left and right, so that if it were heard by itself it would
produce a "phantom" image between the left and right speakers.
 
Whatever the rationale, it seems to have worked.
 
--Steve Graham:
sg2@ub.cc.umich.edu 
     otherwise: 
USERHEFX@UMICHIB.BITNET