Probably the simplest form of stereo mic is represented by the
Rode NT4 which mounts two cardioid capsules at 90 degrees to
each other on a single body. (This angle between the capsules
is known as the mutual angle) The stereo technique employed
by the mic is XY coincident where the capsules are positioned
coincidentally (as close as possible) and at 90 degrees to each
other so that theoretically sound arrives at each capsule at
the same time. With the Rode the output of each capsule comes
out of a separate XLR connector and you simply pan the two outputs,
one left and one right to create the stereo picture. A further
development on this idea is the Sanken CUW 180 where the two
cardioid capsules are housed in pods either side of the body
and can be rotated so that they can face up to 180 degrees from
each other to create a wider stereo picture.
However when manufacturers start incorporating dual diaphragm
capsules then a huge range of stereo configurations becomes
possible. A classic example is the Neumann USM 69I microphone
which has two separate dual-diaphragm capsules which are mounted
vertically and can rotate against each other up to 270 degrees.
The capsules operate independently from each other and the directional
polar patterns can be selected separately for each capsule so
that you can configure it as a pair of coincident cardioids,
a classic Blumlein pair of figure 8 mics or in a Mid-Side configuration
with one capsule looking forward in cardioid with the other
at right angles as a figure 8. However in addition to the usual
polar patterns: omni directional, cardioid, and figure-8, there
is also Hypercardioid and a wide-angle cardioid pattern giving
the engineer unlimited pick up pattern combinations from a single
microphone.
Finally we come to the famous Soundfield
microphone developed by the National Research Development Corporation
and Calrec Audio. The Soundfield concept is based on 4 separate
coincident capsules arranged in a tetrahedron which all contribute
to what the Soundfield designers called the B Format signal
consisting of 4 streams of sound information from any point
around the microphone. The W stream is the omni information,
the X stream forward and rear information, The Y stream left
and right information and the Z stream up and down information.
What's important to realise is that all 4 capsules contribute
to the separate streams and this 4 channel B Format signal runs
from the mic in a special multicore to the microphones control
unit. The control unit allows the user to manipulate these four
streams of audio information enabling the microphone to be remotely
configured to any possible pick up configuration from a single
mic to a pair of mics in any pick up pattern. The direct/ambient
sound balance between front and rear can be changed and the
stereo width can also be adjusted, all without having to physically
move the microphone. But there is much more to it than that
because if you record the B Format signal to 4 separate tracks
you can play these back through the unit in the studio and have
complete control of the mic in post production, spinning it
through 360 degrees if you want or looking straight up or down.
It
really is a fantastically versatile and beautifully sounding
microphone and for the mobile sound recordist it is in a league
of its own.
