Theatre Design & Technology - Winter 1999 - 16

Middle/Side organization (as detailed by Thomas)
does not require three sends at all, just three speaker positions controllable from a stereo output mixer. One signal goes to the center loudspeaker(s) and the other
signal goes to the left and right side speaker(s). The left
and right side speakers have their polarities reversed so
their sound does not collapse to a center mono image
which superimposes itself on the center channel's sound.
In this way, the image from the side speakers stay on the
sides and the center channel fills the void in the middle.
To execute the polarity, reverse the speaker wire
conductors of one side speaker either at the amp or at
the speakers-but not both or the speaker will return to
the same polarity as the other side (see fig. 1). This system economically uses three speaker positions which require only two mixer sends and two anlp channels, and
makes it rather easy for most people to try this system
design with only one additional speaker and length of
speaker wire (assuming that the amplifier can handle the
lower impedance of the two-speaker circuit).
Middle/Side playback is most effective when the
source material has been recorded for MIS playback. In
a typical stereo recording much of the most important information is sent equally to the left and right channels so
that upon playback they will create a virtual center channel halfway in between the speakers. With a good audio
system and the listener perched exactly between the
speakers this effect can be most convincing. Pla)~ng back
a stereo recording with a Mid/Side system is a little hit or
miss. When stereo recordings are played through a

the techniques explored in this article require a mixer
that provides a separate send to the center channel. Such
mixers individually address the three speaker locations
through the input sends either by providing three separate master faders (left, center, and right), or one fader
for the left and right output, and one for the center. While
these features are less critical, a separate fader for each
output is preferable.
Moving from Stereo to MIS Playback
Even with an UCIR speaker configuration many designs
still use stereo playback because most prerecorded
sound is in that format. One effective alternative to stereo
playback is Middle/Side playback. In the fall 1996 issue
of TD&T, Richard Thomas of Purdue University explored
the use of Middle/Side recording and playback systems
for use in the theatre ("Mid-Side Recording and Playback
Techniques," Vol. 32 no. 5). Thomas credited Tom
Mardikes of the University of Missouri at Kansas City for
introducing him to the concept.
Middle/Side (MIS) recording was developed for
situations where a recording may need to supply both a
mono and a stereo output. This technique involves the recording of a center cardioid channel of information and
a side bi-directional channel (using microphones such
as the Shure VP88 or others available today). The center
channel can be used alone as a mono source or in combination with the bi-directional channel to produce a stereo signal.

Figure 1:
A Middle/Side
playback
system wired
from a stereo
master (amplifiers omitted).
Note how
polarity is
reversed in
the "L"
speaker to
avoid collapsing the stereo
image into a
center
monoral
image.

WIT

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MIXER

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Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1999