Theatre Design & Technology - Winter 1999 - 17

Even with an UC/R speaker configuration many deSigns
Middle/Side speaker system equal levels of this virtual
center information are sent to the middle and botb side
positions. Depending upon the location of the listener in
the audience, this technique produces a stereo image between the middle speaker and the side speaker having
the sanle polarity as the center speaker and can produce
a strange lack of image on the side where the middle
speaker and side speaker have reversed polarities.
The aural image of stereo recordings played back
on a Middle/Side system Ct1l1 be poorly defined. When playing stereo recordings WiOl a strong center image, such as
those with a principal vocalist, over an MIS system the
center speaker can disappear completely as a perceptible sound source. Willie the position of the vocalist's
voice in the soundstage is not clearly focused (compared
to stereo playback) it is not unpleasant. The supporting
instruments are similarly vague in their positions.
Stereo recordings that have a clearly focused image
on the side cause greater problems for Middle/Side playback. Arecording of an orchestra in which the violins are
massed on the left illustrates the problem. If the left
channel is sent to the center speaker, the violin section
will seem rather small and in Ole middle of the orchestra.
If the left stereo channel is sent to the side speakers the
violin section will seem to be everywhere and nowhere at
once.
To compensate for these problems, some sound designers use software products from Arboretum Systems
(HyperEngine, Hyperprism, etc., most of which are available for both the PC and Mac computer platforms) to
transform traditionally recorded stereo recordings into
true MIS recordings, which can be played back from traditional two track playback media such as OAT or compact disc. Whether the sound designer chooses to
compensate for these problems or not, the MIS loudspeaker teclll1ique is preferable to standard stereo configurations which fail to provide a center aural image for
most audience members.
Middle/Side sound designs are most effective when
sound designers generate original audio recordings.
They may use an MIS microphone to record acoustic instruments and sounds or mix down their recordings to
take advantage of basic MIS principles. These principles
include recording the primary sound on a channel which
eventually becomes the middle (or center) position, and
anlbient or complementary information which evennlaUy
becomes the side channels. Theatrical playback then
generates a clearly localized image from the center and
more omnidirectional environmental sound from the
side speakers to complement the center image.
For exanlple, try recording the center channel-a
voice or solo instrument-using a close microphone,

stili use stereo playback because most prerecorded
sound is In that format.
and use a more distant mic for the side channels. (You
will want to use digital editing or delay devices to correct
phase canceUations produced by operating mics at various distances from a sound source.) Or, try making a
mono recording for the center channel and run that
sound through a digital processor to create a rich ambiance through the side speaker(s).
Coordinating liddle/Side and
Loudspeaker Panning at
- - the Mixer Outputs
One other limitation of MIS playback is its inability to
provide a fuU stage pan from left to right. An MIS system
only allows panning from the sides toward the center or
from the center toward the sides. Both operations give
each side of the house an opposite perception of which
direction the movement is occurring. UC/R mLxers with
individual sends to the UR and Center masters allow
sending some cues in stereo to the left and right masters
and filling in WiOl the Center if necessary. Tlus will often
provide the best imaging from a standard stereo recording which does not require a strong center image. This
standard stereo mode allows panning completely across
the stage when needed. Some UC/R mixers allow panning from side to side through the center channel. (The
DDA CS-3 and CS-8 mentioned earlier offer panning, as
does the Crest X-8. Panning is also available on more expensive mixers such as the Soundcraft Series 5 and the
Crest CT-X.) Tlus is much more effective than a simple
left to right pan especially on a wide stage wluch nught
otherwise have a center depression in Ole middle of the
sound field. Remember however, that UC/R mixers require a separate anlp channel for each speaker position.
The UC/R mixer also allows Middle/Side playback
by sending one channel of a stereo or MIS source to the
center speaker and the other to both the left and right
speakers (which have one speaker groups polarity reversed). Tilis might require building a polarity switching
box for the outputs (see fig. 2), as most mixers do not
provide this feature on outputs, only inpuls. Also, remember that this technique only works for balanced inputs/OUlputs (or requires isolation transformers for
unbalanced outputs).
I think that the mixer adjustments are gelling a little
tricky at this level and prefer to choose one mode or the
TZl&T

WIN

1 9

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