Theatre Design & Technology - Winter 1979 - 19

vances and techniques for circumventing the need for the inductor are on the horizon. This would lead to a substantial reduction in size and weight of the dimmer.
It now appears that the technology exists to make distributed dimming feasible, but is such a system desirable? This
question is extremely difficult to answer reliably in an abstract
discussion; a more effective way to answer the desirability
question is to build up a system and try it in actual situations.
The aim of this work was to build a prototype with two goals
in mind: 1) To demonstrate the feasibility of distributed dimming, and 2) to explore the desirability of such a system. The
work reported on here covers all the efforts in the area of
communicating the control information between control console and the remote dimmer, together with about half of the efforts in the area of remote dimmer design. The remaining efforts in the laller area are still in progress and will be reported
at a future date.

Transmission Channel Considerations
An all-digital, fully multiplexed system was chosen as the
basis for communicating the control information. Digital transmission was chosen over analog because it provided lower
cost, higher accuracy, greater error immunity, and easier implementation in terms of a fully multiplexed format.
Multiplexing is a technique that enables the control information for more than one dimmer to be placed on a single pair
of wires. (For a detailed introduction to multiplexing see note
3.) Full multiplexing, in which all the information for all dimmers is placed on a single pair of wires, was chosen because
it offers the user complete freedom in allocating the number of
dimmers at various locations throughout the theatre in a manner that is best for each show. Also, full multiplexing makes it
feasible to consider wireless transmission.
The modes that were considered for wireless transmission
are shown in Figure 1. For comparison, a hypothetical "ideal"
mode, and the hardwire mode are shown in the first two col-

Evaluation Criteria

Mythical
"ideal"

Hardwire

of the "ideal." In particular, a system offering the capability to
transmit the control information over either hardwire (HW) or
open-air infrared (lR) covers all the requirements.
The principal disadvantage of IR, of course, is that it is subject to interruptions. Judicious placement of the transmiller
can alleviate the majority of these breaks in transmission. Still,
some interruptions will occur. Hence the system must respond
gracefully should there be a disruption of the control signal.
After considering the alternatives, it was decided for a first try
that each dimmer, upon loss of its control signal, should retain
the level it had just prior to the signal loss, until the control signal returns.
Actually this feature permits the system to overcome a previous objection to multiplexed systems in general: loss of the
control signal when the cable comes unplugged. In previous
systems, all the lights would go out or full or to some random
selling. With the feature mentioned above, should the cable
become unplugged, all the light levels would stay just where
they were, without any flicker, until the control signal is restored.

Dimmer Implementation Considerations
Work on constructing a dimmer suitable for use in a distributed system was concentrated in two areas: performing as
many of the SCR support functions as possible via a single integrated circuit, and reducing the dimmer weight, principally
by replacing the inductor and various other transformers. As
work in the latter area is still underway, the focus here will be
on the results of work in the IC area.
In considering an IC implementation, both analog and digitallC technologies were considered, as both are quite well developed. Mixing the two areas on the same chip generally requires substantial sacrifices in performance or cost. This,
combined with the fact that the data arrives in a digital format,
made it desirable to implement the required circuitry digitally
as much as possible. An added benefit of digital circuitry was

Powerline
carriers
(PLe)

Radiated
Radio
Frequency

Acoustic

Open-air
Infrared
(lR)

Adequate Channel Capacity
Noise and Immunity to
Interference
Completeness of Coverage

yes
high

marginal
low

yes
moderate

no
?

yes
high

total

no
high
free

no
low
low

some dead
spots
yes
high
high

FCC Certification Required
Flexibility
Cost

some dead
spots
yes
moderate
high

line of
sight
no
high
moderate

Figure 1

no
?
?

Comparison among the various wireless modes; a hypothetical "ideal" mode and the hardwire mode are included for reference.

umns, respectively. The criteria for evaluating the suitability of
the various modes, some of which are quantitative and some
qualitative, are listed on the left. Of the four wireless methods,
acoustic was ruled out almost immediately because of insufficient bandwidth.
In surveying the remaining three wireless modes note that
no one mode matches the mythical "ideal." Power line carriers (PLC), while being convenient, suffer from the noise and
marginal bandwidth of the power lines. This laller area would
limit future expansion. Radiated RF does not suffer from bandwidth limitations per se but with interference from other users
in nearby bands. All three modes suffer to some extent from
lack of total coverage, although IR is more limited than PLC or
RF. On the other hand, both of these laller modes require FCC
certification which IR does not.
If a system is considered in which either of two modes are
available, however, it is possible to meet all the requirements

a dimmer design requiring no periodic recalibration. It further
enabled the testing of several novel features.
For example, a feature termed "flexible addressing" is offered. This feature takes advantage of the fact that in a fully
multiplexed system, all the control information for all the dimmers is on the one cable, or IR link, that reaches each dimmer. Flexible addressing allows the user to select the control
information to which a particular dimmer should respond. The
result is that flexible addressing accomplishes the task of having two or more instruments respond to the same control signal. Previously this function has been accomplished in either
of two ways, depending on where it was done: patching when
it was done centrally at the patch panel, and twoforing when it
was done locally at the instrument.
Although one of the goals of distributed dimming is to free
the user from the constraints of patching/twoforing, there will
remain cases-such as washes, cyclorama lights, etc.-

Charles H. Cox is on the staff of Lincoln Laboratory. His work on Light-Link was done as a graduate student at M.I.T.

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1979