The Audio Control of Laser Displays

VIDEO/LASER IV; Signal Routing in the Systems

A collaboration between The University of Iowa and The Adler Planetarium, Chicago, led to the construction of VIDEO/LASER IV. This six–beam system, based on the design of VIDEO/LASER III, was built on the Iowa campus during 1979–80 and installed under the dome of the Planetarium’s Sky Theatre in March 1980. It was used there daily to provide added visual interest to the sky shows and other educational presentations designed by the planetarium staff. VIDEO/LASER IV was commissioned in commemoration of the 50th anniversary year of The Adler Planetarium. (2005 update: the Adler system has since been dismantled.)

V/L Diagram

Fig. 5. VIDEO/LASER block diagram

Figure 5 is a block diagram of one color channel of VIDEO/LASER III. The audio frequency input signals may come from two–channel music of virtually any type, from electronic generators, or from natural sounds in the environment via microphones. These signals enter the system via the input matrix switches, which also determine the horizontal/vertical assignment of the input pair. It should be noted here that stereophonic audio reproduction bears striking similarities to the x–y design characteristics of these laser systems. Anyone familiar with Lissajous patterns displayed on an oscilloscope screen from two–channel audio signals will appreciate the potential applications of projected x–y laser imagery. After the signals have been selected at the input matrix, they are presented to the horizontal and vertical electronic drive circuitry and made available simultaneously for special forms of Z–axis modulation. Undesirable DC offsets in the input signals may be removed by switching in the blocking capacitors; otherwise the system is DC–coupled. The response of the overall scanning system is quite uniform from DC up to the resonant frequency of the scanners (2.2.–2.4 kHz), with useful response extending to above 3 kHz. This range is sufficiently wide to produce satisfactory x–y displays from most forms of music. The uniformity of response has been achieved in the design of the damping and equalization circuitry of the drive electronics. Following the input level controls, a set of switches determines either the scanning or the interference mode. The scanning (x–y) mode is the basic operating condition of the system, for which all design criteria have been optimized. However, another interesting vocabulary of visual effects can be made to operate from the input sound materials by using the interference mode, which takes advantage of the coherent, single–wavelength nature of laser light. The control systems for VIDEO/LASER III include remote switching for sets of solenoids which move various textured translucent materials in front of the scanners; the beams may be either stationary or deflected when passing through these materials. The amplitude detectors cause the resulting interference patterns to evolve at sub–audio rates, in response to the dynamics of the music or other audio information. The polarity switches provide the option of 180–degree phase reversal, inverting the horizontal or vertical coordinates of the scanned imagery. The polarity reversal functions, in combination with the horizontal/vertical assignment possibilities, permit the six scanned displays to be oriented in numerous configurations, including several symmetrical arrangements. The position controls function exactly like the horizontal and vertical centering controls on an oscilloscope. The DC–coupled input and power amplifiers incorporate frequency–shaping and damping characteristics to provide control over the mechanical properties of the scanning transducers. To prevent overdriving and possible damage to the scanners, protection circuits limit the output of the power amplifiers (nominal maximum output, approximately 5 watts).

The z–axis or intensity modulation circuitry is very similar to the scanner channels. Chopper signals may be derived from either the X or Y scanner signals or may be obtained from other sources. The AC/DC switches and input level controls perform the same functions as in the scanner electronics. The Z–axis polarity reversal switches determine whether the chopper aperture will open, or will close, from an input signal of given polarity. The “brightness” controls provide a continuously variable opening or closing of the chopper apertures and determine the threshold levels of z–axis modulation. The resonant frequency of the choppers is approximately 700 Hz (useful response to 1 kHz), again controlled by frequency shaping and damping circuitry. While more rugged than the scanners, they are also protected from being overdriven. The optional use of an automatic beam blanker prevents a stationary (undeflected) beam from exiting the system. In realizing these designs and applying them in public performances, I was invaluably assisted not only by Carson Jeffries and David Tudor, but also by Stephen Julstrom, Thomas Mintner, Peter Lewis, Joel Carl, Peter Elsea, Tim Barrett, and Thomas Henry.