All In One
Controller and Sensor Combined
Over a number of years of working with STM-3 and STM-4 systems, I found myself relying repeatedly on distance sensor based instruments—generally in pods of 4—in my live performances. It made sense, then, to combine these sensors with a controller in a single unit, eliminating the need for flat cables to connect the controller to the sensors, and making the setup more compact and portable.
This new approach resulted in the creation of the STM-5 series instruments Mira and Ute (pronounced oo•tuh), that have become a staple in my live performances, as well as those of my longtime collaborator Pamela Z and other performing artists.
Ute features a pair of ultrasonic distance sensors, mounted for horizontal sensing in two directions, and operating at a frequency of 40 kHz, far beyond the range of human hearing. Each sensor has two round elements, each with a grille on front, one of which functions as a transmitter (speaker), and the other as a receiver (microphone). Speakers and microphones are examples of sound transducers that operate according to similar principles such that, in a crude manner, they can be used interchangeably. In this case, the identical transducers function quite acceptably in both roles. The sensors work by sending out a short pulse of high frequency sound from one element, and then monitoring the time it takes for the pulse to be reflected—in this case by the performers hand—and received by its twin.
The software in the microprocessor first instructs the sensor to send a pulse of a short duration (10 µSec). When the reflected pulse is received, the sensor informs the microprocessor, which has been keeping track of the time elapsed since the pulse was sent. The elapsed time is then used to calculate the distance of the reflecting surface, based on the known speed of sound (343 m/s at 20° C). The distance is then encoded in a MIDI message and transmitted to a host computer (or other MIDI device). This procedure is repeated approximately 30 times per second for each sensor (this is not terribly fast, but a rate of 30 Hz has been found to be an acceptable lower limit for useful application to live performance).
Among the sensors and devices that proved most useful in the early STM designs were photocell (photo resistors) light sensors, as shown in the first two images here. In the right lighting conditions—best achieved with an incandescent desk lamp placed above the sensor array—these inexpensive and easily mounted sensors remain unmatched in STM applications, providing a smooth gradient response as shadows are made to increasingly reduce the amount of incident light. In practice, however, these conditions often prove difficult to establish and maintain as lighting conditions in live performance settings fall increasingly outside the control of the performer, with unpredictable and sometimes disastrous results.
An more reliable alternative turned out to be the use of light sensors that operate in a band outside the range of visible ambient light, in the near infrared (NIR) spectrum (there is some overlap since many visible light sources also emit energy in the NIR range; but with proper shielding and lensing, sensor interference from these sources is virtually eliminated). Although these sensors do not possess the smooth response characteristic of photocells, the gain in noise immunity to visible light sources more than offsets the loss in response.
The active infrared sensors used in Mira (last image) function in a manner similar to that of Ute: a signal is sent out—in this case an invisible beam of focused light—and reflected by an object or surface in its path onto the sensor’s light sensing element. The distance to the object, represented by an output voltage in the 0-5V range, is determined by the amount of light reflected, rather than the time it takes to return (time of flight) since the speed of light imposes much greater constraints on speed and accuracy of any device making such a measurement. As with Ute, there is also a limit to the rate at which new readings can be acquired from the sensor; but in the case of the sensors used in Mira, the rate is a very serviceable 60 Hz, twice that of Ute (photocells are still the best performer here, with accurate sampling at rates of 300 Hz or more).
Mira and Ute Live
Suite for Solo Voice & Electronics, Pamela Z (2017)
Pamela Z performs a suite of solo works for voice and electronics to open a concert by Kaleidoscope Chamber Orchestra (in partnership with LA Philharmonic’s Noon to Midnight Festival) on Sunday, November 19th, 2017, in Santa Monica, CA.
Pamela is processing her voice and manipulating sound using custom Max/MSP software and gesture-controlled MIDI instruments designed by Donald Swearingen.