Wasabi Photopickup

See also: Sound Cameras

We are surrounded by light. Our eyes detect it and provide us detailed spatial information about color, brightness, and motion. We are limited though by the physiological effect of persistence of vision, which limits our perception of rapidly changing light intensity to a low bandwidth; for instance, theatrical film runs at 24 frames per second and gives the impression of smooth motion. Compare this to the bandwidth our ears can receive; 20Hz - 20,000Hz (Hz = hertz, cycles per second) being the quoted figure for normal human hearing. In a way, our ears pick up where our eyes leave off; visual events faster than about 20Hz are perceived as continuous, and audible waves below 20Hz are not perceived at all.

Modern technology involves much modulated light at imperceptibly high frequencies; CRT monitors, infrared remote controls, fiber optic communications, laser barcode scanners, etc. are deliberately engineered sources of modulated light. Household lighting also presents a great deal of modulated light at 60Hz and its harmonics. If our eyes were to communicate this lightwave information intact to our brains as sound, we would hear a very strange electronic hum and buzz around us almost all the time.

Modulated light has seen some use in a musical context. At one time, analog audio was encoded onto celluloid movie film as a stripe of varying density beside the image frames. A light shining through this stripe is modulated as the film passes, and the modulated light was picked up by a photosensitive device. From that point, the sound waves return to the electronic realm and are amplified and heard through speakers.

There is also a historical keyboard instrument called the optigan (optical organ) that produced melodic and percussive sounds via a similar technique to film sound reproduction, although the optigan was based on a spinning disc rather than a strip of film. The optigan played discs containing many concentric tracks of audio waveforms encoded as dark/light patterning using a photocell-based light pickup system. It can be considered along side the mellotron as a pioneering step towards sample-based music.

I decided to build a simple lightwave reciever to explore the world of modulated light around me. It is based on a cheap phototransistor that is sensitive to UV, visible, and near-IR light sources; I assembled it inside a small metal can that I purchased some wasabi powder in:

The Wasabi Photopickup plugs into a guitar amp and amplifies modulated light waves. I am not sure what its upper frequency limit is, but it does seem to gather high frequency information well. For instance, pointing it at a computer CRT yields an interesting buzzing tone that varies depending on what the screen is displaying. Incandescent lamps are a strong source of 60Hz hum and harmonics. An infrared remote control gives wild bursts of harsh noise:

infrared remote modulations (TV-B-Gone)

I’m working on a spinning optical disc system to generate musically modulated light, and the Wasabi Photopickup is a key component.