INFORMATION ABOUT ASSISTIVE LISTENING TECHNOLOGIES
On this page we will provide background information about selecting and installing assistive listening systems. This is a work in progress and not ready for posting yet.
For now, here is a basic diagram illustrating the operation of our model SC-186K infrared transmitter's operation.
Sound enters the microphone, is amplified and processed before being used to modulate the infrared emitter. The optical signal leaves the emitter and is picked up by the photodiode on the receiver which then amplifies the sound before sending it to the earpiece transducers.
This has been going on for a lot longer than most of us realize...
Experiments in the transmission of speech by means of light beams were first made by Professor Bell some time ago with an apparatus called the "Photophone. " The transmitter consisted of a plain mirror so arranged as to reflect the light upon a selenium cell in circuit with an ordinary receiver at the opposite station. The mirror served as a telephone diaphragm, a resonating chamber and mouthpiece being placed at the back. Speaking in the mouthpiece vibrated the mirror, the vibration altering the intensity of the beam of light. The changes in the light beam resulted in the selenium cell (acting with its well-known property of altering its electrical resistance under influence of light) setting up corresponding changes in the receiver circuit, and so producing vibrations in the receiver diaphragm like those communicated to the mirror of the transmitter. Professor Ruhmen, of Berlin, has improved somewhat on Bell's device, but the same principle is retained and the system is successfully used on warships of the German navy. It would seem as though we now have the germ of a means of inter-planetary communication.
Excerpted from: The New Idea Self-Instructor edited by Ferdinand Ellsworth Cary, A. M. (Monarch Book Company, Chicago & Philadelphia, 1904) p.51