How Electromechanical Television Worked

(Diagram.)

How Electromechanical TV Worked

The Sending Apparatus

The transmitter employed a rapidly spinning disc with 50 small holes arranged in a spiral to focus a bright light source into a narrow beam, which scanned across the subject 16 times per second. A photoelectric cell received the light reflected off the subject and then converted it into corresponding electrical signals of varying intensity. AT&T then transmitted the waves over telephone wires (from Washington) or by radio (from Whippany, N.J.) to New York. AT&T transmitted a synchronizing signal over a separate circuit. This signal assured that the spinning discs of the sending and receiving equipment would operate in unison. The audio portion of the transmission traveled as a third signal on a conventional telephone line.

The Receiving Apparatus

The electrical signal went to a neon globe tube, which produced a ruddy orange light that changed in brightness rapidly with the fluctuation of the electrical signal. Between the glow tube and the viewer, a rapidly spinning disc identical to that in the sending apparatus directed the light as a narrow beam to a succession of points in a pattern repeating 16 times per second. As a result of the persistence of human vision, this succession of points appeared to the viewer as a clear, 50-line, moving image, approximately 2 inches by 2½ inches. (Today, American TV resolution is at 525 lines of resolution, 30 times per second.)

A second, larger receiver used a long neon tube bent back on itself 50 times and a grid of 2,500 pairs of electrodes to deliver a 2 foot-square picture to the audience. Its image quality was not nearly as satisfactory as the smaller receiver.