CONTROL FORK:
Of course, the sending machine and the receiving machines must run in exact synchronism. This synchronous control of the sending and receiving motors is maintained by the vibration of a rather heavy fork at each station, and adjusted to beat together, with such slight automatic correction by radio as may be required to keep all receiving forks in step with the fork of the station which at the moment is sending. It is a very simple and dependable mechanism, by which any number of motors, of any size, separated by any distance, can be made to run in synchronism.
RADIO MOTOR:
Another scheme of the rotary type, perhaps even better adapted to the distant control of large motors, is a small synchronous radio motor driven by power carried by radio from the broadcasting station to the receiving stations. It is, of course, rotated partly by radio power from the distant station, and partly by local current, just as a loudspeaker is operated. These small motors, rotating in synchronism with the motor at the sending station, control the rotation of a larger motor in each receiving camera, and so all stations keep in step.
STROBOSCOPIC LAMP:
Of course, it would be fatal if it were necessary to wait until the picture was developed before it could be discovered that the receiving camera was getting out of control. So a special “neon” lamp is located to shine on a revolving marker on the motor shaft of the receiving instrument, and flashed by the incoming radio signals, which latter bear a definite relation to the rotation of the sending station motor.
SAME WAVE:
It should be noted that the same radio wave carries both the picture frequency which builds up the photograph and the synchronism frequency which controls the motors, and also that it lights the stroboscopic lamp.
MULTIPLE-SIGNAL RADIO:
A further advance step was made when an audible message was added to the same radio wave which carried the picture. This is done by modulating the carrier wave to give audibility, while interrupting the same carrier wave at a frequency far above the audible range, say, two hundred thousand cycles, to make our picture.