P.T.M.

A new system of radio transmission, called Pulse Time Modulation, has been announced by International Telephone and Telegraph Corp. Outstanding advantage of P.T.M.: its ability to send a number of voices--or musical programs, for that matter--over a radio channel which heretofore could handle only one.

In the diagram above, P.T.M. is sending 24 telephone conversations. The trick is done by allowing each conversation only part-time on the air. A cathode-ray tube acts as a multiple switch. Inside it, a scanning ray revolves like a clock hand, 8,000 times per second. Arranged like the numbers of the clock are 24 contacts, each connected with a different telephone. As the ray sweeps over a contact, it puts on the air a minuscule snatch of the voice passing through the telephone with which the contact is connected. When it moves to the next contact, the next telephone has its chance. To carry the voices the snatches (or "pulses") must be timed properly, which is how "Pulse Time Modulation" gets its name.

At the receiving end, the mingled snatches are sorted out by another special cathode-ray tube and distributed to 24 listening telephones. Each set of one-millionth-of-a-second pulses blends into a smooth, clear voice, for the pulses come so close together that the ear cannot note the silences between them.

The new system works only on very short waves, which cannot be received much beyond the horizon. But this does not worry its developer, which has great hopes for P.T.M. Relayed cross-country by parabolic wave reflectors, it may replace expensive and troublesome long-distance telephone wires.

Adapted to broadcasting, it may solve the problem of how to accommodate all the crooners and soap operas which clamor to get on the air. A single P.T.M. transmitter, for instance, atop the Empire State Building, might serve as a joint outlet for most of the studios in New York City. Listeners could concentrate on a single program, or, if they wanted to take the punishment, connect their receiving set (plus a little additional equipment) with loudspeakers in different rooms, each playing a different program.

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