American television directory (1946)

PTM A NEW TELEVISION TRANSMISSION SYSTEM Pulse Time Modulation introduces the interestng pos¬ sibility to television of broadcasting both sound and picture, and color as well, on a common channel. By H. H. BUTTNER _ President, Federal Telecommunication Laboratories T he lifting of wartime secrecy dis¬ closes a new system capable of trans¬ mitting simultaneously a number of telephone or telegraph channels on micro-wave frequencies. This system can also transmit simultaneously the sound and picture channels of a tele¬ vision program. At a time when the attention of the television industry shifts toward the higher radio frequencies, the disclosure of pulse-time modulation should be of considerable interest to those concerned with television. The new system known as “Multiplex Radio Pulse Time Modulation” holds vast and revolutionary potentialities for multiplex telephone and telegraph services, multiplex broadcasting and television with common picture and sound channel. This PTM system which was made public recently when its wartime secrecy was lifted, is the outgrowth of the pio¬ neering efforts of the International Tel¬ ephone & Telegraph system extending back to 1931 with the Dover-Calais 18 centimeter micro-wave link. This system is based on an original conception of E. M. Deloraine, president of Interna¬ tional Telecommunication Laboratories and Mr. Reeves of the London Labora¬ tory of IT&T and has been developed in the New York labs of IT&T under the direction of E. Labin and the super¬ vision of D. D. Grieg. Although commercial television in recent FCC proposals retains roughly its present position in the frequency spectrum, a new band at much higher frequencies from 480 to 920 megacycles has been allocated for experimentation in high definition, color and other tele¬ vision developments. This trend towards higher frequencies offers spectacular possibilities to PTM. These possibilities in the multi-channel telephone field have already been demonstrated and its ap¬ plication to multi-channel broadcasting and television is awaiting further re¬ finements before it is publicly dem¬ onstrated. A radio multi-channel tele¬ phone system has been demonstrated by IT&T on an experimental link equipped with repeaters and spanning a distance of 80 miles. Tests have been conducted over a 2-way circuit that originates and ends at the top floor of the 35-story IT&T building at 67 Broad Street, N.Y. From there the circuit is beamed to a repeater or relay station at Telegraph Hill near Hazlett, N. J., then to another repeater station at the site of the new Federal Telecommunication Laborato¬ ries at Nutley, N. J., and thence back to the building in N.Y. Since the repeater station boosts the strength of the microwave energy without causing distortion or cross-talk, the circuit could be 1000 or more miles long and still maintain a high quality of transmission. In such a system, the repeater stations should be installed at intervals of approximately 30 miles as the extremely high frequen¬ cy micro-wave energy, similar to light, requires direct line of vision for its transmission. These repeater stations are automatically operated. PTM receiving and transmitting antenna installation atop IT&T Building, New York. In the first public demonstration of the triangular network, 24 persons lo¬ cated in separate rooms of the N. Y. building conversed simultaneously over the micro-wave link. The 24-channel ar¬ rangement operates on the principles of time modulation and time selection. The pulse time method of modulation consists of emitting short pulses of high frequency radio energy. These pulses are of constant amplitude and at the same carrier frequency. Modulation is effected by transmitting a synchroniz¬ ing or “marker” pulse at fixed times and following this pulse with the “sig¬ nal” pulse at variable time intervals. The positon of the “signal” pulse at any given time depends on the modu¬ lation to be transmitted. At the receiving end, the time of arrival of the signal pulses with regard to the marker is reconverted into the corresponding amplitude of the modu¬ lating signal. In effect, the PTM system chops the signal to be transmitted into small bursts which occur at such a rate that the ear is unable to detect the interrup¬ tions. This is similar to a motion picture where the spectator watches a screen which is completely dark during a large portion of the time a film is being shown, and yet the eye does not detect the dark intervals. For its multi-channel operation, the system makes use of time selection as opposed to conventional frequency se¬ lection. In a 24-channel system, 25 pulses are transmitted successively in each cycle. The first one or marker is stationary in time, the other 24 corres¬ pond to each of the channels and their instantaneous position depends upon the signal transmitted on each channel at that particular time. The rate of repe¬ tition of this cycle of 25 impulses is 8000 per second. For reception, a special tube, the “cyclophon,” performs the separation and the demodulation of these 24 channels. In spite of the fact that the terminal equipment can handle 24 high quality telephone channels and is equipped for automatic telephone operation, the en¬ tire apparatus occupies a cabinet stand¬ ing only 8 feet high and a floor space 20 inches square. The radio frequency (Continued on page 123) 33