WIRE TRANSMISSION SYSTEM FOR TELEVISION 629 



transmit a narrow range near 750 cycles, and the single frequency of 

 2125 cycles. These synchronizing frequencies are determined by 

 the speed of the motors, which was chosen so that the frequencies 

 would be suitable for transmission over two channels of a voice- 

 frequency carrier telegraph system,'* but later it was found more 

 convenient to use a separate telephone circuit. 



The circuits labeled "program" provided telephonic communication 

 between the observer at New York and the person being viewed at 

 Washington. A loud speaker was also connected to this circuit at 

 New York to transmit the voice to the audience when the large grid 

 receiving arrangement was employed. A special by-passing con- 

 nection was provided between the amplifiers at the terminals of the 

 circuit so that speech from the local microphone could be heard as 

 well as speech from the distant city. 



The order circuit was for the purpose of providing communication 

 between the engineers operating the television apparatus. 



Line Measurements 



In order to determine that the circuits set up as outlined above were 

 satisfactory, their overall characteristics were measured. Certain 

 matters of interest in this work are noted below. 



Measurements of Efivelope Delay. In order to measure the envelope 

 delay to an accuracy comparable to the requirements for the lines, 

 it was necessary to develop special apparatus. Fig. 8 shows in 

 schematic form the circuits of the apparatus designed for this purpose. 

 The apparatus measures not the absolute envelope delay of a circuit, 

 but the relative delay of one circuit at any frequency from about 

 600 cycles to 20,000 cycles or more with respect to the delay on the 

 other circuit at a fixed frequency. 



The functioning of the apparatus may be briefly described as 

 follows: Simultaneously into each line there was transmitted a 

 carrier current, each carrier being modulated by 250-cycle current 

 from the same oscillator. The modulation was accomplished in push- 

 pull vacuum tube circuits so that the undesired products of modulation 

 were ehminated by balance. The carrier on the line under measure- 

 ment was adjusted to the frequency at which a measurement was 

 desired, and the carrier on the other circuit, used for reference, was 

 kept at a fixed frequency of 5100 cycles. 



At the receiving point identical circuits were provided for amplifying 



^"Voice-Frequency Carrier Telegraph System for Cables," B. P. Hamilton, 

 H. Nyquist, M. B. Long and W. A. Phelps, Journal A. I. E. E., Vol. XLIV, pages 

 213-218, March, 1925. 



