Transmission Lines* 



By H. A. AFFEL, R. W. CHESNUT and R. H. MILLS 



Describing methods whereby high quality sound reproduction in auditory 

 perspective can be accomplished over long distances, this discussion centers 

 largely upon a description of the exact technique employed in providing 

 communication transmission circuits for the Philadelphia-Washington dem- 

 onstration. Problems that might be involved in carrying out such trans- 

 mission on a more widespread scale also are touched upon. 



MICROPHONES have been described that will pick up without 

 noticeable distortion all the sounds given forth by a symphony 

 orchestra. Loud speakers and amplifiers also have been described 

 that will accurately reproduce this highest quality music in its full 

 range of tone quality and volume. Therefore, the situation obviously 

 requires connecting transmission paths so perfect in their character- 

 istics that reproduction 100 or 200 miles away may not suffer in com- 

 parison with reproduction which may be only 100 or 200 feet from the 

 source of music. 



There are several respects in which a long line circuit possibly may 

 distort the speech or music passed over it, unless considerable effort 

 is expended to overcome these tendencies. For example, there may 

 be frequency-amplitude distortion; i.e., all the notes and overtones 

 may not be transmitted with the proper relative volumes. Similarly 

 there may be phase or delay distortion, the different frequencies may 

 not arrive at the receiving end of the line circuit in the same time 

 relationships in which they originated. A line circuit is subject also 

 to possible inductive disturbances from other communication circuits 

 ("crosstalk"), or from power or miscellaneous circuits which cause 

 "noise" at the receiving terminal. If the circuit contains amplifiers, 

 transformers, and inductances having magnetic cores, it is subject to 

 possible nonlinearity effects; i.e., the current at the receiving end of 

 the line may not follow exactly the amplitude variations of the current 

 applied to the transmitting end or, what is more important, spurious 

 intermodulation frequencies may be generated within the transmission 

 circuit and mar the purity of the musical tones. The problem of 

 reproduction in auditory perspective, using two or three paralleling 



* Fifth paper In the Symposium on Wire Transuiission of Symphonic Music 



and Its Reproduction in Auditory Perspective. Presented at Winter Convention of 



A. I. E. E., New York City, Jan. 23-26, 1934. Published In Electrical Engineering, 

 January, 1934. 



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