290 BELL SYSTEM TECHNICAL JOURNAL 



Noise 



In setting up these circuits various tests, including measurements of 

 noise currents picked up by the conductors to be employed, were made 

 prior to the actual installation of the apparatus. It was discovered 

 that on the cable circuits north of Baltimore these pairs were picking 

 up sufficient noise even at the higher frequencies to constitute a 

 possible limitation in the volume range that might be delivered. This 

 noise was generated chiefly as a by-product of relay and other similar 

 operations w'ithin the Baltimore office and was propagated over the 

 longitudinal circuits of various pairs in the cable from which, by in- 

 duction, it entered the special selected pairs. As a remedial measure, 

 longitudinally acting choke coils applied to all but the specially 

 selected pairs in the cable greatly reduced the noise. Shielding and 

 physical separation were employed in the Baltimore office to prevent 

 induction between the repeaters and the connection to the main 

 cable. If it is desired to use existing cables for carrier transmission, 

 particularly for such high grade transmission circuits, it seems likely 

 that filtering arrangements of this kind, or other precautions, generally 

 will be required. 



Carrier Apparatus 



The carrier system employed may be characterized briefly as single- 

 sideband carrier-suppressed, with perfectly synchronized carrier fre- 

 quencies of 40,000 c.p.s. Most present-day commercial telephone 

 carrier systems are of the single-sideband carrier-suppressed type. 

 Suppressing one sideband saves frequency space and suppressing the 

 carrier reduces the load on the line amplifiers or repeaters. Ordinarily 

 the exact synchronization of the carrier frequencies at the sending and 

 receiving ends is not required for message telephone service. 



Obtaining a single sideband after modulation commonly is carried 

 out by providing band filters which transmit the desired sideband and 

 suppress the unwanted sideband. For the requirements of message 

 telephone transmission this does not impose severe requirements in 

 the design of filters because audio frequencies less than about 100-200 

 c.p.s. ordinarily are not transmitted, in which case, if the filter in 

 suppressing the unwanted sideband tends to cut off the lower fre- 

 quencies of the desired sideband, it is not important. 



For the requirements of this new high quality system, however, 

 where it was desired to transmit all frequencies to at least as low as 

 40 c.p.s., the problem was considerably more difficult. Two alter- 

 natives presented themselves in the design of the required filters. 

 The first consisted in attempting to provide the required selectivity 

 in the filters themselves, perhaps supplementing the actions of indue- 



