The Bell System Technical Journal 



Vol. XIX April, 1940 No. 2 



Advances in Carrier Telegraph Transmission 



By A. L. MATTE 



Introduction 



IN the comparatively short period which has elapsed since its com- 

 mercial introduction in a practical form, the voice-frequency carrier 

 method of operating telegraph has risen to a position of preeminence 

 and is becoming the outstanding means for providing telegraph facili- 

 ties over main toll routes. 



Since the original installation, improvements have been made in the 

 carrier supply, level-compensating devices, maintenance facilities, and 

 in numerous other specific physical parts of the system. Operating 

 speeds have also gone up, and the number of telegraph channels per 

 telephone circuit has been increased. Furthermore, this system, 

 originally designed for cable circuits operating at voice frequencies, 

 has been applied to open-wire lines and adapted by remodulation to 

 other frequency ranges, in particular to those occupied by existing 

 carrier-telephone systems. Some of the chief advances, however, 

 have been of a more intangible nature, not the least of these being the 

 clearer insight which experience and extended tests have given into 

 the possibilities and limitations of carrier-telegraph systems with 

 respect to interference and other causes of signal distortion. 



As a result of the success attained by the carrier-telegraph system 

 for open-wire lines,'- ^ which had been in commercial service in the 

 Bell System since 1918, this company's engineers turned their attention 

 to the adaptation to cable circuits of the carrier method of transmission 

 for telegraph purposes. Following this work and extensive field trials, 

 the voice-frequency carrier-telegraph system went into commercial 

 use in the Bell System in 1923.^ The initial installation consisted of 

 ten two-way or duplex channels between New York and Pittsburgh, 

 giving an aggregate channel mileage of 3800 miles (6120 km.) including 

 both directions of transmission. Since then, the application of this 



161 



