PRIVATE LINE DATA TRANSMISSION 1463 



475 725 1125 1875 



FREQUENCY IM CYCLES PER SECOND 



Fig. 2 — Double sideband signal with auxiliary start channel. 



Marking consists of maximum carrier amplitude and spacing is zero 

 carrier amplitude. The pulses are shaped both at the transmitting end 

 and at the receiver before sampling. 



The earlier prototype system as described by Horton and Vaughan 

 was tested over a variety of telephone facilities (not including Nl 

 carrier) running up to over 12,000 miles in length and found to be quite 

 rugged. For reasons which are to be discussed later, the use of these 

 systems over compandored circuits presents certain extra transmission 

 problems regarding noise and level changes. 



2.4 Voice Frequency (VF) Carrier Telegraph 



The opposite extreme to the use of the telephone facility as a single 

 band, to carry short duration pulses, is to subdivide the band into a 

 large number of subchannels, each using longer duration pulses. As 

 already noted, this has advantages against impulse noise, and also 

 delay distortion. 



There is available for this use the VF telegraph system.^ In an AIM 

 form (40C1) this subdivides the space into 18 telegraph channels, which 

 can each carry 100 words per minute (or 74 bits per second). A fre- 

 (luency shift form (48A1) is available, to give 17 channels, each to carr}^ 

 74 bits per second. 



The 18 channels use a band of 200 to 3,200 cycles in the telephone 

 facility, and the lowest channel permits only a lower word speed. The 

 17 channels occupy the band from 350 to 3,200 cycles. 



It has been mentioned that the telephone channels which provide 

 the most serious problem for data transmission are those using compan- 

 dors. The untreated Nl carrier is a principal example of such. Further, 

 the type of circuits which use compandors are apt to be placed in plant 

 which is relatively exposed to impulsive noise. 



The principal interest in the telegraph channels, therefore, is to exam- 



