166 BELL SYSTEM TECHNICAL JOURNAL 



At a later date the extensive introduction of H44 circuits, with their 

 relatively high cut-off, was thought to make it desirable to consider a 

 further extension of the frequency band utilized by the telegraph. 

 However, in view of the fact that the then existing state of the art of 

 filter design made it impractical to produce economical filters of the 

 required narrow band-width but having the desired high mid-band 

 frequency, it was decided to develop a carrier-telegraph system suitable 

 for use between dense traffic centers by superposing two standard 

 12-channel systems over the same cable pair. This was accomplished 

 by causing the various signaling frequencies of one system to modulate 

 a single secondary carrier, thereby transposing all the frequencies of 

 this voice-frequency system to a frequency range above that of a 

 normal voice-frequency system operating over the same cable pair. 



The line circuit used with this double system was required to trans- 

 mit a range of frequencies from about 350 cycles to 4400 cycles, and 

 the stability within this range had to be such as not to cause excessive 

 bias in any channel with the regulating methods available at that time. 

 In order to secure this result, it was necessary to change the transmis- 

 sion characteristic of all repeaters from that used for ordinary four-wire 

 telephone or voice-frequency telegraph transmission and, furthermore, 

 to modify somewhat the regulating repeaters in order to maintain the 

 desired transmission characteristics with changes in temperature. 



No changes of any kind were required in the voice-frequency tele- 

 graph terminals. The channel frequencies on the line were arranged 

 to extend uninterruptedly at 170-cycle Intervals from 425 cycles to 

 4335 cycles. 



This arrangement was called the "double-modulation" system, 

 because operation of two voice-frequency carrier telegraph systems 

 over the same circuit was realized by causing all the channel frequencies 

 of one of these two systems to pass through a common modulator, 

 where a second modulation took place; the individual frequencies of 

 each channel being already considered as having been modulated by 

 the sending relays. A single secondary carrier-frequency was used 

 which was common to all the channels. The allocation of frequencies, 

 which was identical for both directions of transmission, will be readily 

 understood by referring to Fig. 2B. The general principle of operation 

 is illustrated in Fig. 2A, which shows transmission from west to east, 

 it being understood that the arrangement from east to west is identical. 

 The voice-frequency system denoted as No. 2 will be seen not to differ 

 in any way from the earlier arrangement except that signals from all 

 twelve channels traverse low-pass grouping filters at the sending and 

 receiving terminals. In the case of voice-frequency system No. 1, 



