The J-carrier (or Type J) system was developed by Bell Telephone Laboratories in the late 1930s to increase the capacity of AT&T's open-wire long-distance transmission plant. Using analog frequency-division multiplexing, J-carrier combined twelve one-way 4 KHz voice-frequency (VF) channels on a single pair of wires. The same pair could simultaneously carry three channels multiplexed via the earlier C-carrier system, plus one non-multiplexed VF channel on the physical pair (used as an order wire), for a total of sixteen one-way voice circuits on each pair.
Because of crosstalk at 140 KHz, a J-carrier open-wire line consisted of only 16 pairs on four cross arms using an elaborate line transposition plan, with rather precise pole spacing and line tensioning. A line spectrum diagram, from an article titled "A Twelve-Channel Carrier Telephone System for Open-Wire Lines" in a 1938 issue of the Bell System Technical Journal, shows how VF, C-carrier, and J-carrier were combined, yielding a total route capacity of 256 circuits.
The twelve VF channels (called a "group") were translated to/from frequency bands of 36 to 84 KHz for one direction of transmission, and 92 to 140 KHz for the opposite direction. These bands were dictated by cross-talk and line attenuation, 140 KHz being considered the upper limit of economically equalizable long-haul open-wire bandwidth. Modulation was accomplished in two steps because the line frequency overlapped the basic group frequency. The 60 to 108 KHz group frequency band was dictated by available quartz-crystal filter technology. The 340 KHz shift allowed the use of much simpler bandpass filters. A J-Carrier modulation diagram, from the same Bell System Technical Journal article, shows how the VF channels were translated to/from the line frequencies.
The Type A channel bank was developed to form the twelve-channel 60-108 KHz standard group to meet the differing requirements of the J and K (wire-pair cable) carrier systems, and in anticipation of coaxial and other broadband facilities.
Created on September 10, 2005 at 22:19 by Albert LaFrance