Carrier for Telegraph on Open Wire
16-Channel Western Electric “O-Carrier” bay, c. 1980s.
I’m going to discriminate between carrier for voice and telegraph for some very obvious reasons. Another section will proceed to explain in considerable detail, the intricacies of carrier for voice circuits. We have done that already for power line carrier as well, since the equipment and applications were fairly different.
In the world of open wire telegraph, two types of carrier systems were developed:
High frequency carrier telegraphy: take one open wire pair and you could impress ten (10!) d.c. telegraph channels on that single pair. What happened was the wire center, c. o. or would dispense high frequency impulses in the same order, length and dynamic as the comparative telegraph impulses. Each channel enlisted filters to avoid conflict with other signals on the same pair. This was the system used by AT&T under the moniker of “C-Carrier Systems.”
Voice frequency carrier telegraphy: take a carrier system of your choice for open wire; in our case we can use “J-Carrier” with its twelve channel system. Sending and receiving filters remove interference and separate the channels. In the carrier system, an alternator composes the various frequency allotments. By other methods, generator windings are momentarily shorted for the “spacing signal” and the “marking signal” are defined by other carrier impulses. Back in 1938, these carrier impulses were modified by vacuum tube electronics. The telegraph equipment working on carrier was no different. On a “C-Carrier” system, three telegraph carrier systems can ride along the same pair as other voice frequency communications. When combined, astonishingly, 36 (!) telegraph channels can be delivered over one pair of wires along with the low frequency telegraph messages. We’ll talk about “duplex” later, but if such a “duplex” system is also operated on this pair then 72 (!!) telegraph messages can be delivered simultaneously over a C-Carrier pair!