Twinplex and twinmode radiotelegraph systems

Detailed description of a system for converting existing single-channel frequency-shift radio circuits to two-channel nonsynchronous Morse, printer, or mixed-code operation or, alternatively, for single-channel transmission of 3-element cable code. In the latter case, the system has found an important application in the direct connection of radio circuits to ocean cable systems. The system is based on the combination of two 2-element mark-space channels to, form on the frequency scale a single 4-element channel in which each of 4 frequencies represents one of 4 possible mark-space combinations. At present, a separation of 400 c/s is used between the different frequencies, so that the total, bandwidth, including keying sidebands, is 1700 c/s with 60-word/min printers keying on both channels. Conversion to twinplex operation is effected by the addition of one small combining unit at the transmitting station and three conversion units at the receiving station. An extension of the twinplex system, termed twinmode, provides 4-channel operation for a radio transmitter equipped with a high-level amplitude modulator. Standard twinplex equipment is used, with the addition of an audio-shift keying unit which is required to translate the d.c. output pulses from the combiner into any one of 4 audio-frequencies for keying on the subcarrier side. Recently the radio circuit from New York to Lima, Peru, was changed from twinplex to twinmode operation and now provides for the former 3-element cable-code channel for connecting to the cable chain down the west coast of South America to Valparaiso, across the Andes to Buenos Aires and thence to Rio de Janeiro, and also provides two 60-word/mm printer circuits. Possible future developments are discussed.