Time-Frequency-Division Multiplexing

a method of transmitting electric telegraph signals over communications lines, in which a combination of frequency- and time-division multiplexing is used. The technique was proposed and demonstrated by a group of Soviet specialists under the direction of V. I. Kirsanov in the 1960’s. During the next ten years it came into wide use.

Time-frequency-division multiplexing is used mainly in the switching telegraph networks of teletypewriter exchange services and direct circuits that use start-stop telegraphs as terminal equipment. The channel-forming equipment contains devices for creating within one standard telegraph channel up to four grouped frequency-division-multiplexed channels, in each of which 12 individual channels are created by means of time-division multiplexing. The transmission rate is 50 bauds for each time-division-multiplexed channel and 600 bauds for each frequency-division-multiplexed channel.

Time-frequency-division multiplexing combines the advantages of the synchronous and start-stop telegraphy systems. Its advantages include high noise immunity; the efficient use of the frequency spectrum in a standard telegraph channel, especially for the creation of a large number of channel groups; the possibility of extracting a portion of the channels at intermediate points along a communications line; and the ability to correct for envelope delay distortions in discrete signals. The principal operating disadvantage of time-frequency-division multiplexing is the necessity of using start-stop telegraphs having a closely controlled telegraph speed.