Lines carrying both Speech and Teleprinter signals
A very clever system known as Speech plus Duplex Teleprinter, S+D or S+DX was used in the UK Civil Defence network to carry duplex teleprinter signals over the same wires as duplex verbal communication. This system was not exclusive to Civil Defence, but was used in the public network too. A complete description may be found on Page 191 of TELEGRAPHY by R N RENTON 1976, ISBN 0 273 40846 1
Teleprinter Background
A teleprinter of the cold war era was a large Electro-mechanical device (A Creed 7B or Creed 444) resembling an electric typewriter. There was a large public network of machines in the UK, which due to their high cost was almost exclusively for business use. The UK public network was dial up in a similar way to the telephone network, but the two networks were not connected. Some large companies had the own independent network, which more than likely used Post Office private circuits as prior to the privatisation of BT there were few private networks.
Teleprinters were favoured as a communications medium as they produced a hard copy, which was ideal for business and of course defence purposes. The data rate, which was considered fast at the time, was 50 baud, with a 5 bit code plus start and stop bits. The signalling was by sending 80 Volts Positive and 80 Volts Negative along the two wires. Compatible machines that read punch paper tape and sent it contents down the line or produced punched paper tape from the incoming signals were used at relay centres, such as those used at ROC Group or Sector headquarters.
Telephony Background
Human speech consists of a spectrum of audio frequencies from a few Hertz to many tens of Kilohertz. However the human brain can still decipher the words even when a large part of this spectrum is missing. The telephone network makes use of this by human ability by restricting the bandwidth from 300 Hertz to 3400 Hertz. The Hertz unit is the modern name for what was previously known a cycles per second and describes the number of times the vocal chords vibrate.
In the telephone, the microphone converts the sound waves into an electric signal. At the other end the earpiece converts the electrical signal back to sound waves. A single pair of wires carries the signals in both directions from the telephone. If both people speak at the same time they will be heard by each other, this is duplex operation.
Over long distances it is usual to use 4 wire circuit to carry the telephony circuit. Two wires carry the speech from A to B and another two from B to A. Avoiding a long technical explanation, this is done to make the amplification easier. At each end a special device known as a hybrid coil converts from a 4 wire to a 2 wire circuit. Where telephony is carried over radio circuits, as was the case between ROC headquarters, the four wires were connected to a radio transmitter and receiver working on separate frequencies operating in duplex mode.
Speech plus Duplex Explained
The drawing above shows a very much simplified diagram, please refer to the text book "TELEGRAPHY" for a more detailed description. The telephone circuit is filtered to withdraw the frequencies in the range of 1600-2000 Hertz. This degrades the quality of the speech channel a little. The filter in the speech path prevents the modulated teleprinter signals being heard on the telephone circuit and prevents the speech affecting the teleprinter. The plus and minus 80 volt signals from the teleprinter are used to modulate a 1680 Hertz carrier in one direction and a 1860 Hertz carrier in the other. The modulation is frequency shift keying by 170 Hertz. At the other end, these are demodulated and converted back to 80 volt signals for the teleprinter.
Had you been in the position to listen into the circuit at the top thick pink arrow you would have heard the speech in the direction of A to B (half the conversation) with a high pitched warbling tone in the background.