A simplified description of the system for carrying both speech and telegraph signals over the same circuit. This was done to halve the quantity of circuits either landline or radio required thereby reducing the cost of provision. The cost of the additional electronics was outweighed by the saving on line plant or radio.
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 lines 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
Telegraph signals were generated by a teleprinter or tape reader and received on a tape perforator or teleprinter. Telegraph signals operated at 50 baud by sending 80 Volts Positive and 80 Volts Negative along the two wires. Each character consisted of 7.5 elements – one start bit, a 5 bit code, then 1.5 stop bits sent serially down the line.
The range of a directly connected teleprinter was limited by line resistance. To go beyond this limit and to use telegraph over radio circuits where there is no direct metallic path a system of converting the 80 volts signal to voice frequencies was developed. These systems are known as Voice Frequency Telegraph ( VFT ) for a single channel or Multi-Channel Voice Frequency Telegraph ( MCVFT) for usually 18 or 24 channels per circuit.
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 words even when a large part of this spectrum is missing. The telephone network makes use of this 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 telephone wires carries the speech signals in both directions simultaneously. Both people may speak at the same time, which is known as duplex operation.
Over long distances it is usual to use a 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 transformer converts from a 4 wire to a 2 wire circuit. The hybrid transformer prevents the received voice from being sent back towards the originator. Without the hybrid transformer the circuit would 'howl' in the same way as a person on stage with a microphone can get feedback from the auditorium speakers. Where telephony is carried over radio circuits two of the four wires are connected to a radio transmitter and two to the receiver. These work with separate frequencies operating in each direction (duplex mode) so the telephone user is unaware a radio link carries their call.
On a 4-Wire land line, the two wires taking the voice away from this point are designated TX and the pair receiving the voice from the other end are designated RX.
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 that are used for the telegraph signals. Although this degrades the quality of the speech channel most people won’t notice the difference. The filter stops 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 or message centre 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 +/− 30 Hertz either side of the channel centre frequency. At the other end these carriers 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 carrying the telegraph message.
As it isn't possible to send the 25Hz or -50volt DC. switchboard calling signal over the circuit this was converted into a tone of 500Hz modulated by 20Hz. Normal speech signals would not accidentally replicate this signal.
Speech as it would be heard by listening on the wires from the telephone system at location 'A'.
Speech filtered by the Band-Cut filter. This is how it would be received at location 'B'.
Listening at the pink 'Listen Here' point on a radio scanner or landline between bunkers. In this part of the demonstration the modulated 1680Hz telegraph signal and the speech can both be heard.
The final demonstration is the inter switchboard calling signal of 500Hz modulated by 20Hz, known as 500/20 signalling.
Here are two S+DX terminal units mounted on a rack. Each combines one duplex telegraph circuit and one speech circuit onto one destination. The keys and meter on the front panel are for circuit alignment.
The sign writing on the right indicates each circuit's destination 1141 code [top: QHKN bottom: QMAD] and the Private Wire ( PW ) numbers. Nowadays the PW is called a private circuit or leased line.