This is a simplified description of the system for carrying both speech and telegraph signals over the same circuit. Only half the quantity of circuits, either landline or radio are required, thereby reducing the cost of provision. The cost of the additional electronics was outweighed by the savings 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. The two wires taking the voice away from this point are designated Transmit (TX) and the pair bringing the voice from the other end are designated Receive (RX).
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.
Speech plus Duplex Telegraph Explained
The drawing below shows a very much simplified diagram. The GPO / BT S+DX units described here a designed to work on 2-Wire circuits or the 2-Wire section of a 4-Wire circuit. The telephone circuit is filtered to withdraw the frequencies in the range of 1600-2000 Hertz used by the telegraph signals. Although this degrades the quality of the speech channel most people won't notice the difference. The filter stops the teleprinter signals being heard on the telephone circuit and prevents the speech affecting the teleprinter.
The S+DX chassis is a Ericsson / Plessey Equipment Telegraph FMT 16. Two versions of the equipment are required for each end of the circuit. The 'A' end, designated by the GPO as 'Panel Telegraph No. 107A, modulates a 1860 Hz carrier frequency. The 'B' end, designated by the GPO as 'Panel Telegraph No. 108A, modulates a 1680 Hz carrier frequency. The signalling speed is the standard 50 baud adopted by the British GPO.
The +/- 80 volt signals from the teleprinter or message centre, frequency shift the carrier by +/- 30 Hertz either side of the channel centre frequency. At the other end the carrier is demodulated and converted back to a corresponding +/- 80 volt signal for the teleprinter.
S+DX Modulation Frequencies
Nominal Carrier Frequency Hz
Mark / Stop -80 volt
Space / Start +80 volt
A (Panel TG 107A)
B (Panel TG 108A)
Had you been in the position to listen into the circuit, perhaps because it was carried on a radio link in the direction of B to A then you would have heard the speech in the direction of B to A (half the conversation) with a high pitched warbling tone in the background carrying the telegraph message centred around 1680 Hz.
As it isn't possible to send the switchboard calling signal of 25Hz or 50 volt DC, over the circuit this was converted into a tone of 500Hz modulated by 20Hz known as five hundred-twenty signalling. Normal speech signals would not accidentally replicate this signal.
The sound of S+DX
Speech as it would be heard by listening on the wires from the telephone system at location 'B'.
Speech filtered by the Band-Cut filter. This is how it would be heard by the person at location 'A'.
Listening 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.
At sites with one circuit requiring S+DX facilities, such as Local Authority Emergency Centres, a desk mounted unit can be used. Sockets on the rear of the unit accept plugs carrying the station wiring.
The gallery shows the front and rear of a single unit. The keys and meter on the front panel are for circuit alignment. Units can be one of two types 'A' or 'B' relating to the tones they transmit. On each path, one end must be an 'A' and the other 'B'.
At sites with a larger requirement such as UKWMO Group Headquarters or Regional Government Headquarters, the S+DX units can be rack mounted as can be seen in the last image in the gallery. The sign writing on the right indicates each circuit's destination GPO Engineering 1141 code [QHKN QMAD QQOG] while the bottom unit is to Edinburgh ROC Group. Also signwritten are the speech and telegraph Private Wire ( PW ) numbers. Nowadays the private wire is called a private circuit or leased line.
S+DX Terminal Equipment Gallery
S+DX Over Line and Radio
This is an example of S+DX working over landline (main) or radio (standby) often used in the UK Civil Defence network. Typically used on circuits between two Regional Government Headquarters (RGHQ), two ROC Group Headquarters or between a Local Authority Emergency Centre and its RGHQ. A patching panel provides an alternative radio path for the telegraph S+DX should a landline be cut by enemy action.
The switchboard has two speech circuits to the distant end, under normal conditions calls can be connected via either circuit. The links in the changeover panel normally connect the telegraph S+DX equipment into the circuit routed via the landline.
If the landline fails it will cut off the teleprinter link as well as the first speech circuit. Under these circumstances both ends must move the links in their respective changeover panel from Main to Standby. The speech circuit via the radio is now routed via the S+DX equipment and the teleprinter connection is reestablished. Once the landline fault has been cleared the changeover panel can be restored to normal working. Often during exercises, the ROC Group Headquarters of the UKWMO would switch to radio working for an hour or more. Under these circumstances, the first speech circuit via the landline is still useable as its connection is maintained through the changeover panel.
For simplicity only one S+DX circuit is shown here but at RGHQ and UKWMO HQ there would be many circuits routed through one changeover panel. Each circuit can be switched to standby individually. The radio standby to line in the Civil Defence network is not to be seen as a wholesale replacement for the landline network but one that can be deployed on a circuit by circuit basis.