UK TELEX (1932)

An extract from the
Post Office Electric Engineers Journal
Volume 25 - October 1932

Telex

For many years past Engineers have visualized the introduction of a system of working which would permit of any one telegraph instrument connected to a network being put in direct communication with any other instrument on that network.

While it has not yet been possible to develop the public telegraph system on these lines, the introduction of Teleprinters, which require only typewriting skill for their operation, has led to such a service being placed at the disposal of renters of telephone exchange lines who are also prepared to pay for the rental of a Teleprinter.

In America and other countries a service is being developed on special networks associated with the telegraph networks by means of special switching centres. The great economic advantage which would accrue if the existing telephone network and switching facilities could be utilized has led the British Post Office to develop a system which utilizes the public telephone line network in place of a special network. The use of the ordinary telephone circuits and trunk and local switching facilities prohibited the adoption of the direct current methods used in ordinary land line telegraphy for the service and necessitated the employment of alternating currents of a frequency and magnitude comparable with those used in the transmission of telephone speech. With this requirement as a basis, a system has been developed and an official service was opened in London in June last and subsequently extended to Manchester, Birmingham, Liverpool and Leeds. A public service was made available in London on August 15th and is now in process of extension to provincial centres. This Teleprinter exchange service is known as the "Telex" service and will ultimately provide Teleprinter communication facilities between renters wherever telephone communication is possible.

It will be appreciated that the difficulties to be overcome in the development of this system and the methods of operating were very different to those which would have been experienced in developing a system using a separate network. The Telex system had to be grafted on to an existing system, using operating procedure developed solely from the telephone viewpoint, with the basic difference between telephone and Teleprinter requirements, that while telephone intelligibility is not interfered with by momentary disconnexions or interruptions, such as the throwing of monitoring keys, the effect of such operations on the Telex system would be that extra letters, failure of letters and misprinting would occur on the Teleprinter, depending on the time of occurrence and the length of the interruption.

The technical problems involved in the production of a suitable voice frequency transmitter and receiver having been solved, the operating procedure to be adopted in handling Telex calls was studied and tried out experimentally on both the trunk and local systems. The results of these experiments have been carefully examined and the causes of the Teleprinter errors followed up and eliminated.

As now installed, each equipment comprises a Teleprinter No. 7A, a voice frequency converter together with auxiliary apparatus (described later) and with a change over switch which in one position connects the telephone to the line, disconnects the power supply to the Teleprinter and the converter, and in the other connects the Teleprinter to the line, disconnects the telephone and connects power to the Teleprinter and converter.

FIG. 1 - SUBSCRIBER'S TELEX INSTALLATION
Table, Teleprinter No. 1

A subscriber making a Telex local call turns the switch to "Telephone" and proceeds to dial the required number, if automatic, or to pass the call in the usual way if manual. When connexion is established both subscribers turn their switches to ''Teleprinter" and the two stations are then connected by a line which provides a simplex teleprinter service in alternate directions without further operations of switches. A local record of the message being transmitted to the distant end is automatically provided.

The Teleprinter No. 7A, used on this service, is fitted with an automatic "answer back" device which permits the subscriber confirming the correctness of the connexion. As soon as the change-over switch is thrown, the calling subscriber depresses the upper case of the Letter D key labelled "Who are you" which sends a combination of signals to line and the distant teleprinter. The reception of this signal brings into action a series of cams which automatically transmits the first three letters of the name of the exchange and the subscriber's number from the called teleprinter. This is recorded at both ends. The called subscriber depresses the "Who are you" key on his teleprinter and receives the exchange and number of the calling subscriber.

When teleprinter communication is finished both subscribers return their switches to "Telephone." The line is thus cleared in the normal manner; the telephones having been replaced on their cradles when the switches were turned to "Teleprinter."

Telex trunk service is being given as far as practicable on a demand basis and is being gradually extended to all parts of the country. Trunk calls will pass through a Telex position in the trunk exchange and where telephone demand working is available the ordinary demand circuits will be used for the Telex service to give the demand facilities required by that service, although, of course, such circuits will be absorbed into the telephone network as and when telephone demand working becomes available between the centres concerned.

The use of a separate Telex position has been rendered necessary to provide, for the special handling of Telex calls. Monitoring and time announcements, such as are used on telephone trunk circuits, would cause interference to a teleprinter circuit, and it was therefore considered desirable that all trunk Telex work should be handled on a special position will be routed via a direct high grade circuit to the Telex position. Where there are difficulties in providing direct circuits, or where traffic conditions do not warrant their provision, a call will be routed via a mechanical tandem exchange or the appropriate switching centre to the Telex operator, who will immediately reverse the call.

In non-director areas level 98 will be used for the Telex service reversing of calls, except those from main exchanges, this being necessary until such time as the transmission efficiency of the circuits is made suitable for ordinary telephone demand working. When such circuits are available Telex calls from by an operator specially trained for the work.

A subscriber in a director automatic area desiring a trunk connexion will dial "TLX" and the call all automatic exchanges in non-director areas will be dealt with on a recording and completion basis.

Traffic from manual exchanges will reach the Telex position via the keysending positions, or manual positions, as the case may be, and the Telex operator will reverse the call.

Subscribers to the service are required to rent a Telex equipment which; for convenience in installation, is mounted on a steel table supplied by the Department. Each table is completely wired before issue to the fitting staff, leaving only the connexion of the telephone lines and power supplies to be made on the subscriber's premises.

A complete Telex installation is shown in Fig. 1.

The table equipment consists of a Teleprinter No. 7A, (a rectifier being provided if the power supply is A.C.), a Converter Voice Frequency Telegraph No. 1 or No. 2 for A.C. or D.C. supply respectively, a change-over switch, a telephone and auxiliary equipment.

The teleprinter is the standard "Teleprinter No. 7A" using the international alphabet No. 2 and standard keyboard; the; bell signal on the upper case of the J key being provided by an electric bell operated by a pair of contacts.

The voice frequency converter is of Departmental design and, operating at a frequency of 300 cycles, permits of teleprinter working over circuits having an attenuation up to 30 db. at that frequency.

The converter consists of two parts mounted in a metal case, the power panel (No. 1 for A.C. and No. 2 for D. C.) supplying the necessary filament and anode power to the converter panel, which forms the second portion of the voice frequency converter.

FIG. 2 - PANEL, POWER No. 1

The Panel, Power No. 1, Fig. 2, is employed when the power supply is A. C. The power supply is fed to the primary transformer 58A which has three secondary windings. The first of these windings Fr provides filament heating current for the rectifier and push pull valves of the converter, and the second winding F2 provides filament heating for the oscillator valve. The output of the third of the secondary windings is rectified by a copper oxide rectifier and smoothed to provide the high tension supply to the anodes of all the valves in the converter.

The grid return of the oscillator valve on the converter panel is connected to the negative terminal of the high tension supply and the centre point of the filament heating winding via the biassing resistance of 500 ohms.

The grid return of the detector valve is connected to the centre point of the appropriate filament heating winding via the resistances of 500 and 200 ohms, the latter being common to the grid returns of the push pull valves.

Panel, Power No. 2, Fig. 3, is used when the power supply is D.C. In this case the valve filaments are connected in series and the heating current adjusted by a 200 ohm resistance. A biassing resistance of 9 ohms is also connected in series with this circuit.

FIG. 3 - PANEL, POWER No. 2

The grid return of the oscillator valve is connected to the positive side of the filament of the last valve of the series; that of the detector valve to the negative high tension terminal through the common g ohm biassing resistance. The grid return of one of the push pull valves is taken direct to the negative of the high tension supply and that of the other to the common 9ohm resistance.

Tappings are provided on both types of power panels to enable the standard type of power panel to be employed for any supply voltages.

Fig. 4 shows diagrammatically the connexions of the converter with A.C. supply and Fig. 5 that with D.C. supply.

FIG. 4 - CONVERTER WITH A.C. SUPPLY

The converter consists of transmitting and receiving portions. The transmitting portion comprises a valve oscillator having one coil of a Transformer No. 55A in its anode circuit, the coil being tuned to 300 cycles by a 0.5µF condenser. The coupling between grid and anode is provided by the second coil of this transformer : the third winding of this transformer coupling the oscillator output via Transformer 48H to the line when the transmitter tongue moves to spacing. A 300ohm non-reactive resistance is inserted in series with the output winding of the Transformer No. 55A to prevent overloading of the valve and the output to line is adjusted by a variable resistance in the anode circuit to give three volts across a line having a resistance of 600ohms.

A portion of the output is bypassed to the receiving portion of the converter via Transformer 56A and the 0.5uF condenser to give a local record on the teleprinter of the signals being sent to line.

The receiving portion of the converter consists of a rectifier valve and two push pull amplifiers.

The rectifier is coupled to line by the Transformer No. 48H, the 0.5µF condenser being short-circuited by the transmitting tongue of the teleprinter resting on the marking stop. The grid of the rectifying valve is normally biassed to a point at the bottom of its operating curve when a minimum current flows in its anode circuit. An incoming signal produces a rise in current in the anode circuit comprising the primary winding of the Transformer No. 57A. This transformer has two secondary windings, one end of each being connected to the grids of the push pull valves and the other to the respective grid returns. The anodes of these valves are each connected through one coil of the teleprinter electromagnet to the positive of the high tension supply.

The increase in the current flowing in the rectifier anode circuit due to the incoming 300 cycle signals causes a momentary current in the secondary windings of Transformer No. 57A. The grid of one of the push pull valves becomes momentarily positive and an increased current flows in the anode circuit of that valve and the coil of the electromagnet associated with it, moving the relay armature to the spacing stop.

The grid of the other push pull valve is made more negative by the same anode current and its current is in consequence slightly reduced. On cessation of the signal the reduction in detector anode current produces pulses in the opposite direction in the secondary's of Transformer No. 57A. The potentials of the grids of the push pull valves are thus reversed with the result that a momentarily increased current flows through the other coil of the teleprinter electromagnet, moving the relay armature to the marking stop. Thus a double current effect is obtained.

FIG. 5 - CONVERTER WITH D.C. SUPPLY

A monitoring and test set for use with the Telex service is provided in each centre. The facilities provided are as follows (see diagram of connexions shown in Fig. 6):-

1. Measurement of the voltage given by the subscriber's set under test.

2. Test of teleprinter working to the subscriber's set from the test set through an attenuator, enabling the satisfactory working of the subscriber's teleprinter and converter to be tested.

3. Facilities for placing the teleprinter test set in leak across two subscribers' lines connected together at the Telex test desk. An amplifier is thrown into the test teleprinter circuit in this case, the leak impedance being kept high in order to interfere as little as possible with the operation of the two sets under test. (This facility will only be used in special cases where it is necessary to take steps to watch the operation of a circuit in order to trace obscure difficulties in working).
    

FIG. 6 - MONITORING AND TEST SET

A service similar to the phonogram service for telephone subscribers is being provided for Telex subscribers. This is known as the "Printergram" service.

A novel feature provided in this service is that the subscriber calling the printergram service starts teleprinting as soon as ringing tone ceases without calling in the printergram operator. As soon as transmission of the telegram is finished, depression of the J bell key calls in the printergram operator who confirms the subscriber's number by depression of the ''Who are you" key on the printergram instrument and acknowledges the receipt of the message.

FIG. 7 - PRINTERGRAM SERVICE

The circuit arrangements used in this case are shown in Fig. 7. Ringing currents at the position cause the white calling lamp to glow and this lamp is extinguished by the throwing of the teleprinter switch by the operator. This switch, in addition to starting the teleprinter, trips the ringing, causes a cessation of ringing tone to the calling subscriber and changes the exchange line over to the converter from the telephone. The distant subscriber then transmits the message, the teleprinter at the printer­gram position being unattended during this period. The operator, who supervises several instruments, is called into circuit by the glowing of a red lamp, illuminated by the distant subscriber depressing the J bell contacts on his machine. When the message is acknowledged the printergram operator throws the change over switch and clears the connexion. Should the operator fail to throw the change over switch the motor of the teleprinter will continue to run for about 90 seconds after the receipt of the last signal and will then have its power cut off by the operation of the auto start-stop switch on the teleprinter. When power is cut off in this manner relay P in the power circuit falls back and lights a green lamp through the closed contacts K1, thus giving a warning that the switch has not been returned to ''telephone." K1 is broken by the switch being returned to telephone.

This automatic feature of this service will ultimately be carried a further stage by means of a device now being developed to switch on the teleprinter power and change over the telephone line to the teleprinter after three ringing pulses have been received. The printergram operator will, therefore, only be required to attend to the circuit when the lighting of the red lamp indicates that the transmission of the message has been completed by the subscriber.

FIG. 8 - PRINTERGRAM OFFICE AT THE CTO