THE C.T.O. - UK INLAND PHONOGRAM &
TELEPHONE-TELEGRAM INSTALLATION

An extract from the
Post Office Electric Engineers Journal
Volume 30 - April 1937

The C.T.O. Inland Phonogram and Telephone-Telegram Installation

A description is given of the phonogram and telephone-telegram equipment recently installed in the Central Telegraph Office (C.T.O.), London, which is believed to be the largest installation in the world for this class of service.

Introduction
The provision of the installation described completes another step in the comprehensive reorganisation of the phonogram and telephone­telegram sections of the Post Office telegraph system, undertaken as a result of the recommendations of the 'Simon Report.' It allows the replacement of Morse systems by telephone working on short distance circuits of low traffic capacity and provides a central point for the dictation of telegrams by and to telephone subscribers over the telephone network.

As there be some doubt as to the distinction between the Phonogram and Telephone­Telegrams, these are defined below:-

• Phonogram - A phonogram is a telegram which is transmitted by telephone, between a post office and a telephone subscriber.
   

• Telephone-Telegram - A telephone-telegram is a telegram which is transmitted by telephone, between two post offices.

In the C.T.O. the received phonograms and telephone-telegrams are typed down direct from verbal dictation.

Where the traffic and economic conditions are favourable, direct junctions, known as direct telephone-telegram circuits, are provided between the C.T.O. and certain post offices. Where direct telephone-telegram circuits are not provided, the post office gains access to the C.T.O. telephone-telegram section by dialling ''C.T.O.," or, in manual areas, by asking the local exchange for " telegrams." Telephone subscribers in automatic areas gain access to the Phonogram section by dialling "TEL." and subscribers in manual areas ask the local exchange for "telegrams," the calls being completed over direct junctions or via Tandem. A number of has been provided between Tandem and the C.T.O. for the purpose of carrying this traffic.

Outgoing telephone-telegram traffic from the C.T.O. is disposed of over the direct telephone­telegram circuits, or via the exchange system, to post offices for delivery in the respective areas. Out-going phonogram traffic is disposed of by dialling the required subscribers direct, or by obtaining them via manual exchanges where necessary, and then dictating the telegram over the circuit.

Unlike telephone exchange work, where an operator is required to spend only a relatively short time on each call, the handling of a phonogram, or a telephone-telegram, occupies the undivided attention of the operator for an appreciable and widely variable period of time. 1t was therefore necessary to take into account this factor, as well as the incidence of calling, when determining the equipment required to maintain a reasonable standard of service.

Equipment provided
The main equipment is of the standard, double tier, continuous panel type and the whole of the installation work with the exception of the Vee belt conveyors was carried out by the local Post Office engineering staff:

The telephone-telegram equipment comprises six suites of sixteen positions each, with a 5-panel multiple appearing four times on each suit. In addition. there are four suites of de-concentration positions. each suite having fourteen positions. Any one of these positions can be connected to a selected telephone­telegram circuit by means of plugs and cords at a de-concentration switchboard. There are also two distribution positions. which facilitate the distribution of messages that have to be forwarded over the direct telephone-telegram circuits.

The phonogram equipment comprises twelve suites of sixteen positions each, making a total of 192 phonogram positions in all, with a 5-panel multiple appearing four times on each suite.

A general view of the phonogram suites appears in Fig. 1. A 12-position suite handles all enquiry work in connection with the phonogram and telephone-telegram traffic. A portion of this suite can be seen on the left of Fig. 1.

FIG. 1 - GENERAL VIEW OF PHONOGRAM AND TELEPHONE-TELEGRAM SUITES

Foot switch transmitter cut-outs of the non-locking type are fitted on the floor at each position. The operators are thus able to make full use of this important facility while using both hands for typing. ingle stage amplifiers, for increasing the strength of received speech, are associated with each position. A Vee belt conveyor below the centre of each suite of positions carries the received messages to their appropriate circulation points.

Fig. 2 - GENERAL LAYOUT OF EQUIPMENT

The remainder of the equipment consists of one listening-in position, two observation positions, three supervisors' desks, two supervisors' panels and five ancillary switching cabinets.

The accessory apparatus is in a separate room, so that noise from the relays, switches, etc., may not give rise to hearing difficulties in the phonogram room.

Accommodation
In view of the number of operators to be accommodated in the one room and the fact that typewriter reception was to be adopted, a high room noise level was anticipated. To counteract this the whole of the ceiling of the phonogram and telephone­telegram room was treated with sound absorbing material, and the results amply justify this precaution. The general layout of the equipment, which occupied the whole of the West Gallery and part of the North Gallery on the fourth floor of the building, is shown in Fig. 2.

Cabling
The Traffic Department expressed a desire for concealed cabling between the suites and the accessory apparatus, etc. This involved the provision of cable chases in the floor, and as the existing floor was not sufficiently deep to accommodate the required number of cables, a new floor six inches deep was built over the old one. The work of providing the chases was proceeded with concurrently with the building of the new floor. The chases, which extend down the whole length of the West Gallery, are lined with sheet metal, and have removable metal covers.

Owing to the long distance between the apparatus room and the suites of positions it was decided that the cables connecting these two points should be paper insulated. These were tail-ended with silk and wool cable. The use of lead cables made it necessary to provide a terminating point so that connection could be made to the switchboard cables feeding the jacks, lamps, etc., on each suite. This was conveniently done by mounting connection strips on a small iron frame designed lo fit beneath the end of each double table.

Telephone-Telegram Cord Circuit
The whole of the cord circuit apparatus, excepting the keys, plugs and lamps, is contained in an auxiliary apparatus unit of the "jack-in" type, known as a "Unit Auxiliary Apparatus TL 1643." In Fig. 3 two of the units can be seen in position under the desk. The cover has been removed from one of them so that the arrangement of the relays, etc., may be seen. The foot switch transmitter cut-out can be seen immediately beneath the right-hand unit. The transmission portion of this circuit is the well-known Stone system of cord circuit feed. On short direct telephone-telegram circuits the current for the operation of a distant transmitter is supplied from the telephone-telegram cord circuit. For this reason it is essential that, when using a battery of 22 or 24 volts, the combined D.C. resistance of each cord circuit feed shall not exceed 100ohms. This cord circuit is used in conjunction with the distribution positions, and as the arrangement is somewhat exceptional it is proposed to describe it rather fully.

Fig. 3 - TELEPHONE-TELEGRAPH POSITION

Distribution Positions
Each distribution position consists of a panel having a field of position lamps and a field of circuit busy lamps and circuit keys. The keys are of the push button type. A photograph of one of the positions appears in Fig. 4. Each position lamp is associated with a particular telephone-telegram position, and is designated with the number of that position. Each circuit busy lamp is associated with a particular direct telephone-telegram circuit, and is designated with the code of that circuit. A circuit key is associated with and fitted immediately below each circuit busy lamp, the key being labelled with the same circuit code as the lamp with which it is associated. The panels are adjacent to a conveyor drop point where messages arc received for forwarding over telephone-telegram circuits.

Fig. 4 - DISTRIBUTION POSITION

The facilities provided by the distribution positions are:-

1. Visual indication of all direct telephone-telegram circuits that arc engaged with incoming or outgoing traffic.

2. Visual indication on the operation of a circuit busy key of the number of the position which is engaging that particular direct telephone-telegram circuit.

3. Visual indication to the engaging position operator that a further message is being circulated to that position for transmission over the circuit which is engaged there.

The circuit is shown in Fig. 5, and a brief description of its operation follows.

When the calling plug of a telephone-telegram position cord circuit is inserted into the outgoing jack of a direct telephone-telegram circuit the line circuit K relay operates. Contact K3 lights the circuit busy lamp associated with this circuit on the distribution panels. Cord circuit relays HC, HCC, HD and HDD operate. Contact HDD3 operates relay I, which at contact I2 prepares a circuit for the lighting of the circuit lamp on the position keyboard. Contacts HCC3 and HDD2 prepare a circuit for the lighting of the position lamp.

Assuming that a message came to hand for transmission over the above circuit, the operator at the distribution position, observing from the glowing of the circuit busy lamp that the circuit was already engaged, would operate the circuit key associated with the lamp. The operation of this key causes relays A and D to operate to earth via the uniselector wiper W1. Contact A1 provides a holding circuit for these two relays. Contact D2 removes a short-circuit from across the 3,000ohm resistance in the sleeve circuit of the engaged line. This reduces the current flowing in the sleeve circuit and results in relay HDD releasing. Contact HDD2 extends battery via relay HA, metal rectifier, "close down" and "reserve" keys and wiper W3 of the uniselector to earth, thus causing relay HA to operate. Contact HA3 lights the required position lamp on the distribution panel and also operates relay L. Contact L1 operates relay F, which, at F1, lights the circuit hold lamp and operates relay G. Contact G1 locks the G relay, thus maintaining the circuit hold lamp alight.

The operator at the distribution panel notes the required position number and marks it on the message. The message is then placed on a conveyor serving the suite containing this position. When it is received at the drop point serving this suite a circulation officer delivers the message by hand to the position concerned, and at the same time operates the reset key on the position keyboard. This breaks the locking circuit of relay G which releases and thereby extinguishes the circuit hold lamp. The position lamp darkens immediately the operator at the distribution panel releases the circuit key, but the circuit busy lamp remains glowing as long as the circuit is engaged.

The circuit operation, when the incoming side of a direct telephone-telegram circuit is concerned, is similar to the above.

During busy periods it is necessary to staff both distribution positions, and the operators staffing each position must have access to every direct telephone­telegram circuit. The whole of the circuits must therefore appear on each distribution panel. It was not possible to have a simple multiple of the lamps and keys, because under such conditions a key operated on either panel would light the position lamp on both panels. This would cause serious inconvenience when both operators were pressing different circuit keys simultaneously as two position lamps would glow on each panel, and neither operator would be able to determine which of these lamps was associated with her particular circuit key.

The difficulty was overcome by arranging for the whole of the 96 telephone-telegram positions to be connected to each panel alternately. A uniselector stepping under the control of earth pulses supplied by a special cam on the ringing machine is used to effect the switching of the positions, which are connected to each panel in turn for approximately four seconds. Under this scheme, if a key is operated on one panel when the positions are connected to the other it is only necessary to hold the key for a very short period, at the end of which time the positions will be connected to the panel concerned. This period of waiting, when it occurs under actual working conditions, is barely perceptible. A key is provided so that the uniselector . may be switched in or out of circuit as required. Another key is provided to bring in a spare uniselector should a fault develop on the working switch.

Distribution Speaker Circuit
When a message is received at the distribution position for a circuit which does not show engaged, the message is placed, unmarked, on a conveyor. When the message arrives at the drop point it is circulated to a disengaged operator, who disposes of it in the normal way. During busy periods it often transpires that, during the time taken for a message to circulate from the distribution panel to an operator in the above circumstances, the circuit becomes engaged. Therefore the operator desiring to dispose of the message encounters engaged click on the circuit jack. In order to minimise delay arising from this cause a speaker circuit has been provided between the distribution position and the telephone-telegram positions. The circuit appears in the outgoing multiple on every telephone­telegram suite, while at the distribution position it is terminated on a lamp. A loud-speaker associated with a 2-valve amplifier is provided for receiving incoming speech from the operators. The transmitter at the distribution position is fixed to a flexible arm mounted vertically on the desk. This arrangement releases the distribution operators of the necessity of wearing a headgear receiver and breastplate transmitter, thus allowing them full freedom of movement for the circulation of messages. The loud speaker and transmitter can be seen on the right of the distribution position in Fig. 4.

When an operator has a message for a direct telephone-telegram circuit which tests engaged, she plugs into the speaker circuit jack. This causes the calling lamp to glow at the distribution position and draws attention to the fact that an enquiry is about to be made. The operator then quotes the code of the engaged circuit, and, on being informed of the number of the position engaging this particular circuit, marks the message accordingly and arranges for it to be delivered to the required position.

The action of the distribution operator when ascertaining at which position the circuit is engaged results in the glowing of the circuit hold lamp at the engaging position. Thus the operator at this position is warned that another message is being circulated to her for transmission over the circuit she is engaging.

FIG. 5 - DISTRIBUTION POSITION CIRCUIT

Telephone-Telegram De-concentration Positions
Although the 96 telephone-telegram positions are sufficient to handle the normal traffic there are certain days in the year, e.g., Christmas, Easter, race days and Saturday mornings, when they are unable to deal efficiently with the increase in traffic. To ease the load on the normal positions during these exceptionally busy periods the de-concentration positions are staffed. Fig. 6 is a photograph of the de-concentration positions. The de-concentration switchboard can be seen on the right of the photograph. A certain number of selected direct telephone-telegram circuits and incoming exchange telephone-telegram circuits are wired permanently to jacks and lamps on the switchboard. These jacks and lamps are in parallel with the normal jacks and lamps, and a key is provided on each circuit at the switchboard so that the lamp may be cut out when not required. The de-concentration positions are terminated on plugs at the de-concentration switchboard.

In order to connect any one of the selected circuits to a de-concentration position it is only necessary to insert the plug of that position into the required circuit jack on the de-concentration switchboard. A listening circuit is provided so that the officer extending the circuits may, by listening on the line, ensure that a call is not in progress before the circuit is extended. It is possible that a circuit may be left through to a de-concentration position when the latter is unstaffed. In this case the line lamp at the position would glow on receipt of a call but, being on the keyboard, would probably escape notice for some considerable time. As a guard against this a lamp has been provided on each circuit in the switchboard jack field and is switched in whenever the circuit is extended. Should the circuit be left extended after the operator has vacated the position any further calls will light this lamp, which will readily be seen by staff who are always in the vicinity of the switchboard. A telephone circuit is provided on the switchboard so that the controlling office may give assistance when required.

When a direct telephone-telegram circuit is extended to a de-concentration position its appropriate circuit keys on the distribution panels are designated with the number of the particular position to which the circuit is connected, thus indicating to which point messages for this particular circuit should be circulated.

Fig. 6 - DE-CONCENTRATION POSITIONS

Phonogram Cord Circuit
The phonogram cord circuit arrangement is shown in Fig. 7. The whole of the cord circuit apparatus, with the exception of the keys, plugs and lamps, is contained in an auxiliary apparatus unit similar to the telephone-telegram cord circuit unit, and is known as a "Unit Auxiliary Apparatus TL 2041."

The answering side of the circuit is simply the standard C.B. arrangement. The calling side is arranged to discriminate between junctions to automatic exchanges and junctions to manual exchanges. Briefly, the circuit operation is as follows :-

When the calling plug is inserted into a jack outgoing to an automatic exchange it meets a high resistance earth on the sleeve of the jack. This results in the cord circuit relays HC and HCC operating, but not HD or HDD. Contact HCC3 disconnects the engaged test circuit while HCC2 lights the calling supervisory lamp. Loop dialling conditions now exist on the calling cord, and the operator completes the call in the normal manner. When the called party answers the reversal of the battery and earth at the automatic exchange causes the polarised relay DC to operate. Contact DCl extinguishes the calling supervisory lamp, thus giving standard supervisory conditions. If the calling plug is inserted into a jack outgoing to a manual exchange it meets a low resistance earth on the sleeve of the jack. This results in relays HC, HCC, HD and HDD operating. The first two relays perform much the same function as on a call to an automatic exchange, while contacts HDD l and 2 substitute CB conditions for loop dialling conditions on the calling cord.

Fig. 7 - DISTRIBUTION POSITION CIRCUIT

Position Lamps and Traffic Indicators
Two lamps, one red and one green, contained in one fitting, are provided on top of the suites at each operator's position. When the position is staffed, but not actually engaged on a circuit, the green lamp glows. When the operator plugs into an outgoing or incoming jack, the green lamp darkens and the red lamp glows. These lamps facilitate the distribution, to operators, of messages to be forwarded, the officer distributing the message being able to see at a glance which positions are disengaged. Referring to the cord circuit diagram in Fig. 7 it will be seen that it is the condition of relay I which determines whether the red or green lamp glows.

Traffic indicators consisting of milliamperemeters fitted with suitable shunts, and calibrated to indicate numbers of positions, are mounted on the supervisors' desks. One "Positions Staffed" and one "Positions Engaged" indicator are common to a certain number of phonogram and telephone-telegram positions. The indicators serving the phonogram suites can be seen in position on a supervisor's desk, on the right in Fig. 1.

By comparing the readings of the two indicators for each group of positions the supervisors are able to see at a glance whether the staffing arrangements require attention. The circuit arrangement, which is quite simple, is shown in Fig. 7.

Free Line Signalling Facilities
To provide an outlet to exchanges which have no direct junctions to phonograms, 7-digit dialling conditions are provided on a large group of outgoing junctions to National exchange. In order to save time in testing these junctions for engaged click, free line signalling facilities have been provided. The current for the lamps is drawn from the A.C. mains supply, and in order to prevent mains hum interfering with adjacent speaking circuits great care had to be exercised with the cabling and wiring.

Listening Facilities
Facilities are provided for listening-in to any operator on the phonogram or telephone-telegram suites by wiring out the secondary winding of the induction coil at each position via the contacts of a relay to jacks accommodated on a listening-in panel.

As the listening-in position is situated some considerable distance away from the suites of positions it was necessary to take precautions against crosstalk between operating positions arising from the long length of cable carrying the listening-in leads. It is for this reason that a relay has been provided in the circuit at each position. The listening-in taps are thus disconnected from the operator's circuit until listening-in is actually in progress on the circuit. A transformer has been provided in the common listening-in circuit to ensure that the action of listening-in does not introduce any appreciable loss on the circuit under observation.

Any position may be extended from the listening-in panel to the supervisors' desks by means of double­ended cords and extension jacks. The arrangements are such that even when a position is extended in the above manner the listening-in taps are not connected to the operator's circuit unless the supervisor is actually listening on the circuit.

Observation Facilities
In the observation circuits, clip connections on the I.D.F. have been avoided. The points on the line circuits which have to be connected for observation purposes are cabled out to "Jacks, Test No. 17." The observation tapping circuits are also cabled out to "Jacks, Test No. 17." The jacks are mounted on the observation apparatus rack, where connections between the line circuit apparatus and the observation equipment are made by means of "Clips, Test, No. l/18B." The line connections on the M.D.F. are made in the usual manner by means of the standard
test clip.

There are two observation panels equipped to accommodate thirty line circuits. The line jack and line lamp fields of the panels are connected in multiple. Immediately above each strip of line jacks is a designation strip to carry labels bearing the appropriate circuit code for each jack. The labels consist of "Pegs No. 15, White," suitably engraved, and are changed by the traffic staff to agree with the circuits connected to the panels at any time.

The red and green lamps indicating exchange and phonograph operator conditions respectively are fitted at the bottom of the panel.

When a line is connected up for observation, but is not actually being observed, the circuit arrangements are such that the tapping equipment is short circuited. This ensures that there is no loss on the circuit unless it is actually under observation.

Fig. 8 - PHONOGRAM OBSERVATION PANELS

Earthing Scheme
Owing to the distance between the suites and the line relay equipment it was found necessary to devise a special earth wiring scheme for the whole installation, the purpose being to obviate false engaged conditions arising on the sleeves of the outgoing jacks at times when the load was heavy. In general, the arrangement consists of earthing the sleeve circuits of the outgoing junctions near the switchboards and not, as is usual, on the apparatus racks. In this way the Potential Difference between the tip of a plug and the sleeve of the jack of a disengaged junction is kept to a minimum.

Photogram Concentration Facilities
The phonogram section gives a 24-hour service, and must necessarily be staffed day and night. Arrangements have therefore been made to concentrate on two suites (C and D):-

1. all the incoming exchange lines within the phonogram section;

2. all the incoming exchange lines from the foreign section (on the second floor); and

3. all the incoming exchange lines from the telephone-telegram section.

Ancillary Switching Cabinets
In order to make full use of the standard phono­gram and telephone-telegram ancillary scheme, all the keys controlling the ancillaried lamps are concentrated at one point in the phonogram and telephone-telegram sections. The keys are mounted in special cabinets and are adequately labelled.

The operation of the keys cuts in or out the different ancillaries as required. The keys are also used for closing down or opening up suites as the traffic load vanes.

Additional Pictures

Printergram - Old Switchboard Suite (Picture dated 1954)

Printergram - New Switchboard Suite (Picture dated 1954)