REPORT ON COMMON BATTERY AREAS

Telephone Efficiency Committee
Report on Common Battery Areas

August 1932

Terms of Reference

"To investigate the conditions resulting from the perpetuation of the various alternative and obsolescent types of subscribers telephones at present in use from a transmission maintenance and installation standpoint and subsequently to consider the financial effect of gradually introducing, under annual rental the latest types of subscribers apparatus for the various types of
exchanges and to report to the Engineer-in-Chief."

COMMON BATTERY AND AUTOMATIC TELEPHONES

This report follows on the Committee a interim report dated 18th May, 1931 regarding Local Battery telephones and now deals with the question of the transmission efficiency in Common Battery and Automatic areas, and the organisation for transmission testing in general

TRANSMISSION

Internal Plant
Until recently the only means of testing the efficiency of transmitters has depended upon voice ear methods and these were much affected by the conditions under which they had to be performed.  Although the method was known to be unsatisfactory there was no suitable alternative - Recent advances in the art of transmission measurement have however enabled  Quantitative tests to be made of instruments in service and in stock.

Tests on 50 Transmitters No. 1 taken at random from stock and ready for issue to the Districts shewed an average efficiency of 3.2 db worse than standard, whilst 22% were more than 4.6 db worse than standard.

Tests on 1212 Transmitters No. 1 in service in Willesden, Glasgow, Cardiff, Barry and Palmers Green shewed that:-

72% were 3 db worse than standard
44% were 5 db worse than standard
20% were 8 db worse than standard
8% were 12 db worse than standard

The average of all transmitters then tested was 5.8 db, worst than standard.

The Committee is of opinion that the conditions revealed by the special tests are representative of the general state of efficiency of fixed transmitters in all Common Battery areas.

In the course of the investigation attention has been give to the transmission loss on telephones connected to Private Branch
Exchanges, which is occasioned by the necessity for signalling devices across the line on the Private Branch Exchange Switchboard.  This loss cannot be overcome without reconstruction of the Switchboard, which would not only he exceedingly costly, but would involve much temporary inconvenience to Subscribers and would necessitate increased accommodation.  The only alternatives are to improve the line plant or to improve the efficiency of the telephone. The latter is by far the least costly method.  The use of a more efficient telephone on the extensions, will also allow of the recovery of a large number of primary batteries, which are not only expensive to maintain but vary in voltage and adversely effect transmission.

At this point it may be well to make reference to the existing standards of transmission and the desirability of improving the general average.

The present standards of transmission are based on the use of a telephone with a transmitter described as Transmitter No. 1 having a certain efficiency.  Line plant has been laid out on the assumption that this efficiency could be obtained end maintained.

Although the few transmitters used in fixing the original standards had the assumed efficiency, and every effort was made to obtain supplies with the high efficiency, it was necessary in order to avoid excessive cost of manufacture to allow for new transmitters, a tolerance of 5 db worse than standard Moreover it has been found that the No. 1 type of transmitter deteriorated considerably in service as mentioned earlier in the report.

There is no doubt that a large proportion of the transmission complaints which are received arise from the use of transmitters which are inefficient and that a marked improvement to the general standard and a consequent reduction in the number of complaints would result from the introduction of a more efficient transmitter.

Attention has therefore been given to the possibility of using the existing Common Battery telephone the inset transmitter employed on Telephone No. 162, the new microtelephone set.  This inset is efficient; the laboratory tests indicate that it is considerably bettor than the previous standards in efficiency and stability.  The high efficiency has been obtained in the large quantities manufactured during the past two years, and the stability is confirmed by our maintenance reports that these instruments have been remarkably free from deterioration in the 18 months or more of service.

Arrangements have been made accordingly for embodying this inset in the ordinary Common Battery telephone.  When employed in this way its designation is 'Transmitter No. 22' and it can be readily substituted for 'Transmitter No. 1'.

It has been demonstrated that by making this substitution the gain in efficiency would be of the order of 7.5 db., with the result that not only will the actual loss due to the existing transmitter be eliminated, but there will be a gain above the theoretical performance of the existing transmitter. There is another important aspect of the case, viz., greatly improved articulation that results from the use of the new type of transmitter. Although it is not possible at present to estimate the financial value of this improvement, the advantage will be readily appreciated.

The gain due to the introduction of the new type of transmitter will be very marked in the case of telephones connected to Private Branch exchanges.

Photographs are attached of the types of instruments under discussion.

Figure 1 above a typical Wall pattern Telephone fitted with Transmitter No. 1.

Figure 2 shows a typical Wail pattern Telephone fitted with the new Transmitter No. 22.

Figure 3 shows a typical Table pattern Telephone fitted with Transmitter No. 1.

Figure 4 shows a typical Table pattern Telephone fitted with the new Transmitter No. 22. Label on Transmitter.

Figure 5 shows a typical Table pattern Telephone fitted with the new Transmitter. No. 22. Label on Dial Dummy.

In order to give some indication of the advantages to be gained by the adoption of the improved transmitter, an investigation has been made to ascertain the cost of effecting an improvement of 5 db, in one exchange area containing 1260 lines by means of an increase in the size of copper conductor compared with the cost of effecting the same improvement by the substitution of the new type of transmitter, and it is found that the additional conductor costs would be approximately £6,300 while the cost of replacing the transmitters would be only £620.

It will be realised that if it were practicable to make similar calculations for all the various types of exchanges in the country, the total difference in cost would amount to several million pounds.

The Committee has come to the conclusion that the most efficient means of improving transmission is to make a replacement in situ of all Transmitters No. 1 by the new Transmitter No. 22.

If such replacements were carried out on all Common Battery and Automatic telephones throughout the country, the cost would account to approximately £600,000.  There is little doubt, however, that transmission requirements for a considerable number of existing subscribers are met satisfactorily by the existing telephones.  It is, therefore, proposed that the change shall be made on these transmitters which are known to be operating under the least efficient conditions, such as long extensions on Private Branch Exchange switchboards and long direct line telephones which at present require to be augmented by local batteries. At the outset it is recommended that £10,000 be authorised for this purpose to deal with the worst cases.

As regards future subscribers it is recommended as the first step that 25% of the telephones in stock required for one year's  issue by the Controller of Stores shall be fitted with Transmitters No. 22 at a cost of approximately £10,000. This supply will be utilised for the installation of new subscribers circuits with long direct lines and long P.B.X. extensions.

As regards instruments in situ, past records indicate that approximately 60,000 complaints per annum are received due to inefficient transmitters which require replacement. It is proposed that such cases in future shall be replaced in the field with Transmitters No. 22, but as the cost of the new transmitter is less than the present transmitter the actual maintenance replacement cost will be reduced.  Therefore no additional expenditure will be required under this heading. There is little doubt that until the time arrives when Telephones No. 162 are provided without additional cost to Subscribers it will be necessary to make stock provision of telephones fitted with the improved Transmitter No. 22 for existing and new subscribers.

EXTERNAL PLANT
It should be mentioned that in the endeavour to improve transmission, a great deal has been done in recent years; notably the introduction of improved types of cables, loading coils and. repeaters.  This has been accompanied by a progressive reduction in costs.  These improvements have made it possible to reduce the transmission loss in circuits between Zone centres to zero.  Efforts are being made to extend the improvement in line transmission to circuits between Zone Centres and Group end Minor exchanges, but owing to the fact that the cost of the improvement devices remain approximately the same there is not such a big field in these cases for savings in the provision of copper.  Moreover, there is at present, little prospect of introducing devices on local line plant in London and other large cities which will enable transmission to be appreciably improved at moderate cost.  As previously mentioned the only alternatives are to provide heavier conductors or to improve the standard of performance of the apparatus.  Owing to the very large amount of line plant in situ, the cost of replacing this plant by heavier conductors to give a general improvement of the required number of decibels, would be so great as to be altogether prohibitive.

TRANSMISSION TESTING INSTRUMENTS
The investigations of the Committee have also clearly demonstrated the need for introducing more effective methods of testing the efficiency of telephone apparatus before issue and after installation.

Efficient testers for use both at the factories and in situ, have now been evolved, which allow a standard amount of sound equivalent to speech, to be applied to the transmitter and the efficiency of the latter to be read direct from an indicator.

There are three types of tester, as follows:-

1. Fixed type for use at factories for testing the efficiency of transmitters and receivers (These are already in use).
2. Fixed type for use in exchange test rooms to be used in conjunction with a small portable noise generator, weighing a few ounces, which is taken to subscribers premises.
3. A combined portable type for taking from point to point when the installation of a fixed type is not justified.

Appendix 'A' furnishes descriptions of these testers.  Appendix 'B' outlines the present and proposed arrangements for making transmission tests.  It is estimated that the cost of supplying the necessary number of testers at the various centres throughout the country is £45,000.

Although at first sight this appears to be a substantial expenditure, it is small when compared with the value of the telephone plant as new, viz., over £139,000,000 an outlay that has been incurred in order to allow telephone speech between subscribers.  It is highly desirable, therefore, to use means which have been developed to ensure the most efficient use of this expensive plant.

In order to obtain experience with these transmission testing sets, it is proposed at the outset that authority be given for £10,000 to cover the cost of such testers at selected centres.  It is proposed to review the conditions after the new transmission testing sets have been in operation for six months, when further information will be available regarding the number of inefficient transmitters in the field.

SUMMARY OF COMMITTEES RECOMMENDATIONS

1. That 25% of all Common Battery telephones fitted with Transmitters No. 1 now in stock and all future supplies should be fitted with Transmitter No. 22.  The cost of dealing with 25% of the transmitters in stock would be £10,000.
   
2. That transmission testing sets be provided at selected centres a cost of £10,000.
   
3. That an expenditure of £l0,000 be authorised to cover the replacement of the least efficient transmitters on existing subscribers circuits.
   
4. That Transmitters No. 1 in the field proved to be faulty as the result of a complaint be changed in situ for Transmitters No. 22.
   
5. That exchange equipment and subscribers apparatus be periodically tested at selected centres for transmission efficiency.
   

Appendix A

TRANSMISSION TESTING SETS FOR ACCEPTANCE AND MAINTENANCE
OF DEPARTMENTAL APPARATUS

(a) Telephone Instrument Efficiency tester
This instrument is designed to replace voice and ear methods for testing telephone transmitters and receivers in bulk.  With this object the human voice is replaced by a loud speaker actuated from a special type of oscillator and the human ear by a form  of valve voltmeter which gives a visual indication of the efficiency of the transmitter or receiver.

(b) Exchange Tester
The purpose of this tester is to measure the transmission efficiency of Subscribers' telephone in situ.  It consists of a valve amplifier connected to the test desk voltmeter so as to indicate the voltage of received speech.  The actual deflection of the voltmeter is controlled by a potentiometer arranged in steps each of 1 decibel.  The potentiometer arranged is placed across the test desk testing circuit and is joined to the input transformer of the amplifier rectifier.  A standard testing transmitter is carried by the lineman when visiting a subscribers' apparatus for the test, and the voltage of received speech from (a) the testing transmitter connected direct across the lines and (b) the subscriber's telephone is measured and the condition of the latter is deduced from the measurements.

A later development is the use of a Standard Noise Generator which will he carried by the linesman and thus obviate the use of the Standard Testing Transmitter at the subscriber's premises.

This testing instrument is just being developed and the only one at present in use is at Palmers Green Exchange, London, where it is being used for testing the efficiency of every instrument which is visited in connection with a fault.

(c) Small Potable Tester
The instrument consists essentially of a full wave copper oxide rectifier connected in series with a sensitive direct current micro ammeter.  This arrangement gives  visual indication of the value of voice frequency voltages and can therefore be used as  a volume indicator. Used in conjunction with an oscillator, the tester can be used for measuring input and output voltages to indicate the transmission loss in the equipment under examination.

Appendix B

Schedule showing present and proposed arrangements for testing Transmission efficiency of Departmental equipment.

Fig. 1 Typical Wall Pattern Telephone Telephone No. 121

Fig. 1 Typical Wall Pattern Telephone Telephone No. 121 - fitted with Transmitter No. 22

Fig. 3 Typical Table Pattern Telephone Telephone No. 2

Fig. 4 Typical Table Pattern Telephone Telephone No. 2 fitted with Transmitter no. 22

Fig. 5 Typical Table Pattern Telephone Telephone No. 2 fitted with Transmitter No. 22, with label on dial dummy

Last revised: November 05, 2022