Patent

This is the Patent Robert Swift and others took out in 1933:

PATENT SPECIFICATION

No. 19,213 /33. 419,392

Application Date: July 6, 1933.

No. Complete Left: July 6, 1934.

Complete Accepted: Nov. 12, 1934.

PROVISIONAL SPECIFICATION.

Apparatus for Testing Telephones.

419,392

We, EDGAR JAMES BARNES, 15, Graham Eoad, Hendon, London, N.W. 4, British Subject, ROBERT ERIC SWIFT, 44, Oxen-park Avenue, Wembley, Middlesex, British Subject, OSCAR WILLIAM GILL., 40, Wargrave Eoad, South Harrow, Middlesex, British Subject. JOHN WOOD. 50, Waldron Road, Earlsfield, London, S.W. 18, British Subject, do hereby declare the nature of this invention to be as follows :—•

This invention relates to apparatus by which subscribers' transmitters and lines may be tested for transmission efficiency. In order to test telephone transmitters it has been customary to use the voice and ear. This is the only method possible as a final check but direct comparison must-be made with an instrument of known quality and considerable time is required for a test. This method is not possible when the transmitter to be tested is installed on a subscriber's premises. Under these conditions a simple speaking test to an observer at the exchange has been all that was possible and on account of the great variability of the speaker's voice and listener's judgment very great errors may arise and very little reliance can be placed on the test. To overcome these difficulties it has been proposed to use some form of noise producer at the transmitting end and an electrical measuring instrument at the exchange. If this scheme is tried out and comparison be made with accurate voice-ear tests wide divergences between the results will be found when different types of transmitter are involved. These errors are due in the main to the wide difference which may exist between the output-frequency characteristics of the different transmitters. To make the results agree at all with speech it is necessary that the noise used shall contain tones of practically all such frequencies as are present in speech in substantially their correct proportion. The present invention is for a device which fulfils the above requirements in a simple manner and can be made small so that it may be readily carried in the tool kit of the maintenance officer.

The device consists essentially of a [Price I/-]

sound emitting member which may be in the form of a diaphragm or cone. A point substantially near the centre is struck by metal balls carried loosely in recesses in a revolving wheel or alterna lively by metal weights loosely pinned to the revolving wheel. The weights may be annular rings, circular discs or may be elongated in shape. The main, requirements are satisfied by making the resonance frequency of the lowest internal mode of vibration of the diaphragm or cone above the frequencies at which the transmitters to be tested have any substantial output. The frequency of vibration of the cone as a whole must also be oiitside the range at which the transmitters have any appreciable output, or the vibration at this frequency must bo heavily damped. As an alternative or in addition an acoustic resonator or resonators may be added in the neighbourhood of the sound emitter to absorb any nn wanted energy at particular frequencies.

By these means the output is made substantially uniform over the important band of frequencies.

To ensure that this band is effectively covered the balls or weights may be unevenly spaced round the circumference of the wheel. The fundamental frequency of the blows on the emitter is thus indeterminate or in effect varies rapidly between certain limits. The higher tones are provided by overtones produced by the shape of the waye form of the motion given to the emitter by the blows of the balls or of the weights. It is preferable that the minimum spacing of the balls or weights shall be rather less than half the maximum and it may vary uniformly in between.

In order to obtain free action for the balls the recesses in which they are mounted may be set at an angle to the radii of the wheel. It is then possible to obtain a substantially direct blow in place of an oblique one on the emitter.

The wheel may be driven by clockwork and its speed held approximately constant by a simple centrifugal governor.

Alternatively the sound emitter may be struck by a vibrating hammer operated

419,392

by a toothed wheel and pallets, in which case it may be possible to omit the governor.

In the case where a centrifugal governor is used the revolving balls or weights may be allowed t'o ;be in contact during part of each revolution with the inside face of a ring, the friction caused

by the centrifugal force on the balls or weights producing the governing action.

E. J. BARNES.

ROBERT ERIC SWIFT.

OSCAR WILLIAM GILL.

JOHN WOOD.

COMPLETE SPECIFICATION. Apparatus for Testing Telephones.

We, EDGAR JAMES BAKNES, 15, Graham Road, Hendon, London, N.W. 4, British Subject, EoiiBiiT EBIC SWIFT, 44, Oxen-park Avenue, Wembley, Middlesex, British Subject, OSCAII WILLIAM GILL, 40, Wargrave Road, South Harrow, Middlesex, British Subject, JOHN Woon, 50, Waldron Road, Eiarlsfield, London, S.W. 18, British Subject, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:—

This invention relates to apparatus by which telephone subscribers' lines and transmitters may be tested for transmission efficiency. In order to test telephone transmitters it has been customary to use the voice and ear; this is the only method possible as a final check but direct comparison must be made with an instrument of known quality and considerable time is required for a test. This method is not practicable when the transmitter to be tested is installed on a subscriber's, premises. Under these conditions thn only test which it has been so far possible to make without requiring too much time and/or requiring the lineman to carry too much apparatus to the siibscriber's premises has been to speak over a known length of artificial line to a listener at the Telephone exchange and on account of the great variability in the speaker's voice and listener's judgment, large errors may arise and little reliance can be placed on the test. To overcome the difficulties an obvious method is to use some form of noise producer at the transmitting end and an electrical measuring' instrument at the exchange. If this scheme is tried out and comparison be made with accurate voice-ear tests wide divergencies between the results will be found when different types of transmitter or circuit are involved. The errors are due, in the main, to the wide difference which may exist between the output-frequency and/or the input-output characteristics of the.

different transmitters or circuits. To make the results agree at all with speech tests it is necessary that the noise used shall contain tones of practically all such frequencies as are present in speech in substantially the correct proportion, and that the total intensity of sound at the transmitter mouthpiece should be of the same order as that produced by the speaker when using the telephone. The present invention is for a device which fulfils the above requirements in a simple manner and can be made small so that it may be readily carried in the tool kit of the maintenance officer.

The invention employs the well-known device of a sound emitting surface struck by weights attached to a rotating wheel. The weights however are irregularly spaced round the wheel which is rotated by clockwork so that the surface is struck at irregular time intervals, thus causing the production of a sound containing a large number of frequencies. The weights may be placed in slots at such angles to the radii of the wheel that friction between them and the wheel is reduced to a minimum giving reliability and constancy in their behaviour. The speed of the wheel is held constant by means of a centrifugal governor which may be the weights on the wheel themselves in contact with a friction surface or may be a separate unit. Unwanted frequencies in the sound may be absorbed acoustically.

A preferred form of the device is illustrated in Figures 1, 2, 3 and 4 of the accompanying drawing, but it must be understood that alterations in arrangement of the parts and dimensions may be < made without departing from the scope of the invention as set out in the appended claims.

The sound is emitted by the cone (1) when this is struck by the balls or weights ! (2) loosely carried by the wheel (3) when the wheel is made to revolve by means of the spring (fi) wound by the handle (26). The weights may be annular rings, pircular discs, spherical balls or may be

419,392

elongated in shape. The main requirements are satisfied by making the resonance frequency of the lowest internal mode of vibration of the diaphragm or cone above the frequencies at which the transmitters to be tested give the greater part of their output. The frequency of vibration of the cone as a whole must -also be outside the range at which the transmitters have any appreciable output, or the vibration at this frequency must bs heavily damped. A suitable damping device consists of a washer of felt pressed against the emitter by a spring. Thia damping device would also help to damp unwanted internal modes of vibration of the emitter. As an alternative or in addition an acoustic resonator or resonators may be added in the neighbourhood of the sound emitter to absorb any unwanted energy at particular frequencies, as described later. By these means the output is made substantially uniform over the important band of frequencies.

To ensure that this band is effectively covered the balls or weights are unevenly spaced round the circumference 6f the wheel. The fundamental frequency of the blows on the emitter is thus indeterminate or in effect varies rapidly between certain limits. The higher tones are provided by overtones produced by the shape of the wave form of the motion given to the emitter by the blows of the balls or weights. It is preferable that the minj mum spacing of the balls or weights shall be rather less than half the maximum and it may vary uniformly in between.

In order that the friction between, the balls or weights and the wheel when the balls move relatively to the wheel on striking the emitter be minimised it is preferable that the axis of the wheel be off-set so that a line at right angles to the plane of the sound emitter through the point at which it is struck does not cut the axis of the wheel and that if balls are used the recesses (7) in which they are mounted be set at an angle to the radii of the wheel. It is then possible to obtain a substantially direct blow in place of an oblique one on the emitter. By this means the intensity of the blows remains very constant and depends primarily on the speed of the wheel. However a small adjustment of the distance between the cone and the rotating wheel may be provided by means of a movable pip (8) held in place by a locknut (9).

The wheel being driven by the spring (6) may have its speed held substantially constant by means of the centrifugal governor of well-known type (10) driven „ by worm gear (11). The speed may be '° adjusted by bending the springs carrying

the governor weights. Alternatively the balls or weights may be made to form a governor by making them rub against a circular arc of metal placed eccentrically to the wheel as at (12) in fig. 3. The correctness of the speed may conveniently be ascertained by means of a stroboscopic disc (18) mounted on the wheel.

The whole mechanism may be mounted in. a frame (13). In order to start and stop the wheel a spring brake (14) acting, on the governor spindle, operated by a button (.15) may be provided.

In order to prevent transmission of mechanical vibration from the mechanism to the transmitter under test the mechanism may be mounted in a case (16) and separated from direct contact with it by rubber or felt packing as at (17). It is necessary that the distance between the emitter and the telephone transmitter be substantially constant when testing. This is ensured by means of the rubber buffers (19) which project through the perforated guard plate (20) xised to protect the emitter from damage. Further protection may be provided by the gauze (21). In use the device is held with the buffers (19) lightly touching the rim of the telephone mouthpiece.

As mentioned above it may be desirable to absorb unwanted sound at certain frequencies by means of a resonator. A suitable form for this to take is a hollow annular box (22) Fig. 4, surrounding the : emitter and placed between it and the protecting gauze (21) and plate (20). Sound is admitted to the annular space (23) through the narrow annular gap (24) and/or through holes (25). The sizes of : the holes or gap and of the annular space are adjusted according to the frequency of the sound it is desired to absorb. If desired the space may be divided into two or more compartments to absorb a \ corresponding number of sounds of different frequencies.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to \ be performed, we declare that what we claim is :—>

(1) A device for testing telephones consisting of a sound emitting surface which is struck by weights loosely attached at 1 irregular intervals to the circumference of a wheel rotated by clockwork.

(2) A device as in claim (1) in which the weights consist of balls moving in slots placed at angles to radii of the 1 wheel which is itself off-set relativelv to the sound emitter.

(3) A device as in claims (1) or (2) in which the speed of the wheel is held constant by the friction of the weights or 1

419,(

balls rubbing against a circular arc placed eccentrically to the wheel.

(4) A device as in claims (1), (2) or (3) in which sounds of unwanted freqxiency or frequencies are absorbed in an acoustic resonator or resonators.

(5) A device for testing telephones substantially as hereinbefore described.

Dated the 5th day of July. 1934.

E. J. BARNES

R. E. SWIFT.

A. W. GILL.

J. WOOD.

Redhill: Printed for His Majesty's Stationery Office, by Love & Malcomsoiij Ltd.—1934.

419,392 COMPLETE SPECIFICATION