TRANSMITTER INSET No. 16
Issue 1, JAN 1965
TRANSMITTER INSET No. 16
There are three types of Transmitter Inset No. 16:-
Replacement of existing insets
New Carbon Transmitter - Transmitter Inset No. 16
A new transmitter has been developed to supersede the Transmitter Inset No. 13 which has been in service for nearly 30 years. The new transmitter has a slightly improved sensitivity, better frequency response, lower amplitude/amplitude distortion, and freedom from positional effects. Its introduction will complete the development of the 706-type telephone.
THE development of the 700-type telephone circuit envisaged the use of both a new receiver and a new transmitter. New developments in magnetic materials and the replacement of the single magnetic diaphragm by a balanced magnetic armature and separate aluminium diaphragm enabled the new receiver (Receiver Inset No. 4T) to have a considerably greater sensitivity and improved frequency response compared with its predecessor - Receiver Inset No. 2P. Unfortunately, no corresponding basic improvements which could lead to higher sensitivity had taken place in the carbon transmitter. Nevertheless, the low cost, robustness, and high sensitivity of the carbon transmitter had lead to its almost universal use in telephony, and development was mainly directed towards improving its quality of reproduction and its stability.
It was possible, therefore, when the new receiver became available to incorporate it in a new circuit using the Transmitter Inset No. 13, with the knowledge that a new transmitter could be used later without any modification of the circuit. With the introduction of the Telephone No. 706, it was also possible to incorporate a new handset (Handset No. 3) designed to work in conjunction with both the existing transmitter and the new transmitter as soon as it became available. This new transmitter, known as the Transmitter Inset No. 16 and illustrated in Fig. 1, is now being fitted on all new telephone instruments.
Although many of the performance characteristics required of the new transmitter can be specified in absolute terms, it is simpler to consider the design objectives in relation to the known achievements and limitations of the Transmitter Inset No. 13. The latter, which has been in almost universal use during the
post war period both in the United Kingdom and many Commonwealth countries, has on the whole an excellent record of service, and the basic design has not been changed since its introduction about 30 years ago. The main performance characteristics of this transmitter and the improvements required to be achieved by redesign are as follows.
CONSIDERATION OF DESIGN OBJECTIVES
The design of a carbon transmitter may conveniently be considered as divided into two parts: first, the acousto-mechanical transducer that converts sound energy into mechanical displacement of the front electrode; second, the carbon chamber where this displacement compresses the carbon granules to produce resistance modulation.
The design of the acousto-mechanical transducer will control the frequency response and, to some extent, the sensitivity and amplitude/amplitude distortion of the complete transmitter. In its simplest form the diaphragm, electrode and carbon masses form a single series-resonant system with the sum of the stiffnesses due to the diaphragm flange, the air volume behind the diaphragm and the granular carbon. Unfortunately, both the effective mass and stiffness of granular carbon varies with the amplitude of displacement, so the design must ensure that their contribution to the total is sufficiently small to prevent high amplitude/amplitude distortion. The resonant response may be at least partially equalized by superimposing an anti-resonant system consisting of a second air volume behind the diaphragm, connected to the first through an acoustic hole or mass suitably damped. The resulting response is much smoother, but overall sensitivity is lowered and, as will be apparent later, some compromise is necessary. The holes in the handset mouthpiece and in the front cover of the transmitter, together with the air interspacings, form a low-pass filter. It is possible to adjust these to maintain output at higher frequencies, but this results in a fairly sharp drop in sensitivity above the cut-off frequency.
The carbon chamber design is influenced by so many complex interactions and often conflicting requirements that experimental work, past experience and manufacturing methods rather than calculation will largely determine the final configuration of the electrodes and boundary walls.
The general design will determine the sensitivity, the resistance, the distortion, the noise and the stability of the complete transmitter. Since electrode displacements imparted by the sound are exceedingly small, thermal expansions due to unequal or transient heating or ambient temperature change can be of the same order, so that materials must be chosen to eliminate differential expansions which could cause unwanted electrode movements.
Long-term stability is largely determined by the quality of the carbon granules, which normally tend to increase in resistance, become noisier, and decrease in efficiency during service. Considerable research has been carried out in this country and elsewhere to produce a more stable carbon. By suitable oxidation and mechanical pre-aging treatment it is possible to produce granules of greatly improved stability, and such granules have been employed in many telephone sets where a stable transmitter resistance is essential to the functioning of the automatic transmission regulator. Unfortunately, these pre-aged carbons have initial efficiencies somewhat lower than normal granules, and it has been considered that, for use in the 706-type of transmission circuit with a regulator action sensibly independent of transmitter resistance, the efficiency loss would outweigh the advantages. However, both research and field trials of various carbons are currently in progress, and, if satisfactory, new carbons will be brought into service later. It must be emphasized, however, that the risks inherent in introducing any new carbon without the most rigorous field trials are far too great, so progress is necessarily slow.
Lastly, as in all mass-produced articles, a very important consideration at all stages of the design is cost. In the No. 16 transmitter technical performance must be of prime importance, and because it is not always possible to predict stability or end-of-life behaviour with great certainty from laboratory tests, extreme caution must be exercised in the introduction of changes. Nevertheless, with the closest co-operation between the Post Office and the manufacturers, it has been possible to allow sufficient flexibility in the construction to give cost-saving ideas the chance of being tried out under conditions of full-scale production.
A cross-sectional diagram of the transmitter is shown in Fig. 2.
To minimize variation in resistance when the transmitter is used in a handset, it is necessary to maintain a carbon filling of the order of 94 per cent of the total chamber volume. Since normal manufacturing tolerances lead to relatively large variations in the actual volumes of individual chambers, it is not easy to fill with a fixed volume charge, and special techniques are necessary to obtain a 94 per cent fill for each transmitter.
The carbon chamber is closed by a small nylon plug which is a push fit in the central filling hole of the back electrode.
Other Constructional Features
Frequency Response and Amplitude/Amplitude Distortion - Fig. 4 (a) shows a typical frequency response measured under matched load conditions with constant
free field applied sound pressures of 30, 10 and 3 dynes/cm2. The improved frequency response and lower amplitude! amplitude distortion compared with that of the Transmitter Inset No. 13C shown in Fig. 4(b) are immediately apparent. Nevertheless, the response falls off rather than rises at higher frequencies as required by the design objective, but there are two reasons for this. First, a compromise had to be reached between loudness and quality, and to maintain the former some peakiness remains in the region of 1,300 c/s. Secondly, a dip in the frequency response between 2 and
2-5 kc/s is advantageous in avoiding any possible speech interference with the C.C.I.T.T. signalling frequency of 2,280 c/s.
First, limited numbers of No. 16 transmitters have been placed in selected public call-offices on main-line railway stations, where usage is virtually continuous throughout the day and evening, and the atmosphere is liable to be adverse. By making periodical laboratory measurements of frequency response, resistance and noise, it is possible to ascertain within 12 months the deterioration trends. The average life of 2 years obtained under these conditions is considered satisfactory.
Secondly, similar controlled tests were carried out with transmitters placed in selected busy telephones in a commercial office. Under these conditions the measurable deterioration after one year’s service was nil, and there were no transmission complaints such as would be expected if the performance suffered from any short term instabilities.
Thirdly, several thousand transmitters were fitted at random into new subscriber trunk dialling public-call offices. This test lacks, of course, the precision of controlled tests, but allows a much larger scale of testing with wider and more random distribution of operating conditions.
Lastly, No. 16 transmitters have been used to replace No. 13 transmitters where adverse transmission conditions were known to exist because line lengths exceeded normal, or where specific transmission complaints have been made.
MOUNTING THE TRANSMITTER IN THE HANDSET
The transmitter is held in contact with the mouthpiece by a spring ring similar to the one used with the Receiver Inset No. 4T. The spring ring, supported by moulded lugs in the handset, presses against the back surface of the transmitter. The rotation of the transmitter is prevented by lugs which are positioned by ribs on the inside of the mouthpiece cavity.
Two modifications to the original design (Mark 1) have been developed to permit manufacturers to make full use of their own production techniques. Both designs are physically interchangeable with the Mark I and have similar electrical performances.
The Mark II transmitter (Fig. 5) has a modified back cover which extends over the frame and is clamped by the rim of the front cover, thus making it unnecessary to have the three back-cover fixing screws.
The Mark III transmitter, which externally is identical to the Mark I design (Fig. 1), has a modified diaphragm assembly. The moving electrode is sealed to the diaphragm, which is protected by enamel, So that it is no longer necessary to have a membrane in front of the diaphragm. The edge of the diaphragm is sealed with a pliant ring and sealing compound.
The Transmitter Inset No. 16 is now superseding the Transmitter Inset No. 13C for all 700-type telephones. Its slightly improved sensitivity, better frequency characteristics, lower amplitude/amplitude distortion, and freedom from angular positional effects will be of particular advantage under adverse transmission conditions; in all circumstances it will help to bring better quality to telephone speech. With its introduction into the 706-type telephones, the development of this telephone may now be said to be complete and it can the more readily take its place among the few first-grade telephone sets in general use throughout the world.
Last revised: January 16, 2021