British Kinematography (1951)

124 BRITISH KINEMATOGRAPHY Systems of Modulation Voice frequencies from the microphone may be impressed on the radiated carrier wave in one of two principal ways. Amplitude modulation and frequency modulation, as they are known, both have their ardent supporters in the field of application now being discussed. Until quite recently F.M. was widely regarded as being capable of much higher fidelity standards than A.M. and also of being immune from nearly all forms of interference. The former supposition has now been discredited, however, and it is generally accepted that, given equal sideband limits, A.M. and F.M. are able to account for themselves equally well. It is true that because the majority of manmade static interference exhibits A.M. characteristics, a frequency-modulated receiver tends to be freer from certain types of noise than its A.M. counterpart. At frequencies of the order of 180 Mc/s the difference is not appreciable, however, and in practice few can say with certainty which technique is being used when judging the performance of representative equipments. As F.M. technique tends to complicate design and to be rather critical of adjustment, it is not as widely used as is often supposed. In this country amplitude modulation prevails, and its choice, by the Post Office authorities for use in conjunction with the Thames River Service will serve to illustrate its continued popularity. The proportion of A.M. to F.M. in use at present for business radio installations is likely to be about 4 to 1. General Design Through the use of semi-miniature techniques and careful design, manufacturers have succeeded in keeping the size and weight of their equipment within convenient limits. The mobile transmitter, receiver and power unit may be contained in one or sometimes two robust cases, designed for fitting either in the corner of the boot of a car or in any other convenient position in a commercial vehicle. Shock absorbent mountings ensure that vibration is not excessive, and quick release devices enable sets to be removed speedily for servicing and replacement. The Vol. 18, No. 4 built-in loud actual control panel with small built-in lo speaker is mounted near the driving position and is connected with the main equipment by a multicore cable. Some models designed for mounting under the dash-board of a vehicle are completely self-contained, but these, whilst compact, have certain disadvantages. Leg room in a vehicle is seldom generous, and the space occupied by sets of the dash-mounting type is often greater than can be readily spared. Furthermore, the mechanical hum from the rotary transformer is added to the general background noise level. The aerial system is almost invariably of the Marconi quarter-wave type, and takes the form of a short " stick " mounted on the roof of the vehicle. The metal work of the vehicle roof forms part of the aerial system and is essential to it. Where the roofing material is non-metallic, an artificial ground plane is normally fitted, and may take the form of a metal plate or strips of metal suitably arranged. The power output of the mobile transmitter is limited by the battery drain permissible, and does not usually exceed about 15 watts; 5 watts is a representative figure, and the Pye P.T.C. 113 is rated for this output. On the whole, increases of power output do not bring about improvements in performance to the extent that might be expected, and the results obtained with aerial powers of the order of 1 watt and even less have been remarkably encouraging. The fixed equipments use somewhat larger outputs than their mobile counterparts, and although the most popular level is about 15 watts, outputs of 25, 50 and 100 watts are used. Fixed receivers, although basically similar to the mobiles, usually include refinements which would be considered rather ambitious for ordinary mobile work. A Particular Range of Equipment The Pye P.T.C. 704 fixed station, and P.T.C. 113 Mobile Station, are representative of apparatus currently available, and a brief description of the general design of these equipments now follows,