1,147,605. Radio signalling; pulse modulation circuits. PYE Ltd. 13 July, 1966 [16 July, 1965], No. 30233/65. Heading H4L. In a system for selecting a signal having the best quality as determined by the signal strength, the signal/noise ratio, or the noise content, from a plurality of spaced receivers, coding means responsive to each received signal produce a train of pulses which are modulated in dependence upon the quality of the received signal and means for comparing the modulated pulse trains select therefrom the one whose modulation indicates the best quality signal. As described the system is applied to the reception of a signal from a vehicle moving within a given area and the quality of the signal is represented by a variation of the pulse repetition frequency. Alternatively the pulses may be width or amplitude modulated. General description. Fig. 1.-Four spaced receivers RX receive the transmission from the vehicle and considering site A, the audio signal from the receiver is fed to a transmitter TG, TX. The quality of the received signal is assessed in the receiver and a current proportional to the quality is supplied to a coder 103 where it is converted into a pulse train of corresponding repetition frequency which is also fed to the transmitter TG, TX. The outputs of the transmitters TG, TX are supplied via land lines to corresponding receivers TG, RX each receiver having an output for the audio signal which is applied to an audio switch 117, and an output for the pulse train which is applied simultaneously to a code detector 100 and a code decider 101. The code detector is controlled by pulses from a generator 104 having a repetition frequency lower than the lowest possible frequency of the coded signal which in the absence of an input signal cuts off the code detector 100 to prevent operation of the call lamps A, B, C or D. The units 101 each receive two of the pulse trains and supply an output to a common unit 102 corresponding to the pulse train having the higher repetition frequency (representing the higher quality signal). Unit 102 causes only the signal of the highest repetition frequency to be applied to an audio switch 117 and this allows only the corresponding audio signal to be applied to an output stage for utilization. Pulse modulation, Fig. 2.-The input signal at 204 charges capacitor C203 negatively so that transistor TR203 conducts via emitter follower TR202. This switches on transistor TR204 and thus transistor TR205 and when TR205 conducts a bi-stable circuit comprising transistors TR206, TR207, TR208, TR209 is switched over. When transistor TR204 conducts a pulse is fed back via capacitor C206 to ensure a rapid switching action and this pulse is also supplied via capacitor C205 to switch on transistor TR201 to discharge capacitor C203. Capacitor C206 is then charged via resistors R206, R210 to switch off transistors TR203, TR204, TR205. When transistor TR204 is switched off the resulting positive pulse is supplied via capacitor C205 to cut off transistor TR201 so that capacitor C203 commences to recharge and the cycle is repeated. The output is taken from terminals 215, 217, the speed of changeover of the bi-stable circuit and thus the current reversals at the output being dependent upon the magnitude of the current at the input terminal 204, a higher current causing capacitor C203 to be charged to the given level faster than a smaller current. Germanium diode D201 and silicon diode D202 provide temperature compensation for transistor TR201. Miscellaneous features.-The unit 100, Fig. 3 (not shown), utilizes gas tube trigger circuits for each input coded signal and has a corresponding number of output circuits which provide a continuous output current if a coded input is present and are cut off by low frequency pulses from the generator 104 in the absence of an input signal. The units 101 and 102 also make use of gas tube switching circuits, Figs. 4 and 5 (not shown).