Patent GB-935309-A (GB)
Title: Improvements in or relating to radio frequency transmitting arrangements
IPC: H04H20/48
Filed: 1961-06-23
Abstract: 935,309. Radio frequency systems. PHILIPS ELECTRICAL INDUSTRIES Ltd. June 23, 1961 [June 27, 1960], No. 22836/61. Class 40 (5). A radio-frequency transmitting arrangement comprises an amplitude modulating stage controlled by the signals to be transmitted on a radio-frequency carrier oscillation and also a control-voltage generator, controlled by the signals, for producing a control voltage dependent upon the peak amplitude of the said signals, whereby the control voltage varies the amplitude of the carrier oscillation according to the peak amplitude of the signals. The control voltage generator comprises two rectifiers fed by the signals, each having an associated smoothing filter, each of a different time constant such that the first rectifier is associated with a smoothing filter of a longer time constant than that of the filter associated with the second rectifier; a third rectifier being connected between the cathodes of the first and second rectifiers which conducts only if the output voltage of the second rectifier falls below the output voltage of the first rectifier. The output voltage of the control generator is derived from the smoothing filter associated with the first rectifier. The invention may be applied to the transmission of stereophonic signals, where two coherent stereophonic signals A and B originating from microphones 1 and 2 are supplied via pre-emphasis networks and amplifiers to a frequency sum producer 7 and a difference producer 8. The difference A - B is amplitude modulated in a unit 9 on an auxiliary carrier derived from the oscillator 10 and combined with the sum signal A+B in a device 11, the resulting signal being used to frequency modulate a 100 Mc/s. carrier frequency at the unit 12 before transmission. The A - B is also passed along a lead 14 to a control voltage generator 25, the control voltage produced varying with the peak value of the difference signal and passing via a cathode follower 15 to be combined with the difference signal in a unit 16. If there is no difference signal there will be no control voltage and the auxiliary carrier oscillation at the modulator 9 will be suppressed. The generator comprises an input amplifier 25, followed by a transformer 23, the secondary of which has two taps fed to the two rectifying stages defined above. The smaller output from the transformer feeds the rectifier associated with the longer time constant circuit, which provides the control voltage fed to the unit 16. However, when the A - B signal ceases, the diode 22, which was previously held off by the larger output voltage from the transformer through the diode 18, comes into conduction and the condenser 19 rapidly discharges through the short time constant filter 20, 21, so that no control voltage will be supplied to the unit 16 and the carrier will be suppressed. An embodiment wherein a control voltage is obtained from the A and B signals separately, using two such circuits, is shown in Fig. 1, connection 14 being removed and connections 30, 31 being made. The output of the control voltage circuit is taken through a cathode follower 15 and is limited by the diode 26 and the potential divider 28, 29, so that the circuit is mainly sensitive to small input signals to the amplifier 25. Specification 926,357 is referred to.