GB926357A - Improvements in or relating to arrangements for the stereophonic reproduction of signals - Google Patents

Abstract

926,357. Stereophonic reproduction. PHILIPS ELECTRICAL INDUSTRIES Ltd. March 18, 1960 [March 23, 1959], No. 9691/60. Class 40 (5). For stereophonic reproduction of a sum signal A+B modulating a main carrier in frequency and a difference signal A - B modulating a sub-carrier frequency in amplitude, the I.F. stage of the receiver passes the signals to an output band-pass filter 4, 5 forming part of a frequency detector which includes rectifiers 6, 7 and an output impedance 8. The output is taken from the centre tapping of resistor 10 and applied to device 16 which recovers the coherent signals A and B and supplies them through separate low frequency amplifiers 17, 18 to reproducing speakers 19, 20. The device 16 comprises two amplitude detectors, i.e. rectifier cells 31, 32 connected in parallel with opposite polarities and each with output impedances comprising parallel resistors 33, 34 and capacitors 35, 36. The audiofrequency sum signal A+B acts as a threshold voltage for the two rectifiers 31, 32, which together with an amplitude detection of the envelope signal A - B produces at the output stages 33, 35 and 34, 36 respectively an addition and subtraction of the audio-frequency sum and difference signals A+B and A-B. The resultant A and B signals are fed through blocking capacitors 29, 30 to the triodes 17, 18 having negative feed-back. A series resistor 39 at least equal to one-quarter-one-third of the value of discharge resistors 33, 34 reduces detection distortion. Cross-talk reduction is achieved by a resistor 40 and shunting capacitor 15 connected across the output circuits of detectors 31, 32, their time constant being approximately equal to those of output impedances, and the voltage division ratio equal to the cross-talk factor. Similar and additional positive cross-talk correction is given by a variable resistor 41 connected between the cathode resistors of triodes 17, 18. If the sum signal A+B is supplied alternatively with opposite phase to each of the two amplitude detectors, e.g. using a frequency detector connected in push-pull, the amplitude detectors may be connected with the same polarity. A switch 42 connects the output circuits for non-stereophonic reproduction. The arrangement is suitable also for magnetic tape reproduction. In Fig. 2 (not shown), the receiver is transistorized.