SE303310B - - Google Patents

Abstract

949,225. Stereophonic reproduction. PHILIPS ELECTRICAL INDUSTRIES Ltd. Feb. 27, 1961 [March 2, 1960], No. 7095/61. Addition to 926,357. Heading H4L. In order to improve the reproduction quality in the apparatus for the reproduction of stereophonic signals A and B described in the parent Specification, in which the transmitted signal is in the form of a carrier frequency modulated by the sum signals A+B, and frequency modulated by a subcarrier, e.g. 35 kc./s., amplitude modulated by the difference signals A-B, there is connected between the frequency discriminator 13 and the amplitude detectors 31, 32, a frequency-dependent attenuator 45, 46, 47 having a higher attenuation for the frequency band of the audio frequency sum signal than for the frequency band of the difference signal modulated on the sub-carrier, the attenuation being substantially uniform over both bands. By this means non-linear distortion introduced by the detector 31, 32 is reduced, but a frequencydependent cross-talk is introduced. This is removed by shunting the resistor 41 with a series connection of the resistor 48 and capacitor 49. The time constant of this circuit together with the cathode resistors 23, 24 and 25, 26 is arranged to be equal to that of the circuit 45, 46, 47. In practice the resistor 48 may be omitted. The arrangement emphasizes the higher audio frequencies and this is corrected by connecting in series with each of the anode resistors 27, 28 of the amplifiers 17, 18, a capacitor 50, 51 shunted by a resistor 52, 53 respectively, the time constants of networks 50, 52 and 51, 53 being equal to that of the attenuator 45, 46, 47. These measures may be incorporated in a transistorized receiver. In an alternative arrangement, Fig. 3 (not shown), an inductor is connected in series with the resistor 46 and the capacitor 47 dispensed with. A common output impedance, consisting of a resistor and capacitor connected in parallel, is included in the output circuit of the amplifiers 17, 18 to reduce the frequency-dependent crosstalk and produce a smooth frequency characteristic. The modified attenuator may be used in place of the attenuator in the circuit of Fig. 1 and the other correction circuits may be interchanged.