Patent GB-1009265-A (GB)
Title: Monophonic and stereophonic frequency-modulation receiver
IPC: H03D1/22
Filed: 1962-06-06
Abstract: 1,009,265. Stereophonic and monophonic receiver. ZENITH RADIO CORPORATION. June 6, 1962 [June 19, 1961], No. 21873/62. Heading H4L. In a frequency modulation receiver for the reception of a stereophonic broadcast programme in which the transmitted carrier is modulated in accordance with the function M(t) = K 1 (A + B) + K 2 (A - B)S + K 3 S<SP>1</SP> in which A and B are the programme signals, K 1 , K 2  and K 3  are constants, S is the fundamental component of an amplitude modulated suppressed sub-carrier signal and S<SP>1</SP> is a pilot signal related in frequency to the sub-carrier, the receiver automatically adjusts itself, in the absence of a stereophonic signal, for moaura reception. The incoming signal is applied via R.F. amplifier 10, I.F. amplifier and limiter 12 to a ratio detector 15 which develops a push pull output signal comprising the sum component A and B and the difference component A-B amplitude modulated on suppressed sub-carrier S across the resistors 24, 25. The pilot signal, which in this embodiment is half the subcarrier frequency, is extracted from across capacitor 14 and applied to a selective pilot amplifier 30. This amplifier which is quiescent in the absence of a pilot signal drives a frequency doubler circuit including diodes 50, 51 to provide a signal at the sub-carrier frequency at the input of carrier amplifier 67. Positive feedback for the pilot amplifier is provided by connecting the output of the doubler circuit via resistor 54 to the grid circuit of the amplifier. A synchronizing detector including diodes 60, 61, which on receipt of a stereo signal are biased to act as normal rectifiers by a positive bias applied to their anodes from the junction of resistors 71, 72 and similar bias applied to their cathodes from the cathode of the pilot amplifying value, is supplied via capacitor 65 with the ratio detector output signal developed across resistor 25 and with the sub-carrier switching signal produced at the output of carrier amplifier 67. The detector develops across its load resistor 62 an A signal with a small amount of B signal and across its load resistor 63 a B signal with a small amount of A signal. These signals are applied to a matrix circuit comprising potentiometer 75 with capacitor 77 and potentiometer 76 with capacitor 78 to which the ratio detector signal across resistor 24 is also applied. In the matrix, effectively, an adjustable small amount of A + B signal is subtracted from each of the signals developed across the diode load resistors so that substantially separated A and B signals are developed across capacitors 77, 78 and applied via their respective amplifiers to the speakers. The capacitors 77, 78 also filter the sub-carrier frequency components and apply de-emphasis to the A and B signals. In the absence of a stereo signal, pilot amplifier 30 is quiescent so that the positive bias it supplies to the cathodes of diodes 60, 61 disappears and the diodes are biased only from the junction of resistors 71, 72 to act as transmission elements. In this condition a monaural signal applied to the synchronizing detector via capacitor 65 is passed without modification to the amplifier A and B.