Patent GB-945546-A (GB)
Title: Improvements in and relating to radio transmission systems for stereophonic signals
IPC: H04H1/00
Filed: 1960-02-09
Abstract: 945,546. Stereophonic broadcasting systems. PHILCO CORPORATION. Feb. 9, 1960 [Feb. 9, 1959], No. 4529/60. Headings H4L and H4R. A system for transmitting stereophonic signals by radio comprises means for combining first and second stereophonic programme signals to provide signals in a first frequency band representative of an additive combination of the two programme signals, and signals in a substantially adjacent frequency band which are equivalent to the lower sideband components only of a subtractive combination of the higher frequency components of the programme signals modulated on a subcarrier signal, and means for modulating the signal in the first and second frequency bands upon a carrier wave. As shown in Fig. 1, A and B signals, Fig. 3A, are supplied from microphones 10, 12 respectively to cross-over networks 14, 20, which pass the high frequency components to respective adder circuits 26, 38, and pass the low frequency components via an attenuator 28 or 32 to a common adder circuit 30. The low frequency components from the adder 30 are combined at 26 and 38 with the high frequency components to provide signals A<SP>1</SP> and B<SP>1</SP> respectively, Fig. 3B and 3C. The attenuator 28 is such that the gain for lowfrequency components through 14, 28, 30, 26, is equal to one half the gain for high frequency components through 14 and 26, and the attenuator 32 has similar characteristics. The signals from circuits 26, 38 are supplied to balanced modulators 40, 42 and a source 44 supplies oppositely phased signals to the modulators at a frequency approximately equal to twice the highest frequency component which may be present in the output of the adders 26, 38. In a television system the horizontal synchronizing signals may also be provided by the source 44. The outputs of the modulators 40, 42, are added at 46 and the output of 46 includes audio frequency signals representing the sum of the two signals from adders 26, 38, i.e. A<SP>1</SP>+B<SP>1</SP>, and signals at higher frequcncies representing the difference, A<SP>1</SP> - B<SP>1</SP>. In the case of Fig. 3, the A and B signals are equal and A<SP>1</SP> - B<SP>1</SP> = O. The higher frequencies are the equivalent of sidebands of the sub-carrier frequency 72 supplied by the source 44, and contain only the higher frequency components of the audio signals, the sub-carrier being suppressed, and the audio frequency signals plus the lower sideband are passed by a filter 48 to a circuit 50 which inserts a reference signal from the source 44. This reference signal may be omitted if the horizontal synchronizing signal is used as a reference in a television system. The output of the circuit 50 is supplied to, for example, a frequency modulated transmitter 54. Fig. 4 shows the signal supplied to the transmitter with unequal A and B signals. The system provides a compatible signal suitable for reception by a monoaural receiver providing the passband of the receiver is sufficient to pass only the A<SP>1</SP>+B<SP>1</SP> signal. The received signal after normal amplification and detection, Fig. 5, is supplied to a filter 114 which passes signals between zero frequency and the frequency of the source 44 to demodulators 120, 122, and supplies the reference signal to control a demodulating signal source 118. The source 118 supplies demodulating signals in opposite phase to the demodulators 120, 122. Low-pass filters 124, 126, pass only signals in the band up to one half the frequency of the demodulating signal, the A<SP>1</SP> and B<SP>1</SP> signals appearing at the outputs of the filter 124 and 126 respectively. Thus speakers 130, 134, will reproduce the high-frequency components of the A and B signals respectively, but the low frequency components will be common to both speakers. In alternative transmitting arrangements the A and B signals are applied directly to an adding circuit and a subtracting circuit to provide an A+B signal and an A - B signal. The A - B signal is applied via a filter removing the low frequency components to modulate a sub-carrier, and the lower sideband is added to the A+B signal and the transmitter together with a small amount of sub-carrier for reference purposes, modulated as in Fig. 1. In an alternative receiving system, after detection in the normal manner, the A<SP>1</SP>+B<SP>1</SP>, A<SP>1</SP> - B<SP>1</SP>, and the reference signals are selected by filters. The A<SP>1</SP> - B<SP>1</SP> signal is demodulated in a single sideband detector circuit controlled by the reference signal to provide an A<SP>1</SP> - B<SP>1</SP> signal in the audio band to be reproduced, and the A<SP>1</SP> + B<SP>1</SP> and A<SP>1</SP> - B<SP>1</SP> signals are applied to sum and difference circuits to extract the two signals.