Patent GB-946562-A (GB)
Title: Stereophonic radio signal receiver
IPC: H04H1/00
Filed: 1960-03-03
Abstract: 946,562. Stereophonic receivers; valve limiter circuits. RADIO CORPORATION OF AMERICA. March 3, 1960 [March 16, 1959], No. 7616/60. Headings H3W and H4L. Relates to a stereophonic radio signal receiver for receiving multiplicative stereophonically modulated carrier waves, which include modulation components corresponding to a pilot signal of fixed frequency, comprising first and second signal translating channels, one for each of the pair of stereophonically related signals. The receiver has an arrangement in the second channel operable between two conditions, the first of which permits signal translation through the channels whilst the second prevents it, together with an arrangement responsive to the presence of the pilot signal connected so as to cause the arrangement in the second channel to adopt the first condition. A suitable transmitter (Fig. 2) for producing the appropriate signals required by the receiver of the invention produces from stereophonically related input signals, A and B, signals (A+B) and (A-B) in a matrix circuit 14. The (A - B) signal is fed via an amplifier 20 and a pre-emphasis circuit 22 (with a characteristic of 150 Ásec. or more) to a frequency modulating circuit 26, which comprises a carrier wave oscillator and a reactance modulator. The frequency modulated signal is applied to the amplitude modulating circuit 28, to which the signal (A+B), suitably amplified, is applied via a cathode follower. The resulting signal is filtered and then passed to the transmitting circuits and thence to the aerial. A tone from an oscillator 42 is also applied as frequency modulation to the main carrier-this is the pilot signal used at the receiver for identifying the stereophonic signal. The receiver described (Fig. 3) consists of conventional R.F. mixer and local oscillator stages 46, 48, 50 and an I.F. stage 52. The signal from the I.F. stage is passed to an amplitude modulation detector 54, from which AGC is applied back to the preceding R.F. and I.F. stages, the detector 54 producing the (A+B) signal which is passed via the audio amplifier 56 to the matrix circuit 58. The signal from the I.F. stage is also supplied to a frequency modulation detector 82, which includes a static limiter stage and which is followed by a deemphasis circuit 84 which supplies the (A - B) signal. This, after suitable amplification in the audio amplifier 88 is combined in the matrix circuit 58 with the signal from the (A+B) channel. From the matrix circuit the A and B signals are fed to appropriate loud-speakers. A triode 90 is arranged to provide automatic stereo balance in the system-a potential tapped from a potentiometer 96 fed from the AGC line is fed to the grid of the triode and controls the conduction thereof to provide a shunting effect on the (A - B) signal. A further conducting triode 106 is arranged to completely short the (A - B) signal if no pilot tone is detected in the signal from the de-emphasis circuit but to have no shunting effect if a pilot tone is present. This is achieved by extracting the low frequency pilot signal (25 c/s.) from the (A - B) signal by means of a filter 100 and converting it to a D.C. by means of the rectifier stage 102. This voltage is used to cut off the tube 106, thus preventing it from shunting the A - B signal if a pilot tone is present, so that the FM channel in the receiver is only connected if a stereo signal is being transmitted. This ensures that no spurious signals pass along the path, and also enables a compatible system which will detect ordinary AM transmissions to be realised. A detailed circuit for the stereo receiver (Fig. 5) includes limiter stages 194 and 196 in the FM channel prior to the detector stage 198. The second of these limiters 196 has a triode 232 connected in shunt with its anode and controlled by the AGC voltage derived from the amplitude modulator detector circuits, so that the second limiter is controlled according to the AGC.