Patent GB-946707-A (GB)
Title: Improvements in stereophonic broadcasting system
IPC: H04H1/00
Filed: 1961-06-12
Abstract: 946,707. Stereophonic broadcasting systems. GENERAL ELECTRIC CO. June 12, 1961 [July 22, 1960], No. 21086/61. Heading H4L. Relates to a stereophonic broadcasting system of the type comprising a transmitter provided with means for producing and broadcasting a stereophonic signal consisting of a first sound information signal which is frequency modulated on to a principal carrier wave and a second sound information signal which is amplitude modulated on to a sub-carrier wave, whereafter the amplitude modulated signal with suppression of the sub-carrier wave is frequency modulated on to the principal carrier wave; and a receiver for the broadcast signal. According to the invention the transmitter includes means for producing and broadcasting a pilot signal having a frequency that is a sub-harmonic of the frequency of the suppressed sub-carrier wave and is located in the frequency spectrum between the frequency bands occupied by the two sound information signals, while the receiver includes means for producing from the pilot signal a restored sub-carrier wave signal having a frequency that is a harmonic of the frequency of the pilot signal. System transmitter.-A microphone 11 (Fig. 1) picks up sound for the left-hand (L) channel, and the signal therefrom is passed through a low-pass filter 12, a pre-emphasis network 13 and an amplifier 14 to a matrix circuit 16. The signal from the right-hand (R) channel is similarly passed to the matrix circuit, which produces (L+R) and (L - R) signals. The (L+R) signal is fed through an amplifier 22 and a time delay 23 (of the same delay as the band-pass filter 37) to a frequency modulation modulator 24, which is also fed by the 100 kc/s. oscillator 26. The output from the modulator 24 is connected via a frequency multiplier 27 to the input of a second F.M. modulator 28. The (L - R) signal is fed via an amplifier 29 and a time delay network 31 to the input of a balanced amplitude modulator 32. The network 31 has a time delay such as to equalize the time of arrival of signals to the F.M. modulator 28. A 19 kc/s. master oscillator 33 is connected via a frequency multiplier 34, preferably a doubler, to the modulator 32, and also via a phase-shift network 36 to the input of the modulator 24. The output from the balanced modulator 32 is connected via the band-pass filter 37 (whose pass-band is in the range 23 kc/s.-53 kc/s.) and an amplifier 38 to an input of the second frequency modulation modulator 28. The output of this is connected via a frequency multiplier 41 and a power amplifier 42 to a transmitting antenna 43. Circuit details (Figs. 5 and 6, not shown) are given. Receiver.-An antenna 51 (Fig. 2) is connected via a R.F. amplifier 52 to a mixer 53 to which also a local oscillator 54 is connected. The mixer output is connected via an I.F. amplifier 55 and a limiter 56 to a discriminator 57 which passes frequencies as high as 53 kc/s. An amplifier 58 follows the discriminator and passes the signal (L+R), which is fed to a matrix circuit 59. The output from the amplifier 58 is also fed to a band-pass filter 61 whose range is 23 kc/s.-53 kc/s. and also to a pilot signal filter 63 (19 kc/s.) and thence to a frequency multiplier 64 (preferably a doubler). The outputs from these two channels are fed to a detector 62, whose output (L - R), is fed to the matrix 59. This matrix reproduces the L and R signals which are applied, via de-emphasis networks and audio amplifiers, to the loudspeakers 68 and 72. Circuit details of units 58-69 are given (Figs. 3 and 4, not shown, indicating two types of frequency multiplying circuits 64, one a frequency doubler, the other a synchronous oscillator). System operation.-The (L+R) band of signals 201 (Fig. 7) is applied to the FM modulator 24 along with the 19 kc/s. pilot signal 202. These are frequency modulated on to a carrier wave from the oscillator 26, of, say, 100 kc/s. The output signal from the modulator 24 is then frequency multiplied (at 27) and then acts as a carrier wave for the final frequency modulator 28. The (L - R) signal is amplitude modulated on to the 38 kc/s sub-carrier, which is then suppressed, in the modulator 32, resulting in a signal band 203 in the range 23-53 kc/s. This signal is frequency modulated on to the F.M. signal from the modulator 24 in the modulator 28. After frequency multiplication and amplification this signal is then transmitted (perhaps in the 88 mc/s.-108 mc/s. band). In the receiver the output from the discriminator 57 will be substantially the components shown in Fig. 7. The pilot signal is used to control the output from the 38 kc/s. multiplier 64, which is used to provide the carrier for the (L - R) signal at the detector 62. The detected (L - R) signal is combined with the (L+R) signal from the discriminator in the matrix circuit 59, producing 2R and 2L signals which are fed via de-emphasis networks and amplifiers to the left and right loud-speakers. By suppressing the 38 kc/s. carrier signal and transmitting a 19 kc/s. pilot signal instead, the filters necessary in the receiver may be much simpler, and the pilot signal may have relatively low amplitude.