Patent GB-1341673-A (GB)
Title: Transmission system for stereophonic signals
IPC: H04H1/00
Filed: 1972-01-26
Abstract: 1341673 Transmitting stereophonic signals; automatic phase control PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 26 Jan 1972 [29 Jan 1971] 3623/72 Headings H4R H3A and H4L In a transmission system for transmitting stereophonic signals, a respective carrier signal is modulated with the corresponding coherent signal and in one case the upper frequency sideband is transmitted and in the other case the lower frequency sideband is transmitted, there being provided, circuitry by which the two carrier frequency signals are derived from a single signal generator, and an automatic phase correction loop whereby differing phase shifts introduced during the production of the carrier frequency signals do not result in an additional phase difference between the two coherent signals after transmission. Such an automatic phase correction loop is included in carrier frequency production circuitry of both the transmitter and receiver, although only the transmitter will be described in detail.  Transmitter (Fig. 1):-Two microphones 1, 2 are connected via corresponding band pass filters 3, 4 (0À03-15 KHz.) and amplifiers 5, 6 to modulators 15, 16 where the microphone signals are used to modulate a 32 KHz. signal produced, via a multiply by 8 multiplier 24 and a divider by twentyone divider 23, from a generator 55 of frequency 85 KHz. Filters 17, 18 select the lower frequency sidebands of the modulated signals and apply them to respective modulators 7, 8. In modulator 7, the signal derived from microphone 1 is used to modulate a carrier of frequency 112 KHz. and a filter 9 selects the lower frequency sideband.. In modulator 8, the signal derived from microphone 2 is used to modulate a carrier of frequency 56 KHz. and a filter 10 selects the upper frequency sideband. The two sidebands together with a pilot signal of 84 KHz. from generator 55 are combined in a combination device 13 and are amplified and transmitted by transmitter amplifier 14. The carriers of frequency 112 KHz. and 56 KHz. are derived from a single nominally 84 KHz. generator 21 by dividing this frequency by 3 in divider 26 and combining the divided and undivided frequencies in modulator 25. Filters 19, 20 are then used to pick out the sum and difference frequencies 112 HKz. and 56 KHz. Due to the filters 19, 20, different phase shifts, e.g. # 2  in 19 and # 3  in 20 can occur in the carrier signals which can result in the coherent signals being subject to different phase shifts during the transmission. To obviate this problem an automatic phase control loop is used whereby the phase and frequency of generator 21 is controlled in relation to generator 55 such that the phase shifts of the the two carrier signals are equal and opposite. By virtue of the fact that the upper sideband of one signal is transmitted and the lower sideband of the other, the coherent signals suffer equal phase shifts of the same sign and the stereo effect is not affected. The phase control loop comprises modulators 51, 52 which are connected to filters 19, 20 respectively as well as to divide by 3 divider 26. These modulators supply to combination device 53 84 KHz. signals which are out of phase with each other due to the differing phase shifts in filters 19, 20. The combination signal is then supplied via a selection filter 54 to a phase detector 47 which has a second input from pilot frequency generator 55 via a compensatory selection filter 46. This phase detector has an output which is coupled via a low pass filter 49 to the generator 21 to control a frequency determining member 50 thereof. The phase detector 47 controls the phase of generator 21 relative to generator 55 so as to maintain a 90 degree phase difference between the signals received from filters 46 and 54. A mathematical analysis is given which show that the respective phase shifts in the carrier signals from filters 19, and 20 relative to the same frequencies produced by 22 are as a result + # s  and - # s  i.e. the carrier signals are P cos (# 1 t+# s ) and Q cos (# 2 t-# s ). By transmitting the lower and upper sidebands respectively after modulation it is shown that both stereo signals are subject to α - # s  phase shift. The Receiver (Fig. 2, not shown):-The receiver circuitry includes a near identical arrangement for producing the carrier signals for application to demodulators corresponding to modulators 15, 16, 7, 8. The generator 55 is replaced by the pilot signal received from the transmitter and the 32 KHz. signal is derived from a generator (21<SP>1</SP>) corresponding to generator 21 instead of from the pilot frequency derived from generator 55. Figs. 3a and 3b (not shown) each show an alternative way of generating the frequencies for application to filters 19, 20, i.e. alternatives to circuity 22. In Fig. 3a generator 21 is connected to a divide by 3 divider (26) which is connected to filter 19 by a multiply by 4 multiplier (56) and to filter 20 by a multiply by 2 multiplier (57). In Fig. 3b the divider (26) is connected to filter 20 via two multipliers in series, the output of the first of the two multipliers being connected to filter 20. If the divider divides by 3, the first multiplier may multiply by 4 and the second by ¢.