GB945267A - Signal-processing apparatus for an fm/fm multiplex signal - Google Patents

Abstract

945,267. Multiplex radio signalling; limiting circuits. HAZELTINE CORPORATION. Dec. 12, 1961 [Jan. 24, 1961], No. 44431/61. Headings H3W and H4L. Relates to a radio receiver for a stereophonic system in which the transmitted signal consists of a main carrier wave frequency modulated by the L+R signal and by a sub-carrier which itself has been frequency modulated by the L - R signal. In order to use a ratio-detector, which is not completely insensitive to amplitude changes, for demodulating the main carrier wave, the frequency detector for demodulating the sub-carrier is preceded by a limiter having a variable threshold which is dependent on the sub-carrier amplitude. Thus, the required balance between the L and R signals is maintained. As shown in Fig. 1, the receiver apparatus 11 provides an I.F. signal to the frequency detector 12, e.g. a ratio detector providing A.G.C. for the apparatus 11. The composite signal at terminal 13 which is subject to undesirable amplitude variations, consists of an L+R signal which is selected by a low-pass filter 14 including the usual de-emphasis circuit, and fed to a matrix 15, and the subcarrier frequency-modulated by the L - R signal, which is supplied via a band-pass filter 16 and variable threshold limiter 17 to a frequency detector 18 including a de-emphasis circuit. The resulting L - R signal is fed to the other input of the matrix 15 which carries out the appropriate addition and subtraction to provide the L and R signals. The variable threshold limiter 17, Fig. 2, maintains the level of the detected L - R signal proportional to the strength of the sub-carrier at terminal 13, the clipping level being determined by the R.C. circuit 24, 25. Both peaks of the signal are clipped and the amplitude-limited signal is supplied to a pulse-counter frequency detector 18 including a differentiator 26, 27. Diode 28 removes the positive-going pulses and clamps the negative pulses to earth potential, and a de-emphasis circuit 30, 31 also serves to filter out the sub-carrier frequency pulses. The series inductor 29 improves linearity and provi des increased output. In order to balance out any amplitude modulation which may be due to the L+R signal and would be detected by diode 28, an amplitude detector 33 is connected to the point 21 and feeds signals via an adjustable resistor 39 to the point 32. Specifications 945,268 and 945,269 are referred to.