DISCRETE FOUR-CHANNEL DISC REPRODUCING SYSTEM

专利名称：DISCRETE FOUR-CHANNEL DISC

REPRODUCING SYSTEM

申请号：AU7901475申请日：19750313公开号：AU471323B2公开日：19760415

摘要：1498065 Quadrophonic decoders MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd10 March 1975 [12 March 1974 28 March 1974 10 April 1974 (2)] 9903/75 Heading H4RThe adverse effects caused by dropouts in the modulated carrier component in adiscrete LI-CH disc owing to dust &c. are reduced by detecting 9 the carrier component,muting 10 the component during a detected dropout and further attenuating 12 at leastthe high frequency band of the remaining signal for a period longer than the duration ofthe dropout. Fig. 1 shows the system for left or right channels in which a low pass filter 2extracts the main component for a matrix circuit 4 whilst a band pass filter 7 routes thesubcarrier component through the muting circuitry detailed in Fig. 2. Here the FMdetector 9 converts the carrier to a pulse waveform to reset a ramp generator Q1 R1 C1except when pulses are absent owing to carrier dropout. Excursions of the ramp voltageabove a threshold set by D1 turn on transistor Q2 to discharge C2 and thereby open FETswitch Q3 for the duration of the dropout. The waveform at f still contains distortionowing to the switching action. The switch-controlling signal c therefore turns on Q4 todischarge C5 and thereby turn on Q5 which, via C6, shunts to earth at least the highfrequency components of the signal at g for a time dependent on the time sustant of C5,R8, R10. In a modification (Fig. 4, not shown) time sustant circuits (R11, C11) and (C13, R16)

ensure that a pilot lamp is not extinguished during carrier dropout and is not illuminatedby harmonics of a 2.CH stereophonic disc. In a further modification (Fig. 7, not shown) theoutput of the equalizer 11 is fed to the base of a transistor (Q31) in the attenuatorcontrol circuit 15 to produce a collector output which is rectified and which negativelycharges a capacitor (C34) to bias off a transistor (Q33) which, when a dropout occurs,turns on and attenuates the high frequency components. The output from the carrieramplifier is also rectified (D31) and fed through a time constant circuit (C31, R32) to thetransistor (Q31) so that even if the first controlling input from the equalizer 11 is large,high frequency attenuation is still effected should the carrier level fall. Similarly

attenuation is maintained by an output from the switch operating circuit 14 through (Q32)and a time constant circuit (C36, R36).

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