| 摘要 |
PURPOSE:To detect exactly the presence or absence of a carrier signal by passing an SAP carrier signal through a limiter amplifier, integrating the output of the limiter amplifier when a noise is large or the SAP carrier signal itself is small and comparing the integrated output with a reference voltage. CONSTITUTION:When a multichannel sound signal in America is impressed from a signal input terminal 1, the SAP carrier signal is selected by a band-pass filter 2, and then sufficiently amplified by the limiter amplifier 11. When its waveform (a) is supplied to the 1st integrator composed of a capacitor 13 and a resistance 14, it goes to a waveform (b). When a signal becomes weak, a noise is increased and the integrated waveform drops below the reference voltage V2 as shown by arrows (w), (x) and (y). Here the integrated waveform is sliced by the reference voltage V2, and the slice signal is converted into an electric current. Then, the current is integrated by a capacitor 26 and a resistance 27 to obtain a voltage V5 as shown by (c) in the figure. When this signal is compared with the voltage V3 of the terminal 25 of a comparator 24, the output terminal 28 is detected to be at a low level, and the absence and local minimum of the SAP carrier signal are detected.
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