JPH0152921B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an FM demodulator that prevents miscounts in a monomulti type FM demodulator.

Conventional configuration and its problems Figure 1 shows a conventional FM demodulator. Further, waveforms of each part are shown and explained in FIG. To terminal 1 in Figure 1
The FM signal (Fig. 2a) is added, the waveform is shaped by the limiter amplifier 2 (Fig. 2b), and the next differentiation circuit 3
Differentiate the rising and falling waveforms of the limiter output (Fig. 2 c) and apply the signal to the trigger terminal of the monomulti, and when the trigger is applied, the time t determined by the capacitor 6 and resistor 18 in Fig. 1 When there is only one pulse (FIG. 2d), its output is amplified by transistors 11 and 13, and integrated by a resistor 15 and a capacitor 16 to obtain a demodulated output as shown in FIG. 2e. The mono multi output pulse width is constant, but when the modulation signal level is high, the FM
The frequency of the carrier wave of the signal becomes high, and as a result, the output pulse density of the monomulti becomes high, and its integrated waveform becomes a high level. Conversely, if the density is low, the integrated waveform will be at a low level and will be FM demodulated.

Here, in part A of FIG. 2, the CD level may fluctuate due to some influence on the FM signal a (in the case of TV audio multiplex broadcasting, overmodulation of the main signal affects the sub signal, etc.). Then, the limiter amplifier does not operate and there is no output. As a result, no differential pulses are produced, nor is there any monomultiple output. If there is no mono multi output, the demodulated output will be OV. This is much lower than the unmodulated level, resulting in a loud and harsh noise to the audible sense.

OBJECTS OF THE INVENTION The present invention eliminates the above-mentioned problems, and aims to provide an FM demodulator that does not generate noise even if an abnormality occurs in the FM signal.

Structure of the Invention The present invention provides FM demodulation using monomulti
The demodulator is characterized by being equipped with a circuit that detects the voltage of a capacitor that controls the width of the mono multi pulse generated, and a circuit that forcibly triggers the mono multi using the detection output.
Even when the FM signal is abnormal, the mono multi can be operated reliably, and abnormal noise can be prevented.

DESCRIPTION OF EMBODIMENTS FIG. 3 shows an embodiment of the present invention. Further, FIG. 4 shows the operating voltage waveform.

When a differential output occurs at point A in FIG.
It is added as a trigger input to a monomulti composed of capacitor 6 and resistors 5, 8, 9, and 18. Then, transistor 4 becomes conductive, current flows through capacitor 6 in the direction of charging from resistor 18, voltage _V1 rapidly drops in the negative direction, transistor 10 becomes non-conductive, and the collector voltage further conducts transistor 4. state. After that, capacitor 6 continues to be charged from resistor 18.
When the potential of V ₁ rises, transistor 10 becomes conductive (this period is the monomultiple output pulse width). Then, the transistor 4 becomes non-conductive and the capacitor 6 is now charged through the resistor 5. This charging is continued until the next differential output is produced.

Therefore, when the limiter output disappears, as shown at point B in FIG. 4, no differential pulse is generated. Then,
Capacitor 6 continues to be charged. Here, the comparator (transistors ₂₂ and ₂
4 and a resistor 23), and the output (collector of transistor 22) current is applied to the trigger input of the monomulti to operate the monomulti. As a result, the monomulti operates even when the FM signal is abnormal, and the Px output shown in Figure 4 e is obtained.
The demodulated output requires only a slight level fluctuation (in the conventional case, the output is O _v ). As a result, it is possible to greatly reduce the harsh noises caused by overmodulation at broadcast stations and the like.

Effects of the Invention As described above, according to the present invention, an abnormality in the FM signal is detected from the voltage across the capacitor constituting the monomulti, and when an abnormality occurs, the monomulti is triggered by the detection output of the detection means. , the demodulated output requires only a slight level fluctuation, and abnormal noise can be prevented from occurring.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional FM demodulator, FIG. 2 is a waveform diagram for explaining the operation of FIG. 1, FIG. 3 is a circuit diagram of an FM demodulator according to an embodiment of the present invention, and FIG.
This figure is a waveform diagram for explaining the operation of FIG. 3. 2... Limiter amplifier, 3... Differential circuit, 4,
10, 6...Transistors and capacitors constituting a mono-multi, 22, 24, 23... transistors and resistors constituting a detection means for detecting the voltage across the capacitor 6.

Claims (1)

[Claims] 1. A means for shaping the waveform of an FM signal, a means for differentiating the rise and fall of the output pulse signal of the waveform shaping means, and a mono-multivibrator triggered by the differential output, and a pulse generated by the mono-multivibrator. A detection means for detecting the voltage across the capacitor for controlling the width is provided, and the mono-multivibrator is forcibly triggered by the output of the detection means when the capacitor is charged to a predetermined level or more. FM demodulator.