JPS61147604A - Fm demodulation system - Google Patents

Abstract

PURPOSE:To obtain a stable video signal output by taking a dropout caused in an FM signal as a higher level than the pedestal level of the video signal not as a large amplitude noise when the dropout period is demodulated. so as to attain easy and stable synchronizing signal separation. CONSTITUTION:A voltage comparator 2 outputs an FM signal pulse train only when an FM signal level at an input terminal 1 is higher than the preset level and outputs no pulse during dropout period. A pulse generator 3 generates a pulse at a prescribed width at both leading edge and trailing edge of the pulse train outputted from the voltage comparator 2 as shown in figure (c). Then a pulse period detection circuit 4 detects the period of the pulse generated by the pulse generator 3 and when the period is longer than the predetermined period, the circuit 4 generates a pulse by itself. That is, the circuit 4 generates a pulse only during dropout period as shown in figure (d). When an integration circuit 6 demodulated an FM signal, the pulse width generated in the circuit is selected so that the level of the dropout period is higher than the pedestal level of the demodulated video signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an improvement in an FM demodulation method for demodulating an FM signal modulated by a video signal.

[Technical background of the invention and its problems]

When demodulating an FM signal modulated by a video signal, F
If there is a drop or dropout in the M signal level (hereinafter referred to as dropout), large-amplitude noise will be mixed into the demodulated video signal, which will prevent stable synchronization signal output and cause problems such as pedestal clamp failure and AGC operation. This causes various malfunctions such as malfunctions, making it impossible to obtain a stable video signal.

For this reason, conventionally the envelope of the FM signal is detected,
Efforts have been made to prevent malfunctions, such as by detecting dropouts and prohibiting the output of synchronization signals, but the circuitry becomes complicated and the effectiveness is not sufficient.

[Purpose of the invention]

The confidence of the present invention is that it is a relatively simple circuit for demodulating the FM signal modulated by the video signal, and it enables stable synchronization signal separation even if there is a dropout in the FM signal, resulting in stable synchronization signal separation. It is an object of the present invention to provide an FM demodulation method suitable for use in a recording and reproducing apparatus capable of obtaining a demodulated video signal.

[Summary of the invention]

The present invention detects the level of the FM signal reproduced from a recording/reproducing device etc. using a voltage comparator, and generates an FM signal pulse train only when the level of the FM signal exceeds a certain value, thereby eliminating noise during the dropout period. At the same time, the pulse generator generates pulses at both the rising edge and the falling edge of the pulse train output from the voltage comparator, and the period of this pulse is detected by the pulse period detection circuit to detect dropout. If there is a
The pulse width of the pulse generator and the pulse width of the period detection circuit are selected so that the period of the M signal is a regular video signal, and the dropout period is a constant level higher than the pedestal level of the video signal. An integrator integrates the logical sum of the pulses generated by the pulse generator and the pulses generated by the pulse period detection circuit during the dropout period.
This is a demodulation method that demodulates video signals.

According to this method, the dropout period does not become a large-amplitude noise and becomes a constant level equal to or higher than the pedestal level, so that synchronization signal separation can be performed stably.

[Embodiments of the invention]

An embodiment in which the present invention is applied to a magnetic recording/reproducing apparatus (VTR) will be described. FIG. 1 is a block diagram outlining the invention.

The FM signal read out from the magnetic transducer passes through a preamplifier, an equalizer, etc. (not shown), and then enters the input terminal 1 of the demodulator. This FM signal includes so-called dropouts, where the signal level is partially reduced or deleted due to scratches on the recording medium or adhesion of foreign matter.

The voltage comparator 2 outputs a pulse train of the FM signal only when the level of the FM signal at the input terminal 1 is higher than a preset level, and does not output pulses during the dropout period. FIG. 2(a) shows that the FM signal (b) at the input terminal 1 is the output waveform of the voltage comparator 2, and no pulse train is output during the dropout period.

A specific example of a voltage comparator is shown in FIG. Figure 3 7, 8
is a voltage comparator, which outputs a pulse when the level of the FM signal is higher than the reference voltage range in the positive direction and negative direction set by 9°10. The output of the voltage comparator 7 is connected to the set terminal of the R8 flip-flop 11, and the output of the voltage comparator 8 is connected to the reset terminal of the R8 flip-flop 11. By appropriately setting the reference voltages 9 and 10, the drop Output during the out period is prohibited, and FM signal pulses with good symmetry can be output.

FIG. 1 is a pulse generator, which generates pulses of a constant width at both the rising edge and the falling edge of the pulse train output from the voltage comparator 2, as shown in FIG. 2(C).

By doubling the frequency of the FM signal,
This reduces spurious interference due to the lower sideband of the FM signal, and improves detection accuracy in a pulse period detection circuit, which will be described later. Furthermore, by generating pulses of a constant width at both edges of the pulse train, it is possible to demodulate a video signal using an integrator, which will be described later.

FIG. 14 shows a pulse period detection circuit, in which the pulse generator 3
It is configured to detect the period of the pulse generated by the sensor, and to generate a pulse itself if the period is longer than a predetermined period.

The predetermined period mentioned here is a period in which the period T satisfies T ) 1 /2 fcmin , where fcmin is the lowest frequency among the carrier frequencies of the FM signal, and as shown in Fig. 2(d). The period detection circuit generates pulses only during the dropout period.

Further, the width of the pulse generated here is selected so that when the FM signal is demodulated by an integrating circuit to be described later, the level during the dropout period is higher than the pedestal level of the demodulated video signal.

A specific example of the period detection circuit is shown in FIG.

In FIG. 4, the agate 13 is a retriggerable monomulti 14 which operates at the rising edge of a pulse, and the NOR gate 15 is a delay line. The pulses from the pulse generator of FIG. 1 are connected to one side of the NOR gate 12 16 . IJ) As mentioned above, the time constant of the IJ gable mono multi 13 is selected so that the mono multi is always in operation during the normal FM signal period, and the Q output is high level/L/.

Q output is at low level. In this state, NOR
The output of the gate 14 is at a low level, and the retriggerable monomulti 13 operates only on the falling edge of the input terminal 16. When a dropout occurs in the FM signal, the signal at the input terminal 16 is cut off, and the IJ gable mono multi returns to the non-operating state after a certain period of time determined by the time constant, and the Q output becomes low level and the Q output becomes low level. Becomes a high level. At this moment, the NOR gate 14 and the delay line 15 generate a bipulse at the output of the NCR gate 14,
Retriggerable mono multi 1 at the falling edge of this pulse
3 is triggered and becomes operational again.

As long as this pulse does not enter the input terminal 161 of the NOR gate 12, that is, during the dropout period, the circuit will run free with the pulse fed back from the output of the retriggerable monomulti 13, and the retriggerable signal will be activated as the Q output i. A pulse with a pulse width determined by the time constant of the monomulti and the delay time of the delay line 15 is generated.

When the dropout period ends and a pulse appears at the input terminal 16 again, the retriggerable monomulti 13 is in a constant state of operation, and the Q output pulse disappears and becomes a low level. When a dropout occurs in the FM signal in this manner, the period detection circuit generates a free-running pulse.

FIG. 15 shows an OR circuit, which calculates the OR between the pulse generator 3 and the period detection circuit 4 and sends the result to the integrator 6, as shown in FIG. 2(e).

As mentioned earlier, the integrator 6 integrates the logical sum of the pulse generated from the FM signal and the output pulse of the synchronization detection circuit, and calculates the FM signal.
During the period when the signal is normal, it is demodulated to a regular video signal, and during the dropout period, it is demodulated to a level higher than the pedestal level of the video signal.

As described above, according to the present invention, even if a dropout occurs in the FM signal, when that period is demodulated, it does not become a large-amplitude noise and becomes a level higher than the pedestal level of the video signal. Separation is easily and stably performed, and as a result, stable video signal output can be obtained.

FIG. 5 is a diagram showing the state of the demodulated video signal when dropout occurs, (a) shows the case where the FM signal is demodulated as is including the dropout period, and (b) shows the demodulation method according to the present invention. This is the case when demodulating with .

[Brief explanation of the drawing]

Figure 1 is a block diagram showing an overview of the present invention, and Figure 2 is a block diagram showing the outline of the present invention.
3 is a diagram showing a specific example of the voltage comparator in the present invention, and FIG. 4 is a specific example of the pulse period detection circuit in the present invention. FIG. 5 is a diagram for explaining the present invention in detail. 1 is an FM signal input terminal, 2 is a voltage comparator, 3 is a pulse generator, 4 is a pulse period detection circuit, 5 is an OR circuit, and 6 is an integrator. Agent Patent Attorney Noriyuki Chika (one idiot) Fake
-- 111] Kori

Claims (1)

[Claims] An FM demodulator that demodulates an FM signal modulated by a video signal includes a voltage comparator that detects the FM signal level, and a pulse generator that generates pulses at both rising and falling edges of the output signal of the voltage comparator. a pulse period detection circuit configured to generate a pulse by itself when the pulse period of the output signal of the pulse generator exceeds a certain value; An FM demodulation method characterized in that a logical sum of pulses from a period detection circuit is demodulated into a video signal through an integrator.