JPS60117449A - Amplitude reduction detecting circuit - Google Patents

Abstract

PURPOSE:To shorten considerably the detection time of amplitude reduction of information signals by adding the second signal, which is obtained by shifting the phase of the first signal at about 90 deg. near the carrier frequency and multiplying this phase-shifted signal, to the first signal obtained by multiplying an information signal. CONSTITUTION:An FM luminance signal (a) from an amplifier 14 is supplied to a phase shifter 20. The phase shifter 20 is so set that its shift quantity is about +90 deg. or -90 deg. near the carrier frequency of the supplied FM luminance signal (a). An FM luminance signal (c) obtained by shifting the phase of the signal (a) in the phase shifter 20 is supplied to a multiplier 21, which performs the same operation as a multiplier 15, and is multiplied by 2 to obtain a signal (d) which has a maximum amplitude for a minimum amplitude (namely, zero amplitude) of an output signal (b) of the multiplier 15. The output signal (b) of the multiplier 15 and the output signal (d) of the multiplier 21 are supplied to an adding circuit 22. If a minimum value of shorter amplitude periods of an output signal (e) of the adding circuit 22 is lower than a reference voltage E0, amplitude reduction of the FM luminance signal (a) is discriminated.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to an amplitude reduction detection circuit for detecting an amplitude reduction in an information signal such as a frequency modulation (F'M) signal or a phase modulation signal, and particularly relates to an amplitude reduction detection circuit for detecting an amplitude reduction in an information signal such as a frequency modulation (F'M) signal or a phase modulation signal. The present invention relates to an amplitude drop detection circuit suitable for use in a dropout compensation circuit of a playback system such as a recorder.

[Background of the invention]

In video signal recording and playback devices, such as video tape recorders, dropouts occur in the played video signal due to scratches on the magnetic tape, greatly deteriorating the quality of the playback image, so dropouts occur in the signal playback system. A compensation circuit is provided.

FIG. 1 is a block diagram showing the main parts of the signal reproduction system of such a conventional video tape recorder.

2 is a video head, 3 and 4 are preamplifiers, 5 is a changeover switch, 6 is a control pulse input terminal, 7 is an output terminal, and 8 is an A
A GC (automatic gain control) amplifier, 9 a delay line, 10 a dropout detection circuit, 11 a changeover switch, and 12 an output terminal.

In the figure, video heads l and 2 alternately scan and reproduce a magnetic tape (not shown), and a color video signal is reproduced l fields at a time. These color video signals are amplified by preamplifiers 3 and 4, respectively, and supplied to a changeover switch 5. The selector switch 5 is switched for each field by a control pulse from an input terminal 6, and a continuous color video signal is formed. This color video signal is F~
1 modulated luminance signal (hereinafter referred to as F'M luminance signal)
The F and M luminance signals are separated from the color video signal by a bandpass filter or the like and supplied to the AGC amplifier 8.

Further, the color video signal is supplied from the output terminal 7 to a color signal processing circuit (not shown).

The AGC amplifier 8 is necessary to compensate for level fluctuations in the FM brightness signal due to variations in characteristics of the video heads 1 and 20, and the FM brightness signal from which such level fluctuations have been removed is sent to the delay line 9. The signal is supplied to a dropout detection circuit 10 and a changeover switch 11.

These delay lines9. The dropout detection circuit 10 and the changeover switch 11 constitute a dropout compensation circuit, and the delay line 9 converts the FM luminance signal into one horizontal period (hereinafter referred to as I
In addition, the selector switch 11 normally passes all the FM luminance signals supplied from the AGC converter 8. When the dropout detection circuit 10 detects a dropout of the F'Mff1 degree signal, the selector switch 11 is switched during the dropout period to pass the IH-delayed FM luminance signal from the delay line 9. Therefore, an FM signal with dropouts removed is obtained at the output terminal 12.

The dropout detection circuit 10 includes an amplitude drop detection circuit, detects a drop in the amplitude of the FM luminance signal supplied from the AGC amplifier 8, and generates a control signal for controlling switching of the changeover switch 11.

FIG. 2 is a block diagram showing an example of such a conventional amplitude drop detection circuit τ, in which 13 is an input terminal, 14 is an amplifier, 1
5 is a multiplier, 16 is a time constant circuit, 17 is a comparator, 18 is a reference voltage source, and 19 is an output terminal.

Further, FIG. 3 is a waveform diagram of signals at each part in FIG. 2,
Signals corresponding to those in FIG. 2 are given the same reference numerals.

In FIGS. 2 and 3, F from the input terminal 13
The M luminance signal a is amplified by an amplifier 14 such as a buffer amplifier and supplied to a multiplier 15. The multiplier 15 is composed of a wave detector and the like, and generates a signal obtained by doubling the FM luminance signal a. This signal C is supplied to a time constant circuit 16 consisting of a low-pass filter or the like, which outputs a signal C representing the envelope of the signal S. This signal C is fed to a comparator 17 and a reference voltage E from a reference voltage source 18. The amplitude is compared with The comparator 17 outputs a low level (hereinafter referred to as 11) when the amplitude of the signal C is smaller than the reference voltage E0. An output signal di which is at a high level (hereinafter referred to as H''') is generated when the voltage is lower than , and this output signal d is supplied from the output terminal 19 to the selector switch 11 (FIG. 1) as a control signal.

By the way, the reason why the time constant circuit 16 is provided is that the output signal of the multiplier 15 is directly supplied to the comparator 17.
Reference voltage E even during periods of normal amplitude with no dropout of the luminance signal. There is a period Tt in which the output signal d of the comparator becomes "H" during this period and it is mistakenly determined that the amplitude has decreased. Therefore, the output signal bl of the multiplier 15 is smoothed and the period rl
+, and since the output signal C of the time constant circuit 16 must have an amplitude that exactly corresponds to the amplitude of the FMM luminance signal, needs to be set relatively large.

On the other hand, the detection time of the amplitude drop of the FMM luminance signal is mostly determined by the time constant of the time constant circuit 16. For example, as shown in FIG. 3, when the amplitude of the FMM luminance signal suddenly decreases at time t1, if the time constant of the time constant circuit 16 is set relatively large, the time constant circuit 16 will Since sudden changes cannot be followed, the amplitude of the output signal C gradually decreases from time t1 and approaches the amplitude of the signal S.

Therefore, when the comparator 17 compares the output signal C of the time constant circuit 16 with the reference voltage Eo, it is found that the amplitude of the FMM luminance signal rapidly decreases at time t; The decrease in t is detected at time t, which is delayed by the period TD from time t1. That is, F
The amplitude decrease of the M luminance signal a is detected later than the actual amplitude decrease, and moreover,
The amplitude of the signal C whose amplitude is compared with the reference voltage E at FM
The more accurately it is attempted to correspond to the luminance signal a, the longer the delay in detecting this amplitude drop (i.e., detection time TD) becomes.

When using an amplitude drop detection circuit similar to the dropout detection circuit of a home video tape recorder, the detection time for the amplitude drop of the FM luminance signal reaches approximately 80 Q n s 6 (approximately 80 Q n s 6 ), and with such a long detection time, Dropouts cannot be sufficiently removed from the FM luminance signal,
For this reason, a background noise caused by residual dropouts appeared on the playback screen, which significantly deteriorated the quality of the UMJ.

[Purpose of the invention]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an amplitude drop detection circuit which eliminates the drawbacks of the prior art described above and can significantly extend the detection time of an amplitude drop in an information signal.

[Summary of the invention]

To achieve this objective, the present invention provides a first signal obtained by multiplying an information signal by shifting the phase of the information signal by approximately 90° in the vicinity of its carrier frequency; By adding the signals of the first . The present invention is characterized in that a decrease in the amplitude of the information signal is detected based on the added signal of the second signal.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a block diagram showing an embodiment of the amplitude drop detection circuit according to the present invention, in which 20 denotes a phase shifter.

21 is a multiplier, 22 is an adder circuit, and parts corresponding to those in FIG. 2 have the same symbols.

Further, FIG. 5 is a waveform diagram of signals at each part in FIG. 4,
Signals corresponding to FIG. 4 are given the same symbols.

In FIGS. 4 and 5, amplifier 14. Multiplier 15
is amplifier 14. of FIG. The multiplier 15 operates in the same manner as the multiplier 15 in FIG.
A signal having the same waveform as the output signal of No. 5 is obtained.

Additionally, the FMM luminance signal from amplifier 14 is supplied to phase shifter 20 . The phase shift amount of the phase shifter 2o is the supplied FMM
Approximately 190 degrees or 190 degrees near the carrier frequency of the luminance signal
It is set so that the

The F'M luminance signal C phase-shifted by the phase shifter 20 is transmitted to the multiplier 1.
The signal d is supplied to a multiplier 21 which performs the same operation as that of 5, and is multiplied by 2, thereby obtaining a signal d which has a maximum amplitude at the minimum amplitude (ie, zero amplitude) of the output signal of the multiplier 15.

The output signal d of the multiplier 15 and the output signal d of the multiplier 21 are supplied to an adder circuit 22, and the adder circuit 22 outputs the input signal with the larger amplitude of these two input signals. Then, these two input signals are logically added. For this purpose, the adder circuit 22 operates for a predetermined period (period Tt in FIG. 3) including the lowest amplitude portion of the output signal of the multiplier 15.
is interpolated using the output signal d of the multiplier 21.
High output. The steady amplitude period of this signal e (i.e. FM
The minimum amplitude V min of the period corresponding to the period in which there is no decrease in the amplitude of the luminance signal +l + 7^ is 1 of the maximum amplitude Vmax.
/r2 times.

The output signal e of the adder circuit 22 is supplied to the comparator 17,
Reference voltage E from reference voltage source 18. The amplitude is compared with

Reference voltage E. is set lower than the lowest amplitude in the steady amplitude period of the signal e, and for this reason, the comparator 17 becomes ゛L par during the steady amplitude period of the signal e, °゛H'' during the low amplitude period of the current signal e
A signal f is output.

In this way, a decrease in the amplitude of the FMq degree signal a is detected, and at least the lowest value of the low amplitude period of the signal e is the reference voltage E. If it is lower than , it is determined that the amplitude of the F'M luminance signal a has decreased. Therefore, the maximum amplitude of signal e is (2
Eo (However, Eo < Vmln = Vmax/(2
Therefore, when (2"'o<Vmax) yo') becomes low, it is determined that the amplitude of the FMM luminance signal has decreased. Also, the amplitude of the FM luminance signal has decreased (at time 1. 7) until it is detected (time 12'), that is, the detection time TD is expressed in phase angle as cos (Eo/Vrnax), which is "/4" of the carrier wave of the FM luminance signal a. An amplitude drop is detected within a wavelength.

A specific example of the circuit configuration of this embodiment is shown in FIG. In the same figure, parts corresponding to each block in FIG. 4 are surrounded by broken lines and given corresponding symbols.

In FIG. 6, when the output signals of multipliers 15 and 21 are added and supplied to comparator 17, there is a delay due to the input capacitance of transistor Q that constitutes comparator 17, but this amount of delay is usually ignored. It is possible. Also, the 6th
The circuits shown in the figure all have a configuration that can be integrated into a monolithic integrated circuit, although it depends on the frequency band used.

FIG. 7 is a block diagram showing another embodiment of the amplitude drop detection circuit according to the present invention, in which 23 is a time constant circuit, and parts corresponding to those in FIG. 4 are given the same reference numerals.

Further, FIG. 8 is a waveform diagram of signals at each part in FIG. 7,
Reference numerals corresponding to those in FIG. 7 are given the same reference numerals.

This embodiment differs from the fourth embodiment only in that a time constant circuit 23 is provided between the adder circuit 22 and the comparator circuit 17. Therefore, from the adder circuit 22, a signal having the same waveform as the output signal e (FIG. 5) of the adder circuit 22 in FIG. 4 is obtained.

Now, in FIGS. 7 and 8, the output signal C of the adder circuit 22 is supplied to the time constant circuit 23, and a signal C representing its envelope is obtained. The amplitude of this signal C is compared with the reference voltage Ho in a comparator 17, and the amplitude of the signal C is the reference voltage E.

A signal d is obtained which becomes "L" when higher than the reference voltage E and "H" when lower than the reference voltage E.
This represents a decrease in the amplitude of the M luminance signal a.

The time constant circuit 23 includes a transmission system including a multiplier circuit 15 and a phase shifter 20 . This is provided to compensate for variations in characteristics between the multiplier 21 and the transmission system, and serves to eliminate the amplitude difference between two signals passing through each transmission system. In this case, the time constant of the time constant circuit 230 determines the detection time T
is the detection time TD' (
(Fig. 5), but the time constant of this time constant circuit 23 can be set sufficiently small, and for this reason,
This is sufficiently shorter than the detection time TD of the conventional amplitude drop detection circuit shown in FIG. 2 when the amplitude changes as shown by the broken line C' in FIG.

According to this embodiment, when this was used in a dropout detection circuit for a home video tape recorder, the detection time could be reduced from the conventional approximately 800 nsec to approximately 5QQnsec.

Here, in general, as a time constant circuit, in addition to the usual low-pass filter type, there is also a time constant circuit in which the rise time constant and the fall time constant are different. When detecting an amplitude drop, the sum detection time is shortened when the falling time constant is small, but the time constant circuit 23 of this embodiment uses a time constant whose falling time constant is smaller than the rising time constant. It may also be a circuit, and the detection time can be similarly shortened.

A specific example of the circuit configuration of this embodiment is shown in FIG. In the figure, parts corresponding to each block in FIG. 8 are surrounded by broken lines and given corresponding symbols. It goes without saying that all of this circuitry can be integrated into a monolithic integrated circuit.

Although the embodiments of the present invention have been described above, it is clear that these embodiments can be modified without departing from the scope of the claims. For example, in both embodiments, the reference voltage E of the reference voltage source 18. does not need to be fixed, and the reference voltages for detecting the start and end of the amplitude decrease may be different to provide a hysteresis effect. When these embodiments are used in a dropout detection circuit for a video tape recorder for mobile use, the reference voltage of the reference voltage source 18 is raised at the end of the amplitude drop than at the beginning, thereby preventing dropout at the end of the dropout. The only thing that can be done is to eliminate the residual.

Further, as an input signal supplied to the input terminal 13, F
By passing the FM luminance signal a through a limiter circuit normally provided in the M signal processing system, it becomes possible to detect the amplitude drop more efficiently.

It should be noted that the present invention can be applied not only to detecting a decrease in the amplitude of an FM luminance signal, but also to other information signals modulated to F, and can be similarly applied to information signals that are phase modulated. Applicable.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to significantly shorten the detection time while preventing erroneous detection of an amplitude drop in an information signal, and the amplitude drop detection has an excellent function except for the drawbacks of the above-mentioned conventional technology. The circuit can be provided.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the main parts of a signal reproduction system of a conventional video tape recorder, Fig. 2 is a block diagram showing an example of an amplitude drop detection circuit used in the dropout detection circuit of Fig. 1, and Fig. 3. are the waveform diagrams of the signals at each part in Figure 2, and Figure 4.
The figure is a block diagram showing an embodiment of the amplitude drop detection circuit according to the present invention.
The figure is a circuit diagram showing one specific circuit of Fig. 4, Fig. 7 is a block diagram showing another embodiment of the amplitude drop detection circuit according to the present invention, Fig. 8 is i#, and waveforms of signals at various parts in Fig. 7. figure. FIG. 9 is a circuit diagram showing one specific circuit of FIG. 7. 15... Multiplier, 17... Comparator, 1
8... Reference voltage source, 20... Phase shifter,
21...multiplier, 22...addition circuit,
23...Time constant circuit. o 4 o”

Claims (1)

[Claims] An amplitude drop detection circuit that detects a drop in the amplitude of an information signal, a first multiplier that multiplies the information signal, a phase shifter that shifts the phase of the inert* signal by approximately 90 degrees near its carrier frequency; a second multiplier for multiplying the output signal of the phase shifter; An adder circuit that adds the output signals of the second multiplier is provided, and by detecting that the amplitude of the output signal of the adder circuit has become equal to or less than a preset threshold, the amplitude of the information signal can be increased. An amplitude drop detection circuit characterized in that it is configured to be able to detect a drop.