JPH0234108B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an amplitude reduction detection circuit that detects an amplitude reduction of an information signal such as a frequency modulation (FM) signal or a phase modulation signal, and is particularly applicable to a video tape recorder, etc. The present invention relates to an amplitude drop detection circuit suitable for use in a dropout compensation circuit for a reproduction system.

[Background of the invention]

In video signal recording and reproducing devices, such as video tape recorders, dropouts occur in the reproduced video signal due to scratches on the magnetic tape, greatly deteriorating the quality of the reproduced image, so dropouts occur in the signal reproducing 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.
1 and 2 are video heads, 3 and 4 are preamplifiers, and 5
is a changeover switch, 6 is a control pulse input terminal, 7 is an output terminal, 8 is an AGC (automatic gain control) amplifier, 9
is a delay line, 10 is a dropout detection circuit, and 11 is a dropout detection circuit.
1 is a selector switch, and 12 is an output terminal.

In the figure, video heads 1 and 2 alternately reproduce and transmit a magnetic tape (not shown), and a color video signal is reproduced one field at a time. These color video signals are amplified by preamplifiers 2 and 4, respectively, and supplied to a changeover switch 5. The changeover 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 an FM-modulated luminance signal (hereinafter referred to as FM luminance signal)
The FM luminance signal is a mixed signal with a color signal, and the FM luminance signal is separated from the color video signal using a bandpass filter or the like and is supplied to the AGC amplifier 8. Further, the color video signal is supplied from the input terminal 7 to a color signal processing circuit (not shown).

The AGC amplifier 8 compensates for level fluctuations in the FM brightness signal due to variations in characteristics of the video heads 1 and 2, and the FM brightness signal from which such level fluctuations have been removed is sent to the delay line 9, dropout detection circuit 10, and changeover switch. 11.
These delay line 9, dropout detection circuit 10, and changeover switch 11 constitute a dropout compensation circuit, and the delay line 9 delays the FM luminance signal by one horizontal period (hereinafter referred to as 1H) and supplies it to the changeover switch 11. In addition, the changeover switch 11 is normally
The FM luminance signal supplied from the AGC amplifier 8 is passed through. When the dropout detection circuit 10 detects a dropout of the FM luminance signal, the selector switch 11 is switched during the dropout period to pass the 1H delayed FM luminance signal from the delay line 9. Therefore, an FM signal from which dropout has been 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 switches the changeover switch 11.
A control signal is generated to perform switching control.

FIG. 2 is a block diagram showing an example of such a conventional amplitude drop detection circuit, in which 13 is an input terminal;
4 is an amplifier, 15 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, and signals corresponding to those in FIG. 2 are given the same symbols.

2 and 3, the FM luminance signal a from the input terminal 13 is input to an amplifier 1 such as a buffer amplifier.
4 and supplied to a multiplier 15. The multiplier 15 consists of a detector, etc., and divides the FM luminance signal a into 2
A multiplied signal b is generated. This signal b 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 b.
This signal c is supplied to a comparator 17 and compared in amplitude with a reference voltage E ₀ from a reference voltage source 18 . The comparator 17 has a low level (hereinafter referred to as "L") when the amplitude of the signal c is higher than the reference voltage _E0 , and
High level ( _hereinafter referred to as
This output signal d is output from the output terminal 19 to the selector switch 11.
(FIG. 1) as a control signal.

Incidentally, the reason why the time constant circuit 16 is provided is that if the output signal b of the multiplier 15 is directly supplied to the comparator 17, the reference voltage E will be less than or equal to the reference voltage _E0 even during a normal amplitude period without dropout of the FM luminance signal a. There is a period T _l during which the output signal d of the comparator becomes "H" and it is erroneously determined that the amplitude has decreased.
This is to smooth the output signal b of the multiplier 15 to eliminate the period T l, and also to eliminate the period T _l .
6 must have an amplitude that exactly corresponds to the amplitude of the FM luminance signal a, it is necessary to set the time constant of the time constant circuit 16 relatively large.

On the other hand, the detection time for the amplitude drop of the FM luminance signal a is determined mostly by the time constant of the time constant circuit 16. For example, as shown in Figure 3, FM
When the amplitude of the luminance signal a suddenly decreases at time _t1 , if the time constant of the time constant circuit 16 is set relatively large, the time constant circuit 16 cannot follow this sudden change in amplitude, so its output signal The amplitude of c is
The amplitude gradually decreases from time _t1 and approaches the amplitude of signal b.

Therefore, when the output signal c of the time constant circuit 16 and the reference voltage _E0 are compared in the comparator 17,
Although the amplitude of the FM luminance signal a suddenly decreases at time _t1 , the comparator 17 detects this decrease in amplitude at time _t2 , which is delayed by the period T _D from time _t1 . . That is, the amplitude drop of the FM luminance signal a is detected later than the actual amplitude drop, and the comparator 17 detects the reference voltage.
The more accurately the amplitude of the signal c whose amplitude is compared with E ₀ is made to correspond to the FM luminance signal a, the longer the delay in detecting this amplitude drop (ie, the detection time T _D ) becomes.

When using the 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 is
With such a long detection time of approximately 800 ns, dropout cannot be sufficiently removed from the FM luminance signal, and as a result, noise due to residual dropout appears on the playback screen, significantly degrading the image quality. Ta.

[Purpose of the invention]

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art,
An object of the present invention is to provide an amplitude drop detection circuit that can significantly shorten 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° near its carrier frequency, and furthermore, a second signal obtained by multiplying the information signal by a phase shift of approximately 90° near its carrier frequency. By adding the signals, the low amplitude period of the first signal corresponding to a predetermined period including the zero crossing point of the information signal is interpolated with the second signal, and the low amplitude period of the first signal and the second signal are The feature is that a decrease in the amplitude of the information signal is detected based on the added 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 is a phase shifter;
21 is a multiplier, 22 is an adder circuit, and parts corresponding to those in FIG. 2 are given the same reference numerals.

Further, FIG. 5 is a waveform diagram of signals at each part in FIG. 4, and signals corresponding to those in FIG. 4 are given the same reference numerals.

In FIGS. 4 and 5, the amplifier 14 and the multiplier 15 operate in the same way as the amplifier 14 and the multiplier 15 in FIG. A signal b having the same waveform as the output signal b of No. 15 is obtained.

Further, the FM luminance signal a from the amplifier 14 is supplied to the phase shifter 20. The amount of phase shift of the phase shifter 20 is approximately + in the vicinity of the carrier frequency of the supplied FM luminance signal a.
It is set to be 90 degrees or -90 degrees. The FM luminance signal c phase-shifted by the phase shifter 20 is supplied to a multiplier 21 which operates in the same way as the multiplier 15.
After being multiplied, a signal d having the maximum amplitude at the minimum amplitude (ie, zero amplitude) of the output signal b of the multiplier 15 is obtained.

The output signal b 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. , these 2
Logically add two input signals. For this purpose, the adder circuit 22 outputs a signal e obtained by interpolating the predetermined period (period T _l in FIG. 3) including the lowest amplitude portion of the output signal b of the multiplier 15 with the output signal d of the multiplier 21. do. The steady amplitude period of this signal e (i.e.
The minimum amplitude V _nio during the period corresponding to the period in which the amplitude of the FM luminance signal does not decrease is 1/ of its maximum amplitude V _nax .
√2 times.

The output signal e of the adder circuit 22 is supplied to a comparator 17 and compared in amplitude with a reference voltage E ₀ from a reference voltage source 18 . The reference voltage E ₀ is set lower than the lowest amplitude during the steady amplitude period of the signal e, and therefore, the comparator 17 becomes "L" during the steady amplitude period of the signal e, causing the amplitude of the FM luminance signal a to decrease. A signal f which becomes "H" during the low amplitude period of the accompanying signal e is output.

In this way, a decrease in the amplitude of the FM brightness signal a is detected, and if at least the lowest value of the low amplitude period of the signal e is lower than the reference voltage _E0 , it is determined that the amplitude of the FM brightness signal a has decreased. . Therefore,
The maximum amplitude of signal e is √2E ₀ (however, E ₀ <V _nio =
Since V _nax /√2, when the amplitude becomes lower than (√2E ₀ <V _nax ), it is determined that the amplitude of the FM luminance signal a has decreased. Also, this is detected after the amplitude of the FM luminance signal decreases (time t ₁ ′) (time
t ₂ '), that is, the detection time T _D is expressed in phase angle as cos ^-1 (E ₀ /V _nax ), and within 1/4 wavelength of the carrier wave of the FM luminance signal a. An amplitude drop is detected.

A specific example of the circuit configuration of this embodiment is shown in FIG. In addition, 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 the multipliers 15 and 21 are added and supplied to the comparator 17, there is a delay due to the input capacitance of the transistor Q constituting the comparator 17, but the amount of delay is Usually can be ignored. Further, the circuit shown in FIG. 6 has a structure 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, and the same reference numerals are given to the symbols corresponding to those in FIG. 7.

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, addition circuit 2
2, the output signal e of the adder circuit 22 in FIG.
A signal b having the same waveform as (FIG. 5) is obtained.

Now, in FIGS. 7 and 8, the output signal b of the adder circuit 22 is supplied to the time constant circuit 23,
A signal c representing the envelope is obtained. The amplitude of this signal c is compared with the reference voltage E ₀ by a comparator 17,
“L” when the amplitude of signal c is higher than the reference voltage E ₀ , and “H” when it is lower than the reference voltage E ₀
A signal d is obtained. This signal d represents a decrease in the amplitude of the FM luminance signal a.

The time constant circuit 23 is provided to compensate for variations and fluctuations in characteristics between the transmission system using the multiplier circuit 15 and the transmission system using the phase shifter 20 and multiplier 21. It functions to remove the amplitude difference between two signals. In this case, time constant circuit 2
3, the detection time T _G ″ is longer than the detection time T _D ′ (Fig. 5) in the embodiment shown in Fig. 4, but the time constant of this time constant circuit 23 is sufficiently small. Therefore, the detection time T D is sufficiently shorter than the detection time T _D of the conventional amplitude drop detection circuit shown in Fig. 2 when the amplitude changes as shown by the broken line c' in Fig. 8. It turns out.

According to this example, when this was used in a dropout detection circuit for a home video tape recorder, the detection time was 800 ns, which was conventionally about 800 ns.
It was possible to shorten the time to about 600nsec.

Here, in general, as a time constant circuit, in addition to the usual low-pass filter type, there is also a type in which the rise time constant and the fall time constant are different. When detecting an amplitude drop, the smaller the falling time constant, the shorter the detection time, 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 the ninth section.
As shown in the figure. 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 either embodiment, reference voltage source 18
The reference voltage E ₀ does not need to be fixed, and the reference voltage 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 home video tape recorder, by increasing the reference voltage of the reference voltage source 18 at the end of the amplitude drop rather than at the beginning, the residual dropout at the end of the dropout can be eliminated. It is possible.

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

Note 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 FM-modulated information signals, and is similarly applicable to phase-modulated information signals. It is.

〔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 drawings]

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. FIG. 4 is a block diagram showing an embodiment of the amplitude drop detection circuit according to the present invention. FIG. 5 is a waveform diagram of signals at various parts in FIG. 4. FIG. FIG. 4 is a circuit diagram showing one specific circuit; FIG. 7 is a block diagram showing another embodiment of the amplitude drop detection circuit according to the present invention; FIG. 8 is a waveform diagram of signals at various parts in FIG. 7;
FIG. 9 is a circuit diagram showing one specific circuit of FIG. 7. 15... Multiplier, 17... Comparator, 18... Reference voltage source, 20... Phase shifter, 21... Multiplier, 2
2...addition circuit, 23...time constant circuit.

Claims (1)

[Claims] 1. In 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 information signal by approximately 90 degrees near its carrier frequency; A second multiplier that multiplies the output signal of the phase shifter and an adder circuit that adds the output signals of the first and second multipliers are provided, and the amplitude of the output signal of the adder circuit is set in advance. An amplitude drop detection circuit characterized in that the amplitude drop detection circuit is configured to be able to detect a drop in the amplitude of the information signal by detecting that the amplitude falls below a set threshold value.