JPH051188Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a signal level detection device, and more specifically, it is used in video tape recorders and video disc players to detect the occurrence of noise bars during variable speed playback. The present invention relates to a signal level detection device that detects a period in which the level significantly decreases over a long period of time and the S/N deteriorates.

(Prior art and its problems) Conventionally, the level detection circuit shown in Fig. 4 has been known as a means for detecting periods such as dropouts where the signal level drops significantly, the S/N ratio deteriorates extremely, and noise is generated. It is being

That is, in FIG. 4, 1 is a half-wave rectifier, which extracts only the (+) half-wave (FIG. 5-b) from the FM luminance signal Y-FM shown in FIG. 5-a. Next, COM1 is a level comparator, whose comparison input is supplied with the half wave of the FM luminance signal Y-FM obtained from the half wave rectifier 1, and the reference input is supplied with the reference voltage Vm from the reference power supply V1. It will be done. Then, a level detection output LC (FIG. 5C) is output in ranges A and B (FIG. 5A) where the level of the FM luminance signal is below the reference voltage Vm (FIG. 5A). This level detection output LC (FIG. 5C) is It is output according to the frequency of the FM luminance signal during the generation period.

Reference numeral 2 denotes an integrating circuit, which integrates the level detection output from the comparator COM1 to obtain the integrated output shown in FIG. 5D.

Next, COM2 is a level comparator, and the output from the integrating circuit 2 is used as a comparison input, while the reference voltage Vn is supplied from the reference power supply V2 to the reference input, and a comparison output is made when the level of the integrated output is in the range of Vn or higher. In other words, the detection signal D representing the period of noise occurrence
(Fig. 5 E) is output.

In addition, in Fig. 5A, the noise generation period A is an instantaneous noise generation period, that is, something like a dropout, and the S/N ratio over a long period like period B is
The deterioration period is, for example, the period during which noise bars occur during variable speed reproduction.

Therefore, the level detection output LC is obtained only during the noise generation period, and since the generation period of the level detection output LC varies depending on the length of the noise generation period, by increasing the time constant of the integrating circuit 2, A predetermined integral output can be obtained only during a long period of noise generation, and is insensitive to instantaneous noise generation periods such as dropouts.

(Fig. 5 D, E, detection signal D).

However, the main components of this detection circuit, namely the detection signal forming part, namely the integrating circuit 3 and the comparator
The configuration of COM1 and COM2 is an analog processing circuit and includes an integration circuit with a large time constant, so it is difficult to use an IC.
This makes it difficult to simplify the circuit and downsize the configuration.

In addition, both the integrating circuit and the comparator require adjustment to enable the specified operation, and the comparator requires sufficient countermeasures against DC drift in order to accurately perform the level comparison operation. This made the configuration more complicated and increased costs.

(Purpose of the invention) The present invention aims to improve the drawbacks of the prior art described above, and provides a signal level detection device for detecting a long period of noise generation, which can be integrated into an IC and eliminates adjustment. It is.

(Summary of the invention) In this invention, the primary pulse is triggered by the level detection output of the FM luminance signal obtained during the noise generation period in the reproduced video signal, and has a pulse width longer than the instantaneous noise generation period such as dropout. a first monostable multivibrator that outputs; a second monostable multivibrator that outputs a secondary pulse that is triggered at the trailing edge of the primary pulse and represents noise generation period information held in the primary pulse; A third monostable multivibrator that is triggered by the level detection output and outputs a time window pulse with a pulse width shorter than the primary pulse, and a secondary monostable multivibrator that is output from the second monostable multivibrator during the output period of the time window pulse. A signal level detection device is provided that includes a gate circuit that outputs a pulse as a detection signal for a long period of noise generation.

(Example) Before describing the example, the basic technology of the present invention will be explained first.

A single and instantaneous noise occurrence period such as a dropout is at most 2H (H...horizontal scanning period,
1H = 63.5μsec). On the other hand, long-term noise occurrence periods such as noise bars that occur during still playback or slow playback last for about 8 hours.
Focusing on this time difference, after detecting the rise of the noise generation period (level detection output), the type of noise generation period can be identified by checking the presence or absence of the level detection output again after 2H has elapsed.

Therefore, the detection information of the level detection output present at the rising edge of the noise generation period is stored and held for a period represented by 2H < T ₁ , and during that period, 2H < T ₂
A time window pulse is obtained by detecting the arrival of the level detection output in the period <T ₁ , and if the period T ₁ elapses during the duration of this time window pulse and the stored detection information is output, this The output directly indicates the existence of a long period of noise generation.

The basic technology of the present invention has been described above, and the present invention will now be specifically explained based on the illustrated embodiments.

FIG. 1 is a block diagram showing an example of a signal level detection device according to the present invention, and parts having the same functions as those of the conventional example shown in FIG. 4 are given the same reference numerals.

Now, in FIG. 1, FR is a waveform shaping circuit, which shapes the level detection output LC of the FM luminance signal, which indicates the existence of a noise generation period, and uses it as a trigger for a monostable multivibrator, which will be described later. MM1
is the first monostable multivibrator, which is triggered by the waveform-shaped level detection output LC to generate a period of instantaneous noise such as a dropout.
Outputs a primary pulse having a pulse width shorter than 8H, which is a long-term noise generation period such as a noise bar that occurs during still playback, which is longer than 2H. This primary pulse stores and holds information representing the rise of the noise generation period for a period corresponding to the pulse width. Next, MM2 is a second monostable multivibrator, which is triggered by the trailing edge of the primary pulse and outputs a secondary pulse representing the rising edge information of the noise generation period that has been stored and held. MM3 is a third monostable multivibrator, which is triggered by the level detection output LC from the waveform shaping circuit FR and outputs a time window pulse. In addition,
The pulse width of the time window pulse must be shorter than that of the primary pulse.

Next, AND is an AND gate, each input of which is supplied with a secondary pulse and a time window pulse, and outputs the secondary pulse that is input during the duration of the time window pulse.

The configuration is as described above, and its operation will be explained next using the waveform diagrams shown in FIGS. 2 and 3.

First, Fig. 2 is for explaining the case of dropout. Fig. 2A shows the level detection output LC obtained during the dropout period 2H, and MM1 is determined by the differential output at the rising time _t0 . It is triggered and then outputs a primary pulse P that continues for 4 hours (Figure 2 B). Note that this primary pulse P
The pulse width T ₁ =4H is set as an example from the range of 2H (dropout) < T ₁ as described above. Further, regarding the upper limit, it is better to set it close to 2H in order to output the detection information as soon as possible when it is confirmed that the noise generation period that has arrived is not a dropout.

At the same time, the rising level detection output LC is
It is supplied to MM3 and outputs a time window pulse R that continues for 3H as shown in FIG. 2D. Note that this pulse width T ₂ = 3H is determined by T ₂ <
This was selected as an example from the range of _T1 .
If the pulse width of time window pulse R is
This is because if T ₂ ≧T ₁ , a secondary pulse will always exist within the duration of the time window pulse R even with only the level detection output LC1 wave, regardless of the length of the noise generation period.

Now, since the noise generation period disappears at time _t1 , which is 2H after _t0 , the time window pulse R disappears at time _t2 , which is 3H after _t0 , and is not output again. Therefore, even if the secondary pulse Q is output from MM2 at time _t3 , which is 4H after _t0 , no detection signal can be obtained from the gate AND because the time window pulse R has already disappeared ( Figure 2 (e).

That is, instantaneous noise occurrence periods such as dropouts are not detected as noise occurrence periods.

On the other hand, Fig. 3 explains the case of a long-term noise generation period such as a noise bar that occurs during still playback or slow playback, and Fig. 3A shows the level detection output LC obtained during the noise generation period in this case. MM1 and MM3 are triggered by the level detection output LC at the rising time _t4 ,
MM1 outputs a primary pulse P that continues for 4H (FIG. 3B), and MM3 outputs a time window pulse R that continues for 3H. and 1 after t ₄
At the trailing edge _t5 of the next pulse P, MM2 is triggered to output the secondary pulse Q.

On the other hand, time window pulse R is 3H from t ₄
However, in this case, since the level detection output LC is still obtained even after 2H has passed from _t4 , MM3 is immediately triggered again to generate the time window pulse R.

Therefore, when the secondary pulse Q is occurring,
A time window pulse R is also output, and a detection signal D indicating that a long period of noise generation has been detected is output from the gate AND.

(Effects of the invention) According to the invention, since the signal level detection device is composed of a monostable multivibrator and a gate, it can be integrated into an IC, and no adjustment can be made.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.
2 and 3 are waveform diagrams for explaining the operation of the embodiment shown in FIG. 1, FIG. 4 is a block diagram showing the prior art, and FIG. 5 is for explaining the operation of the conventional example shown in FIG. 4. FIG. Explanation of symbols, FR...Waveform shaping circuit, MM1...
...First monostable multivibrator, MM2...Second monostable multivibrator, MM3...Third monostable multivibrator, AND...And gate.

Claims (1)

[Scope of utility model registration request] a first monostable multivibrator that outputs a primary pulse triggered by a level detection output of an FM luminance signal obtained during a noise generation period in a reproduced video signal; a second monostable multivibrator that outputs a secondary pulse; a third monostable multivibrator that outputs a time window pulse when triggered by the level detection output; and a third monostable multivibrator that outputs a time window pulse when triggered by the level detection output; When the secondary pulse arrives at the input of the 2
a gate circuit that outputs the next pulse as a detection signal of the noise generation period, the pulse width of the primary pulse is longer than the instantaneous noise generation period such as dropout, and the pulse width of the time window pulse is the pulse width of the first pulse. A signal level detection device characterized in that the pulse width is shorter than the pulse width of.