JP2674401B2 - Image playback device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reproducing device having a noise reduction circuit.

[0002]

2. Description of the Related Art In a VTR (video tape recorder), the S / N at the time of reproduction and the frequency characteristic relating to the resolution (hereinafter referred to as f characteristic) become problems. If the S / N is bad, the screen feels rough, and if the f characteristic is bad, the resolution drops and it becomes impossible to faithfully reproduce a fine boundary line. Generally, if the f characteristic is improved and the video signal is reproduced up to a high frequency region, the absolute amount of high frequency noise also increases.
Under the condition that the same tape is used, improving f characteristics and improving S / N are contradictory requirements.

The noise components include tape sliding noise and circuit noise, and there are the following two typical techniques for reducing them. One is a technique of increasing the gain in the high frequency region during recording (pre-emphasis) and recording the gain to attenuate (de-emphasis) during reproduction. However, there are many noises in the VTR reproducing system that cannot be removed even by this. For this reason, a method called non-linear emphasis, which detects the strength of the image signal and changes the emphasis level, is also used. The other technique is to remove noise only in the playback system. By checking the correlation between the current signal and the immediately preceding signal using a field memory or 1H delay line, noise components are removed by removing uncorrelated signal components. Is a noise reduction system using a so-called delay line (delay element).

[0004]

There is a noise component that cannot be removed by the former noise reduction method by pre-emphasis and de-emphasis. The latter noise reduction method using the correlation of image information is a very effective means for a video signal with little motion on the screen,
If the image moves violently, there is a drawback that the correlation is reduced and the effect of noise reduction is halved. In order to solve this drawback, a method of detecting the movement of an image and changing the noise reduction level has been proposed. However, the noise reduction method using the correlation requires an expensive circuit such as a field memory, which inevitably increases the cost of the device.

Therefore, an object of the present invention is to provide an image reproducing apparatus capable of reducing high frequency noise with a simple circuit configuration.

[0006]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a reproducing head for obtaining a reproduced FM modulated signal by relative scanning movement with a recording medium on which an image signal is FM modulated and recorded. A demodulator for demodulating the reproduced FM-modulated signal obtained from the reproducing head, a voltage control low-pass filter connected to the demodulator, and a period for which the reproduced FM-modulated signal is in a specific frequency range. Between the playback head and the demodulator
A specific frequency section detection filter connected to the line branched from, and detects the output of the specific frequency section detection filter, the detection output to reduce the cutoff frequency or high frequency of the voltage control low-pass filter The present invention relates to an image reproducing device having means for controlling a voltage control filter.

[0007]

[Operation] According to the present invention, the high frequency noise component contained in the image signal in the specific frequency range can be removed in real time.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a VTR according to an embodiment of the present invention will be described with reference to FIGS. In FIG. 1, a pair of video heads 1 and 2 as signal conversion heads are rotary drums 3.
The recording medium magnetic tape 4 is scanned while rotating together with the recording medium to record or reproduce image signals. A video signal is FM-modulated and recorded on the tape 4 in the NTSC system, for example. When the relative scanning movement between the tape 4 and the heads 1 and 2 occurs, reproduced image signals are obtained from the heads 1 and 2.

The output stages of the heads 1 and 2 are provided with well-known rotary transformers 5 and 6, preamplifiers 7 and 8, and a switcher 9 in order. The output line 10 of the switcher 9 from which the reproduced FM modulation signal is obtained is the well-known FM demodulator 1
1 connected. Here, the FM demodulator 11 is shown as including a well-known buffer circuit, a high pass filter, a dropout compensation circuit, and a limiter.

The voltage controlled low pass filter 12 connected to the FM demodulator 11 is newly provided according to the present invention, and its frequency characteristic (high cutoff frequency) changes in response to detection of a specific frequency. To do. A well-known de-emphasis circuit 13, a low-pass filter 14 and an amplifier 15 are sequentially provided at the output stage of the voltage control low-pass filter 12.

The branch line 16 connected to the reproduction FM modulation signal transmission line 10 has an AGC as a specific frequency detecting circuit.
An (automatic gain control circuit) 17, a trap filter 18, an amplifier 19 and a detector 20 are connected in this order. In addition,
The AGC circuit 17 and the trap filter 18 are shown as incorporating a buffer circuit. The AGC circuit 17 is a circuit that automatically adjusts the level of the reproduced FM modulation signal obtained from the heads 1 and 2 to an appropriate value. As shown in FIG. 4, the trap filter 18 is a band attenuating filter (BEF) having a frequency characteristic that the output level decreases at a specific frequency (gray level frequency in this embodiment) f1.
It is. The detector 20 is an F that has passed through the trap filter 18.
This is a circuit for envelope-detecting an M signal and converting it into a DC voltage, which includes three transistors 21, 22, 40, two capacitors 23, 24, and four resistors 25, 26, 41,
42, two diodes 43, and a bias circuit 27. The output line 28 of the wave detector 20 is connected to the control terminal of the voltage controlled low pass filter 12.

As shown in FIG. 2, the voltage control low pass filter 12 includes two resistors 31, 32 and a capacitor 33.
And a field effect transistor 34 as a variable impedance element, the resistance value of the field effect transistor 34 changes in accordance with the change in the voltage of the detector output line 28, and the frequency characteristics of the characteristic lines A and B of FIG. It is configured to change. In FIG. 3, the characteristic line A is the characteristic of the low-pass filter required in the case of an image such as a test pattern (monoscope) having many high frequency components,
A characteristic line B is a characteristic when the cutoff frequency of the low pass filter is lowered in response to the detection of the gray frequency according to the present embodiment.

[0013]

[Operation] The operation of the apparatus of FIG. 1 will be described with reference to FIG. 5 showing the waveforms at points A to E of FIG. Tape 4 and heads 1 and 2
The reproduced FM-modulated signal obtained by the relative scanning motion of and is demodulated by the FM demodulator 11 into a composite video signal shown in FIG. In this example, since it is assumed that a signal indicating gray is included, 1H is also included in the video signal after demodulation.
A waveform showing gray appears in the middle of the (1 horizontal scanning) period,
Others are almost black level.

The reproduced FM modulation signal on line 10 contains a frequency component corresponding to the luminance of the image. That is, it includes high frequency components corresponding to white levels, low frequency components corresponding to black levels, and intermediate frequency components corresponding to gray levels. The AGC circuit 17 controls the reproduced FM modulated signal so that its amplitude becomes constant, and outputs a frequency signal (FM modulated wave) illustrated in FIG. 5A. It should be noted that FIG. 5A does not show the change in the frequency of the FM signal. That is, the FM signal of FIG. 5A is a signal including a frequency change, but this is omitted. Since the trap filter 18 has a characteristic that traps occur at the frequency (fundamental wave frequency) corresponding to the gray component, the signal shown in FIG. 5C is obtained at this output stage.

The signal of FIG. 5 (C) passing through the trap filter 18 has an amplitude center voltage of FIG. 5 (D) by the transistor 21 of the emitter follower in the detector 20.
Can be lowered as shown in. The signal in FIG. 5D is PNP.
It is input to the base of the transistor 22 and detected between the base and the emitter, and then inverted by the silane transistor 40 to become the DC voltage of FIG. This DC voltage is input as a control voltage to the gate of the field effect transistor 34 of the voltage controlled low pass filter 12. As a result, the resistance value of the field effect transistor 34 becomes small in the gray level period with the highest potential in FIG. 5E, and the cutoff frequency of the voltage control low-pass filter 12 is low as shown by the characteristic line B in FIG. Become. As a result, noise having a frequency higher than the cutoff frequency is removed by the voltage control low-pass filter 12, and the S / N is improved. That is, the noise in the gray reproduced image area is reduced. Since the cutoff frequency of the voltage-controlled low-pass filter 12 is not in the entire reproduced image but only in the gray area of t1 to t2 in FIG. 5B, the resolution is hardly reduced due to the cutoff frequency. That is, t1 to t in FIG.
Black level signals other than those in section 2 pass when the voltage-controlled low-pass filter 12 is in a state where the cutoff frequency shown in the characteristic line A of FIG. 3 is high, so that the resolution does not decrease here.

The FM demodulated output is sent to the output terminal through the de-emphasis circuit 13 and the low pass filter 14 after passing through the voltage controlled low pass filter 12. The processing by the voltage control low pass filter 12 is performed in real time every second.

The present embodiment has an effect that noise in a specific luminance area (gray area) can be reduced by a simple circuit configuration.

[0018]

[Second Embodiment] FIG. 6 shows a part of a reproducing system of a VTR of the second embodiment, that is, a part of a voltage control low-pass filter 12a and its control circuit. In the second embodiment, only the range from the trap filter 18 to the voltage control low pass filter 12 of FIG. 1 is modified, and the other configurations are similar to those of FIG. Therefore, illustration of common parts is omitted and the description thereof is omitted. The AGC circuit 17 of FIG. 6 is connected to the transmission line 10 by the line 16 as in FIG. The AGC circuit 17 includes a plurality of band attenuation filters (BEF) 18a-1.
8n are connected. The filters 18a to 18n have the same functions as the trap filter 18 of FIG.
The frequency bands are different. Therefore, different frequencies corresponding to different brightness levels are detected. Filter 18a-
The output of 18n is connected to each of the detection circuits 20a to 20n via amplifiers 19a to 19n. Each detection circuit 20a
.About.20n have essentially the same configuration as the detection circuit 20 of FIG. 1, but output voltages V1 to Vn of different levels in response to the detection outputs of the filters 18a to 18n. The output of each detection circuit 20a-20n is connected by a line 28 to a field effect transistor 34a. The field effect transistor 34a is connected in parallel to the resistor 31 of the signal transmission line and functions as a variable resistor. That is, the resistance value of the transistor 34a is the detection circuit 20a-2
It changes according to the level of the control voltage supplied from 0n,
The cutoff frequency of the low-pass filter 12a also changes accordingly. This makes it possible to remove noise of various brightness levels. Note that the filters 18a-18
Since the cutoff frequency cannot be lowered in the frequency band other than the frequency band specified by n, the resolution does not decrease in the region other than the frequency band specified. Therefore, in the area other than the specified frequency band, the reproduction signal processing is performed with the resolution being more important than the noise (SN ratio).

[0019]

[Third Embodiment] FIG. 7 shows a part of the reproducing system of the VTR of the third embodiment. In this embodiment, the detection circuits 20a to 20 of FIG.
0n and the low-pass filter 12a are modified. Therefore, in FIG. 7, the same parts as those in FIG. 6 are designated by the same reference numerals and the description thereof will be omitted. The detection circuits 20a to 20n of this embodiment output voltages of the same level in response to the detection signals of the filters 18a to 18n, which are output to the transistors 34a to 34n of the low pass filter 12b.
Send to The low-pass filter 12b has a plurality of circuits including resistors 32a to 32n having different values, capacitors 33a to 33n having different values, and transistors 34a to 34n in order to switch the cutoff frequency in multiple stages. Transistor 34a is responsive to the output of output lines 28a-28n of detection circuits 20a-20n.
~ 34n is alternatively turned on. As a result, the capacitors 33a and 33n are selectively connected between the line and the ground, and the cutoff frequency changes. In FIG. 7, the resistor 40 is connected between the transmission line and the ground.

[0020]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) Instead of the trap filter 18, a bandpass filter that passes a specific frequency region (gray region) can be used. (2) The voltage controlled low pass filter 12 can be connected after the de-emphasis circuit 13. (3) In order to prevent a reduction in resolution due to a reduction in the cutoff frequency of the voltage control low pass filter 12, 1
The cutoff frequency of the voltage control low-pass filter 12 is lowered slightly after the start time t1 of the specific brightness level region (gray level region) in H, and the voltage control low-pass filter 1 is started at the same time as the end time t2 of the gray level region.
The cutoff frequency of 2 may be restored. In addition, the fifth
The cutoff frequency of the voltage controlled low-pass filter 12 may be lowered only in the central portion of the section from t1 to t2 in the figure excluding the vicinity of t1 and t2.

[0021]

As is apparent from the above, according to the present invention,
The following effects are obtained. (A) Fill a specific frequency section in front of the demodulator
Voltage-controlled low-pass filter based on
Since it is a configuration that controls the
Therefore, the noise in the specific frequency section can be reduced.
You. (B) In parallel with the demodulation operation by the demodulator, a specific frequency band
The noise is detected in real time because it detects and detects
Can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a VTR according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing the voltage controlled low pass filter of FIG.

FIG. 3 is a diagram showing changes in frequency characteristics of the voltage controlled low pie filter of FIG.

FIG. 4 is a characteristic diagram of the trap filter of FIG.

5 is a waveform diagram showing in principle the states of points A to E in FIG.

FIG. 6 is a circuit diagram showing a part of a VTR according to a second embodiment.

FIG. 7 is a circuit diagram showing a part of a VTR of a third embodiment.

[Explanation of symbols]

 1, 2 heads 4 tapes 11 FM demodulator 12 voltage control low-pass filter 18 trap filter 20 detector

Claims (1)

(57) [Claims] 1. A reproduction head for obtaining a reproduction FM modulation signal by relative scanning motion with a recording medium on which an image signal is FM-modulated and a demodulation of the reproduction FM modulation signal obtained from the reproduction head. And a voltage control low-pass filter connected to the demodulator, for detecting a period during which the reproduced FM modulated signal is in a specific frequency range, the reproducing head and the demodulator
And a specific frequency section detection filter connected to the line branched from between the detector and the output of the specific frequency section detection filter, the cutoff frequency or high frequency of the voltage control low-pass filter by this detection output. An image reproducing device comprising means for controlling the voltage control filter so as to lower the voltage.