JPH0528527B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a noise reduction circuit for reducing noise contained in a luminance signal of a video signal. [Prior Art] As is well known, a magnetic recording device (hereinafter referred to as "VTR") is used to record a video signal on a magnetic tape or to reproduce a video signal recorded on a magnetic tape. FIG. 2A is a schematic diagram showing the relationship between the magnetic tape and magnetic head of a conventional VTR. In the figure, a magnetic recording section 10 includes a head drum 11, on which two magnetic heads 12 and 13 for recording and reproducing video signals are provided 180 degrees apart on the circumference. The magnetic tape 1 supplied from a supply reel 14 is wound around the head drum 11 over a predetermined circumferential angle. Thereafter, the tape is driven in the direction of arrow x via the capstan 16 and the pinch roller 17, and is wound onto the take-up reel 15. FIG. 2B schematically shows the recording pattern of the video signal recorded in this manner on the magnetic tape. In the VHS system, the recorded A track 2 is recorded at an azimuth angle of 6 degrees, and the currently recorded B track 3 is recorded at an azimuth angle of -6 degrees. Note that the arrow y indicates the tracing direction of the magnetic head. FIG. 3 is a block diagram showing the circuit configuration of the VTR. As shown in the figure, the VTR has recording system block 21 ,
Head changeover switch 22 , rotary transformer 2
3. Reproduction system block 24 , tape running drive circuit 2
5 and a mode selection switch 26 . The recording system block 21 includes a video signal source 211 ,
It includes a luminance signal passing filter 212 , a color signal passing filter 213 , an FM modulator 214 , an adding circuit 215 and a low frequency converter 216 . Regeneration block 2
4, head amplifiers 241 and 242 , adder circuit 2
43, FM video signal passing filter 244 , FM demodulator 245 , low frequency color signal passing filter 246 , high frequency converter 247 , addition circuit 248 , output terminal 24
9 and a noise reduction circuit 250 . The mode selection switch 26 is the recording command switch 26.
1, a regeneration command switch 262, a stop command switch 263, a high speed regeneration command switch 264, and a temporary stop command switch 265. Next, the operation of this VTR will be explained with reference to FIGS. 2A, 2B, and 3. First, in the recording mode, the switches 221 and 222 included in the head changeover switch 22 are switched to the upper position a. The video signal supplied from the video signal source 211 consists of a luminance signal of up to about 3 MHz and a chrominance signal of 3.58 MHz. The luminance signal passes through a luminance signal passing filter 212 .
FM modulator 214 . FM modulator 21
4 modulates the luminance signal from 3.4 MHz to a 4.4 MHz FM signal and supplies it to the adder circuit 215. On the other hand, the color signal is applied to a low frequency converter 216 through a color signal passing filter 213 . The low frequency converter 216 converts the color signal into a 629kHz signal and supplies it to the addition circuit 215 . An adder circuit 215 adds the FM-modulated luminance signal and the low-frequency converted color signal. This added signal is applied to the magnetic head 11 via the head changeover switch 22 and the rotary transformer 23 . Magnetic head 12 records the modulated video signal on the A track, and magnetic head 13 records on the B track. Next, in the reproduction mode, the reproduction command switch 262 is pressed. In response to this, the tape running drive circuit 25 switches the magnetic head changeover switch 22 to the lower end b, and connects it to the reproduction system circuit. In this state, the signal recorded on the A track 2 of the magnetic tape 1 is selected by the azimuth, is reproduced by the magnetic head 12, and is applied to the head amplifier 241 via the transformer 231 and switch 221 . The head amplifier 241 amplifies the reproduced signal and supplies it to the adder circuit 243 . Similarly, the recording signal recorded on the B track 3 of the magnetic tape 1 is reproduced by the magnetic head 13 and transferred to the transformer 232.
and is applied to the head amplifier 242 via the switch 222 . The head amplifier 242 amplifies the reproduced signal and supplies it to the adder circuit 243 . Addition circuit 243
adds the reproduced signals of both A track 2 and B track 3 and supplies the result to an FM video signal passing filter 244 and a low frequency color signal passing filter 246 .
The FM video signal passing filter 244 passes the FM video signal in the frequency band modulated by the FM modulator 214 and supplies it to the FM demodulator 245 . The FM demodulator 245 demodulates the FM video signal to derive a luminance signal and supplies it to the addition circuit 248 via the noise reduction circuit 250 . On the other hand, the low frequency color signal passing filter 246 passes the low frequency color signal of the frequency converted by the low frequency converter 216 and supplies it to the high frequency converter 247 . The high frequency converter 247 converts the low frequency color signal to 3.58MHz.
It is converted into a color signal and applied to the addition circuit 248 . Adder circuit 248 adds the demodulated luminance signal and color signal and outputs the result to output terminal 249 as a video signal. FIG. 4 is a block diagram showing the configuration of this conventional noise reduction circuit. In this figure, a high-frequency component of a luminance signal input to an input terminal 40 is extracted by a high-pass filter 41, and then sent to a phase correction circuit 42 to match the phase with the original signal passing through a line 44. Given. Phase correction circuit 42
The amplitude limiter 43 allows only low-level signals corresponding to noise signals to pass through the output. By subtracting the noise signal thus obtained from the luminance signal obtained via line 44, a noise-cancelled and noise-reduced luminance signal is obtained. Here, some additional information will be given regarding the phase correction circuit 42 . As mentioned above, the noise signal is a high-frequency low-level signal, which can be thought of as approximating a sine wave. In order to extract this high-frequency noise component, the signal is passed through a high-pass filter 41 , and a differentiating circuit can be considered as this filter. However, if the sine wave is passed through a differentiating circuit, the phase advances, so a phase correction circuit 42 is required to match this advanced phase with the phase of the original signal.
An integrating circuit can be considered as this phase correction circuit. When a sine wave is passed through an integrator circuit, its phase lags behind that when it is passed through a differentiator circuit. Therefore, the characteristics of the integrating circuit must be selected so that the phase is delayed by the phase advanced by the high-pass filter 41 . [Problems to be Solved by the Invention] Figures 5A to 5E show each stage of signal processing in the conventional noise reduction circuit as described above, where the vertical axis V represents the signal strength and the horizontal axis t represents the signal strength. corresponds to time. FIG. 5A shows a case where a luminance signal with small amplitude noise rises. If such an input signal is passed through the high-pass filter 41 of FIG. 4, a noise signal as shown in FIG. 5B is obtained, but a high peak remains at the portion corresponding to the rising edge of FIG. 5A. Next, by passing the signal through the phase correction circuit 42 , the signal shown in FIG. 5C is obtained. In FIG. 5C, two parallel horizontal lines h and l indicate the next limiting range of the amplitude limiter 43 . That is, when a signal as shown in FIG. 5C is passed through the amplitude limiter 43 , it appears as a noise signal as shown in FIG. 5D. In FIG. 5D, a plateau portion where there is no noise appears in region w. This plateau portion appears as a result of the portion extending outside the amplitude limiting range lh in FIG. 5C being limited. Finally, in the subtraction circuit 45 , the noise signal shown in FIG. 5D is subtracted from the luminance signal shown in FIG. 5A given from line 44 to reduce noise. FIG. 5E shows this result. In FIG. 5E, noise remains as is in the area w immediately after the rise of the luminance signal. This is because there is no subtractable noise vibration in the area w of the noise signal in Figure 5D, and as a result of subtracting the plateau part, a step g occurs after the rise in Figure 5E. There is. As described above, in the conventional noise reduction circuit, noise remains for a certain period immediately after the luminance signal rises, and there are disadvantages such as steps in the signal level. Furthermore, it is difficult to adjust the phase correction to the phase change of the high-pass filter, and a phase correction circuit is also required in addition to the high-pass filter, which is economically disadvantageous. The present invention has been made to solve these problems, and aims to improve the noise reduction effect immediately after the signal rises and to eliminate the step in the signal level immediately after the signal rises. Another object of the present invention is to provide a low-cost noise reduction circuit that does not require phasing by achieving noise reduction using a single filter. [Means for Solving the Problems] A circuit for reducing noise contained in a luminance signal of a reproduced video signal according to the present invention includes a differential amplification means to which a first input signal and a second input signal are input. and a level detection means which detects the level of the output signal of the differential amplification means and becomes conductive when the level is outside a predetermined level range, and a level detection means which is connected in parallel with the level detection means and whose output signal of the differential amplification means is comprising an input low-pass filter and output means for deriving both the outputs of the level detection means and the low-pass filter, and using the luminance signal of the reproduced video signal as a first input signal of the differential amplification means; It is characterized in that a signal corresponding to the output signal of the output means is used as the second input signal of the differential amplification means. [Function] In the noise reduction circuit according to the present invention, a large amplitude useful signal passes through the level detection means constituted by a diode circuit, but a small amplitude high frequency noise signal is removed by a low pass filter. Noise components in the luminance signal are reduced. [Embodiment of the Invention] FIG. 1 is a diagram showing an embodiment of a noise reduction circuit according to the present invention. In the figure, a differential amplifier 31
The reproduction signal given to one input terminal 30 of
After being amplified, the signal is supplied in parallel to level detection means consisting of diodes 32 and 33 and a low-pass filter 34 . The low pass filter tie 34 consists of a resistor 50 and a capacitor 51. In such a configuration, the level detection means utilizes the switching characteristics of the diodes, and if these diodes are silicon diodes, this level detection is performed when the signal amplified by the differential amplifier 31 has an amplitude of 1.2V _pp or more. It can pass through means 32, 33. Signals with amplitudes below this are passed through the low-pass filter 3.
4, and a signal from which high-frequency noise components have been removed is output. That is, by appropriately selecting the amplification degree of the differential amplifier 31 and the high-frequency rejection rate of the low-pass filter 34 , noise in the reproduced video signal can be reduced. Level detection means 32, 3
3 or the low pass filter 34 is output from the emitter of the transistor 35 with low impedance. The output from this transistor is led out via an output terminal 39, and a portion is given as another input to the differential width amplifier 31 via dividing resistors 36 and 37. The amplitude ratio of feedback input 38 to the output from transistor 35 can be arbitrarily set by selecting the ratio of these dividing resistors 36, 37. Now, it is assumed that the high-frequency blocking ability of the low-pass filter 34 consisting of the resistor 50 and the capacitor 51 is 1/100, and that the amplification factor of the differential amplifier 31 is 10. Assume also that an input signal having a noise amplitude of 0.1V rises to a level of 1V, as shown in FIG. 6A. In such a case, the amplitude of the noise at the first output of the differential amplifier is approximately 1V, which is the product of the noise input and the amplification factor. This is because the noise output of the low-pass filter is 0.01V, and there is almost no feedback.
That is, the output of the low-pass filter is a useful output of this noise reduction circuit, and its noise is 1/ compared to the noise of the signal applied to the input terminal 30.
It will be reduced to 10. Next, consider when a noise-free input signal as shown by the dashed line in FIG. 6A rises. In Figure 1, the feedback input when both the input at terminal 30 and the output at terminal 39 are set to 1 is
Suppose that the dividing resistors 36 and 37 are selected so that the ratio is 0.9. When the input signal rises to 1V, the output of the differential amplifier 31 increases, but at the moment of rise, no output appears on the output side until 0.6V, when the level detection means 32 and 33 are energized. That is, the level detection means is in a non-conductive state until the input reaches 0.06V, but since the feedback input is approximately 0, the output of the differential amplifier 31 increases rapidly. When the output of the differential amplifier 31 becomes 0.6V or higher, the level detection means 32 and 33 become conductive, and the output of the detection means follows the input. Then, when the output of the differential amplifier 31 rises to 1.54V, 0.94V is output due to the voltage drop of 0.6V caused by the level detection means 32 and 33. Currently, the feedback amount is 90%, so feedback input 3
0.846V is input to 8. At this time, 1V of input terminal 30 and 0.846V of feedback input 38
The output of the differential amplifier 31, which amplifies the difference of 0.154V by 10 times, becomes 1.54V and stabilizes. Next, since there is a potential difference of 0.6V between the input and output sides of the level detection means 32 and 33, the potentials are tried to be the same through the low-pass filter 34 . The moment this potential difference becomes 0.6V or less, the level detection means 32, 33 become non-conductive, and the moment the output of the differential amplifier 31 reaches 1V, it stabilizes. This is because when 1V is output to the output terminal 39, the feedback input 38 is 0.9V.
This is the difference from the input to terminal 30.
01V is multiplied by 10 and used as the output of differential amplifier 31.
This is because 1V is output. The same applies when the input to the terminal 30 contains noise. As shown in Figure 6B, the output of the differential amplifier 31 is 1.54V as in the case without noise.
rises to Until the output changes from 0.6V to 1.54V, the level detection means 32 and 33 are conductive and the output follows the input, so that the noise is output as is. At this time, since the base voltage of the transistor 35 should be 0.6V lower than the output of the differential amplifier 31 , the current level of the diode 32 becomes as shown by h _D in FIG. 6B. Since this energization level is affected by the input up to the rising edge of 1.54 V, the characteristic has a transition region with a smooth shoulder by the width indicated by the arrow n. Therefore, in this transition region, the output is affected by rising noise and 1
The resulting waveform has fiber regions indicated by dotted and dashed lines. Level detection means 32, 3 via reduction filter 34
When the potential difference between the input side and the output side of diode 3 starts to decrease and starts to decrease from the peak of the noise, diode 3
2 is in a non-conducting state, and the influence of the rising edge disappears. Thereafter, the diode 32 remains non-conductive and the noise is magnified ten times and output from the differential amplifier 31, but is reduced to 1/100 by the low pass filter 34 and output. Therefore,
The average value of this output is approximately equal to and parallel to the average value of the input. That is, according to the example of FIG .
If the output of 1 becomes 0.6V or more higher than the average value, the diode 32 becomes conductive and the output follows the input, and if it is lower than that, noise is reduced by a low-pass filter. Also, the output of the differential amplifier 31 is lower than the average value.
If it drops below 0.6V and below l _D in Figure 6B,
It can be easily inferred that the diode 33 conducts and the output follows the input, and as a result, the differential amplifier 31
output is below 1.2V _PP , i.e. 0.12V _PP at input
The following signals are subjected to noise reduction by the low-pass filter 34. Further, although the present invention uses a low-pass filter in the noise reduction circuit, it has more selectivity with respect to signal amplitude than a noise reduction circuit using only a low-pass filter. That is, a noise reduction circuit using only a low-pass filter will reduce high-frequency components regardless of the magnitude of the signal amplitude. For this reason, not only noise but also useful high-frequency signals are reduced. On the other hand, it is constructed of a low-pass filter connected in parallel with a level detection means consisting of a diode circuit according to the present invention. Therefore, as explained above, when the signal amplitude is large, that is, when the signal is 0.12V _PP or more, the diode becomes conductive and the output follows the input. When the signal amplitude is small, i.e.
For signals below 0.12V _PP , the diode becomes nonconductive and the signal passes through a low-pass filter to reduce noise. That is, in the present invention, compared to the case where only a low-pass filter is used, only the small amplitude signal of the high frequency component, that is, only the noise is removed. [Effects of the Invention] According to the present invention, the part of the input signal in which there is no noise reduction effect is a very short range of about 1/4 period of noise after the rise or fall of the input waveform, and conventional noise reduction cannot be achieved. A significant improvement is achieved compared to the circuit. In addition, the level difference in the signal level after the rise or fall of the input waveform seen in conventional noise reduction circuits is also 1/1 of the noise.
It is shortened to a short period of about 4 cycles. moreover,
A noise reduction circuit can be realized without requiring a phase correction circuit as in conventional noise reduction circuits.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a noise reduction circuit according to an embodiment of the present invention. FIG. 2A is a schematic diagram showing the relationship between a magnetic tape and a magnetic head of a VTR. Second
Figure B is a schematic diagram of a recording pattern on a magnetic tape. Figure 3 is a block diagram of a VTR. FIG. 4 is a block diagram showing a conventional noise reduction circuit. 5A to 5E are diagrams showing waveforms at each stage of a signal processed by a conventional noise reduction circuit. FIG. 6A is a diagram showing an example of a waveform applied to the input terminal of FIG. 1. 6th B
This figure is a diagram explaining the operation of the circuit of FIG. 1 when a signal like that of FIG. 6A is input. In the figure, 30 is an input terminal, 31 is a differential amplifier, 32 and 33 are diodes, 34 is a low-pass filter, 35 is a transistor, 36 and 37 are resistors 38 is a feedback line, 39 is an output terminal,
50 represents a resistor, and 51 represents a capacitor.

Claims (1)

[Scope of Claims] 1. A circuit for reducing noise contained in a luminance signal of a reproduced video signal, comprising: differential amplification means to which a first input signal and a second input signal are input; level detection means that detects the level of the output signal of the differential amplification means and becomes conductive when the level is outside a predetermined level range; and a level detection means that is connected in parallel with the level detection means and receives the output signal of the differential amplification means. a low-pass filter; and output means for deriving both outputs of the level detection means and the low-pass filter, the luminance signal of the reproduced video signal being input to the first input of the differential amplification means. a signal, and a signal corresponding to the output signal of the output means is used as the second input signal of the differential amplification means. 2. The noise reduction circuit according to claim 1, wherein the output means includes impedance reduction means. 3. Claim 1, wherein the output means includes a dividing means for dividing the output signal, and the signal divided by the dividing means is used as a second input signal of the differential amplifying means. Described noise reduction circuit.