JP2538316B2 - Noise removal circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a noise removing circuit for removing noise included in a reproduction luminance signal in a home VTR (video tape recorder), and particularly to 1H (H Relates to a noise removing circuit for removing noise by using a delay line of a horizontal synchronizing signal).

(B) Conventional technology The luminance signal reproduction circuit of a home VTR is provided with a noise removal circuit for canceling noise components that are easily perceived by human eyes and improving apparent S / N. Such a noise removing circuit is described in, for example, Japanese Patent Laid-Open No. 61-61572. According to the noise removal circuit described in the above publication, a reproduction luminance signal and a signal obtained by delaying the reproduction luminance signal with a 1H delay line are subtracted, and the subtraction output is amplitude-limited and the reproduction luminance signal. Subtraction of
The noise component in the reproduction luminance signal is canceled.

(C) Problems to be Solved by the Invention By the way, when the method described in the above publication is used,
Noise cannot be accurately detected unless the levels of the reproduction luminance signal and the reproduction luminance signal obtained by delaying the reproduction luminance signal are matched, but it is generally used as delay means.
The conversion efficiency between input and output of D (charge coupled device) etc. is not constant, and its value changes due to temperature change. Therefore, level control means such as a variable resistor must be provided after the delay means to adjust the levels of both signals. However, since the level control means is manually adjusted by the operation knob, there is a drawback that the adjustment takes time and the number of parts increases. Even if the adjustment was performed once again, the readjustment was necessary depending on the ambient temperature change.

(D) Means for Solving the Problems The present invention has been made in view of the above points, and includes a first video signal having a luminance component and a second video obtained by delaying the first video signal by 1H. In a noise removing circuit for detecting a level difference from a signal, subtracting the detected output and the first video signal to remove a noise component of the first video signal, A first level detection circuit which inserts a variable gain amplifier circuit into the delay loop and performs level detection of the first video signal; a second level detection circuit which performs level detection of the second video signal; A comparator for comparing the detection outputs of the first and second level detection circuits is provided, and the gain of the variable gain amplification circuit is controlled according to the comparison output of the comparator to equalize the levels of the first and second video signals. like Characterized in that was.

(E) Operation According to the present invention, the first video signal and the second video signal obtained by delaying the first video signal by 1H are respectively level-detected, and the level detection is performed according to the difference between both detection outputs. Since the level of the second video signal is adjusted, the levels of the first video signal and the second video signal can be automatically matched at all times.

Further, according to the present invention, when the dropout of the first video signal is detected, the level detection operation of the first video signal is stopped accordingly, so that malfunction due to noise is prevented. You can do it.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention.
Is a dropout detection circuit for detecting a dropout of the FM luminance signal from the input terminal (2), (3) is an FM detection circuit for performing FM detection of the FM luminance signal from the dropout detection circuit (1), (4 ) Is the de-emphasis circuit,
(5) is a first clamp circuit that clamps the first luminance signal from the de-emphasis circuit (4), (6) is a first switch that switches according to the detection output of the dropout detection circuit (1), ( 7) is a variable gain amplifier circuit for amplifying the luminance signal from the first switch (6), (8) is a CC
1H delay line composed of D, (9) is a second clamp circuit for clamping the second luminance signal from the CCD (8) at the same clamp level as that of the first clamp circuit (5), ( 10 )
Is a noise detection circuit including a first comparator (11), an attenuation circuit (12), a correlation detection circuit (13), and a second switch (14), and (15) is a luminance signal from the first switch (6). A subtraction circuit for canceling the noise component according to the output signal of the noise detection circuit ( 10 ), ( 16 ) is a first detection circuit (17)
And a first capacitor (18) that operates as a holding means, and a first level detection circuit that performs level detection of the first luminance signal, and ( 19 ) is a second detection circuit (20) and a second level detection circuit that operates as a holding means. A second level detection circuit comprising a capacitor (21) for detecting the level of the second luminance signal, (22) comparing the detection outputs of the first and second level detection circuits ( 16 ) and ( 19 ), A second comparator that controls the gain of the variable gain amplifier circuit (7) according to the difference.

Next, the operation will be described. From the playhead (not shown)
The FM luminance signal is applied to the dropout detection circuit (1) via the input terminal (2) and the dropout is detected. If it is not in the dropout state, the first switch (6) is switched to the contact a side, the second switch (14) is switched to the closed state, and the third switch (23) is switched to the closed state. The FM luminance signal from the dropout detection circuit (1) is FM
FM detection by the detection circuit (3), the detected first luminance signal is applied to the first clamp circuit (5) via the de-emphasis circuit (4) so that the tip level of the synchronization signal becomes constant. Clamped. The clamped first luminance signal is applied to the 1H delay line (8) via the first switch (6) switched to the contact a side and the variable gain amplifier circuit (7), and after being delayed for 1H period. , And is applied to the second clamp circuit (9) via the low-pass filter (24). Therefore, as shown in FIG. 2 (b), the output terminal of the second clamp circuit (9) is delayed by 1H as compared with the first luminance signal of FIG. 2 (a), and the same clamp level ( The second luminance signal having Vref) can be obtained.

The first and second luminance signals obtained from the output terminals of the first and second clamp circuits (5) and (9) are level-detected by the first and second detection circuits (17) and (20), respectively. The detection outputs are the first and second capacitors (18) and (21), respectively.
Is held. Therefore, the positive input terminal (+) of the second comparator (22) receives a DC voltage corresponding to the level of the first luminance signal, and the negative input terminal (−) receives a DC voltage corresponding to the level of the second luminance signal. A voltage is applied, and an output signal corresponding to the level difference between the two signals is generated at the output terminal. Therefore, if the gain of the variable gain amplifier circuit (7) is controlled by using the output signal, the second luminance signal having the same level as the first luminance signal is obtained at the output terminal of the second clamp circuit (9). You can The first and second luminance signals whose levels are controlled to be equal are subtracted by the first comparator (11). At that time, since both signals have the same level, the first and second luminance signals are output to the output terminal of the first comparator (11). Produces an output signal containing noise (α) as shown in FIG. It should be noted that β in FIG. 2A is not noise but a part of the change in the luminance signal. And the second
The output signal of FIG. 3C is attenuated by the attenuation circuit (12) and applied to the second switch (14) as shown in FIG. 2D. The correlation detection circuit (13) determines whether the signal generated from the first comparator (11) is a noise component or a luminance component, and when it determines that it is a noise component, closes the second switch (14). If it is determined that it is a luminance component, the control for opening the second switch (14) is performed.
Therefore, the correlation detection circuit (13) has two reference power source, now when the level between the voltage V ₁ and V ₂ of FIG. 2 (c), those from the voltage V ₁ of the large or Those smaller than V ₂ are regarded as luminance signals, and a control signal as shown in FIG. 2 (e) is generated, and the second switch (1) is operated during the “H” level period of the control signal.
4) Open. Therefore, during the "L" level period of FIG. 2 (e), the subtraction circuit (15) subtracts the luminance signal of FIG. 2 (a) from the signal of FIG. Offset. In addition, in the "H" level period of FIG.
Since the switch (14) is opened, the luminance signal from the first switch (6) is directly led to the output terminal (25) without being subtracted by the subtraction circuit (15). As a result, the noise component is attenuated at the output terminal (25) as shown in FIG. 2 (f), and the luminance information can be accurately derived and the luminance signal can be derived.

FIG. 3 is a circuit diagram showing a specific circuit example of the first and second detection circuits (17) and (20) of FIG. 1, in which the luminance signal from the clamp circuit is transferred to the transistor via the input terminal (26). It is amplified by the first differential amplifier circuit ( 29 ) composed of (27) and (28). A horizontal synchronizing signal is applied to the terminal (30) and is delayed by the delay circuit (31) so that the transistors (32) and (3
It is applied to the third differential amplifier circuit ( 34 ) composed of 3). The horizontal synchronizing signal is separated from the luminance signal, and the transistor (33) of the third differential amplifier circuit ( 34 ) is turned on and the transistor (32 ) Is turned off, the first differential amplifier circuit ( 29 ) operates and the second differential amplifier circuit ( 37 ) including the transistors (35) and (36) stops operating. The delay amount of the delay circuit (31) is set to such a value that the horizontal synchronizing signal is located in the back porch period of the luminance signal. When the horizontal synchronizing signal is applied to the third differential amplifier circuit ( 34 ) and the transistor (32) is turned on and the transistor (33) is turned off, the first differential amplifier circuit ( 29 ) stops operating and the second differential amplifier circuit ( 29 ) is stopped. The amplifier circuit ( 37 ) operates. The second differential amplifier circuit (37) gain is
It is set sufficiently higher than that of the first differential amplifier circuit ( 29 ). Therefore, at the output terminal (38), it is possible to obtain a luminance signal of a type in which a pulse is superposed on the back porch portion as shown in FIG. Is detected. Therefore, it is possible to obtain an output signal according to the amplitude of the luminance signal that has arrived at the output terminal (40).

Now, the third switch (23) built in the first level detection circuit ( 16 ) opens and closes according to the detection output of the dropout detection circuit (1), and when in the dropout state, the third switch (23) (23) is opened. This is because when the FM luminance signal is in a dropout state, the luminance signal applied to the first level detection circuit ( 16 ) becomes abnormal, and when it is level detected, the output terminal of the second comparator (22) becomes abnormal. Control signals are generated.
At this time, since the first switch (6) is switched to the contact b side according to the detection output of the dropout detection circuit (1), the variable gain amplification circuit (7) is responded to according to the abnormal control signal. This is because if the gain of is controlled, a luminance signal of an abnormal level will be generated at the output terminal (25). When the third switch (23) is opened, a direct current voltage corresponding to the level of the luminance signal immediately before the dropout state can be held in the first capacitor (18).
The gain of the variable gain amplifier circuit (7) can be controlled without being affected by the dropout, and a good luminance signal can be obtained at the output terminal (25).

Further, according to the present invention, the third switch (23) is opened even in response to VTR special reproduction (still image reproduction, double speed reproduction). When a control signal indicating that the VTR is in the special reproduction state is applied to the terminal (41), it is switched to the contact a side of the first switch (6) and the third switch (23).
Opens. This is because a lot of noise is included in the playback luminance signal during VTR special playback, so if the third switch (23) is closed, the first capacitor (18) will be charged according to the noise. This is because the second comparator (22) malfunctions at the restart after the end of the special reproduction.

The second switch (14) is also switched according to the output signal of the dropout detection circuit (1), which is arranged to stop the subtraction operation of the subtraction circuit (15) at the time of dropout. is there.

(G) Effect of the Invention As described above, according to the present invention, the levels of the first video signal and the second video signal obtained by delaying the first video signal by 1H can always be automatically made equal. Therefore, the labor of adjustment can be saved and the number of partial points can be reduced.

Further, according to the present invention, since the switch is provided in the first level detection circuit and the switch is switched according to the control signal, a malfunction of the detection operation occurring at the time of dropout or special reproduction of the VTR. Can be prevented.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIGS. 2 (a) to 2 (f) are waveform diagrams for explaining FIG.
FIG. 3 is a circuit diagram showing a specific circuit example of the detection circuit of FIG. 1, and FIG. 4 is a waveform diagram for use in explaining FIG. (1) …… Dropout detection circuit, (6) …… First switch, (7) …… Variable gain amplification circuit, (8) …… 1H delay line, ( 10 ) …… Noise detection circuit, (15) …… Subtraction circuit, ( 16 ) …… First level detection circuit, ( 19 ) …… Second level detection circuit, (22) …… Second comparator, (23) ...
… Third switch.

Claims (1)

(57) [Claims] 1. A level difference between a first video signal having a luminance component and a second video signal obtained by delaying the first video signal for 1H period, and the detected output and the first video signal. In the noise removal circuit that removes the noise component of the first video signal by subtracting
A variable gain amplifier circuit which is arranged in a delay path for delaying a period and which adjusts the level of the second video signal, a first level detection section which performs level detection of the first video signal, and a first level detection section of the first level detection section. An output signal of the first level detection unit according to a control signal indicating that the reproduction signal of the video tape recorder is dropped out or a control signal indicating that the video tape recorder is in the special reproduction state when the output signal is applied. A first level detection circuit including a switch for connecting or disconnecting the switch and a holding unit for holding an output signal of the switch;
A second level detection circuit including a second level detection unit that performs level detection of the second video signal and a holding unit that holds an output signal of the second level detection unit, and an output signal of the first level detection circuit And a comparator for comparing the output signal of the second level detection circuit with each other, and controlling the gain of the variable gain amplifier circuit according to the comparison output of the comparator to make the levels of the first and second video signals equal. A noise removal circuit characterized in that