JPH06296251A - Noise eliminating circuit - Google Patents

Abstract

PURPOSE:To use a simple constitution to obtain the signal, which includes less foldover noise and pulse jump-in noise and has the correlation noise sufficiently removed and to suppress the loss of signal components as much as possible with respect to the circuit which eliminates the noise of a solid-state image pickup element. CONSTITUTION:An input image pickup signal (a) is delayed in a first delay line 2 by a prescribed time, and the difference between this delay signal and the original image pickup signal (a) is operated by a subtractor 3. This difference signal (c) is compared with a reference voltage Vref by a voltage comparing circuit 4 to output only a signal component (d) where the correlation noise is sufficiently reduced. The output signal (d) is delayed in a second delay line 5 by a prescribed time, and this delay signal and the original output signal (d) are added by an adder 6. A multiple signal (f) obtained in this manner passes a low pass filter 7 to obtain a final output signal (g).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise removing circuit for removing a noise component superimposed on an image pickup signal in a signal processing process of a solid-state image pickup device.

[0002]

2. Description of the Related Art With the advent of high-definition systems in recent years,
Higher image quality and higher definition are required. The size of the solid-state image pickup element has been reduced and the number of pixels has been increased. As a result, even in terms of pixels, the area of the photoelectric conversion unit is greatly reduced, and the signal amount thereof is drastically reduced. Therefore, there is an increasing demand for further noise reduction. The noise of the solid-state imaging device mainly includes transfer noise, amplifier noise, reset noise, dark current, shot noise due to light, and the like, and among these, reduction of the dominant reset noise is strongly desired.

As a method of reducing and removing reset noise, a correlated double sampling circuit (Television Society of Japan, Vol. 39, No. 12, 1176 to 1181) has been conventionally used.
Page, December 1985) is widely used, and as another well-known one, a delay difference noise elimination circuit (Television Society, Vol. 39, No. 12, No. 1176).
~ 1181, December 1985). Less than,
These methods will be briefly described.

The correlated double sampling (CDS) circuit is
As shown in FIG. 4, the sample-hold circuit 43 including the feedthrough level sampling pulse generation circuit 431, the switch 432 and the capacitor 433, the signal level sampling pulse generation circuit 451 and the switch 45.
Sample-hold circuit 4 consisting of 2 and capacitor 453
5, the feed-through level and the signal level of the input image pickup signal 4a are sampled and held, and the subtractor 46 takes the difference between them to remove the reset noise.

Next, the delay difference denoising (DDS) circuit is
A delay element 52 as shown in FIG. 5, a subtracter 53, a gating circuit 55 including a gate pulse generation circuit 551 and a gate circuit 552, and an LPF 56 are provided. , And the difference from the signal obtained by delaying the image pickup signal by half of one pixel output period. At this time, a reset noise-free signal component appears on the low level side with respect to a certain constant voltage, and conversely, a noise-free signal component appears on the high level side. Only the signal from which noise has been removed is taken out by using the gate circuit 552, and the low pass filter 56 removes the extra pulse component to obtain the signal component. Of course, instead of using the gate circuit, the sample and hold may be performed to extract the signal component.

[0006]

As described above, in the prior art, both the CDS circuit and the DDS circuit require the switching operation circuit by the operation control pulse like the sampling circuit and the gate circuit. However, when the number of pixels of the solid-state image pickup device is increased (the band of the image pickup signal is widened), a wideband switching circuit which operates at a very high speed has to be constructed, and the circuit configuration tends to be complicated. Further, there is a drawback that these switching pulses jump into the signal component. In particular, in the method of performing sample and hold, in addition to the above-mentioned problems, there is a problem that high frequency components of signals and noise are folded back to a low range, and folding noise occurs.

The present invention has been made in order to solve these conventional problems, and it is possible to easily use a circuit having a simple structure to eliminate high-frequency aliasing noise, switching pulse jumping noise, and correlation noise. It is an object of the present invention to provide a noise elimination circuit capable of obtaining a sufficiently removed signal component and suppressing the loss of signal output as much as possible.

[0008]

In order to achieve this object, a noise removing circuit of the present invention comprises a first delay means for delaying an image pickup signal output from a solid-state image pickup element by half of one pixel output period. And a subtraction means for taking a difference between the delayed signal and the image pickup signal, and a voltage comparison circuit for selecting and outputting either one of the difference signal obtained thereby and a predetermined reference voltage.

Further, the second delay means for further delaying the output of the voltage comparison circuit by half of one pixel output period of the image pickup signal, and the sum of the delayed output signal and the output signal of the voltage comparison circuit. It is composed of an addition means.

[0010]

With the above arrangement, the noise removing circuit of the present invention does not perform sample and hold, so that a signal component free from high frequency aliasing noise is obtained. Further, since a complicated circuit such as a high-speed switching circuit required for sample hold and gating is not required, a circuit having a simple structure can easily obtain a sufficient correlation noise removing effect. Moreover, since no switching operation is performed, there is no jumping noise such as a switching pulse.

Further, according to the present invention, since the noise-removed output signal is delayed to be multiplexed even in a portion where the output signal does not exist and then smoothed, a signal component loss due to the noise removal due to an increase in the signal amount. It is possible to obtain a signal output with little deterioration by significantly reducing.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an example of a noise removing circuit according to the present invention. FIG. 2 is a diagram showing an example of a signal waveform in each part of FIG. FIG. 3 is a circuit diagram showing an example of the voltage comparison circuit in FIG.

In FIG. 1, 2 is a first delay line having a function as a first delay means, 3 is a subtractor having a function as a subtracting means, and 4 is a voltage comparing circuit having a function as a voltage comparing means. 5 is a second delay line having a function as a second delay means, 6 is an adder having a function as an adding means, and 7 is a low-pass filter.

The operation of the present embodiment having the above-described structure will be described. First, the image pickup signal a input from the input terminal 1 is half of one pixel output period of the image pickup signal a by the first delay line 2. Is delayed only to become the delayed image pickup signal b.
The image pickup signal a and the delayed image pickup signal b are input to the subtractor 3 and the difference between the two is taken to generate a difference signal c. Since the reset noise is a change in the feedthrough level, the signal level from which the noise is removed is obtained by subtracting the feedthrough level during the synchronization period from the signal level during one pixel period.

Therefore, in this embodiment, as shown in FIG. 2, a reset noise-removed signal component appears on the lower level side than a certain constant voltage of the differential signal c. On the other hand, on the high level side, signal components in which noise is not removed appear alternately. Here, the reference voltage V _ref is set in advance so as to have the same level as the constant voltage.
The differential signal c is compared with the reference voltage V _ref by the voltage comparison circuit 4, and only the signal component on the low level side in which the reset noise is sufficiently reduced is output as the output signal d.

As the voltage comparison circuit 4, for example, an existing clip circuit as shown in FIG. 3 can be used. In the noise removal circuit of this embodiment, operation control is performed like a sampling circuit and a gate circuit. No complicated circuits such as fast switching circuits that require pulses are required. Also, because sample hold is not performed,
It is possible to obtain a signal output with no aliasing noise of high frequency components together with a sufficient correlation noise removing effect.

The output signal d is used as it is for the low-pass filter 7
The final output can also function as a noise removal circuit that achieves the above-mentioned effects.

In the present embodiment, the output signal d is further delayed by the second delay line 5 by a half of one pixel output period of the image pickup signal a to be a delayed output signal e. The output signal d and the delayed output signal e are input to the adder 6 and added to generate a multiplexed signal f. This multiplexed signal f is passed through the low pass filter 7 to obtain the final output signal g.

By adding the output signal d and the delayed output signal e before smoothing by the low-pass filter 7, the signals are multiplexed even in the period in which no signal component exists,
The amount of output signal can be increased. As a result, the signal loss due to noise removal can be significantly reduced, so that a signal output with less deterioration can be obtained.

The present invention is not limited to the above-described embodiment, and a differential signal between the image pickup signal and the phase-adjusted delayed image pickup signal can be obtained, for example, in a reflection type delay difference noise elimination circuit (RDS). It is needless to say that a noise removing circuit can easily obtain a sufficient noise removing effect by applying the present embodiment and inputting the difference signal to the voltage comparing circuit 4.

Further, even if the output of the other noise removing circuit is an intermittent output such as the output signal d, the same delay addition processing as in the present embodiment is performed, It is possible to significantly reduce signal loss.

[0022]

As described above, the noise removing circuit of the present invention does not require a sampling circuit or a gate circuit unlike the conventional noise removing circuit. Therefore, folding noise of high frequency components and jumping noise of switching pulses are not required. Can be obtained. Further, since a complicated high speed operation (wide band) circuit is not required, a sufficient noise removing effect can be obtained with a circuit having a simple structure.

Furthermore, since the amount of signal is increased by multiplexing the delayed signal even during the period in which no signal component exists, the loss of signal component due to noise removal can be greatly reduced. As a result, a signal output with almost no deterioration can be obtained, and signal processing in the subsequent stage can be advantageously performed.

[Brief description of drawings]

FIG. 1 is a block diagram of a noise removing circuit according to an embodiment of the present invention.

FIG. 2 is a signal waveform diagram of each part in the noise removal circuit of the same embodiment.

FIG. 3 is a circuit diagram showing an example of a voltage comparison circuit in the same embodiment.

FIG. 4A is a conventional noise removal circuit (CDS circuit).
Block diagram (B) is a signal waveform diagram of each part

FIG. 5A is a conventional noise removal circuit (DDS circuit).
Block diagram (B) is a signal waveform diagram of each part

[Explanation of Codes] 2 first delay line 3 subtractor 4 voltage comparison circuit 5 second delay line 6 adder 7 low-pass filter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryuichiro Kuga 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A first delay means for delaying an image pickup signal output from a solid-state image pickup device by a predetermined time, a subtracting means for taking a difference between the image pickup signal and the output from the delay means, and an output from the subtracting means. And a predetermined reference voltage, and a voltage comparator that outputs only the signal component of either the high level or the low level of both signals. 2. A second delay means for delaying a signal output from the voltage comparison means by a predetermined time, and an addition means for taking a sum of the output from the voltage comparison means and the output from the second delay means. The noise removing circuit according to claim 1, further comprising: