JPH05103314A - Moving picture encoding device - Google Patents

Abstract

PURPOSE:To prevent wasteful information transmission even when an interframe difference signal becomes larger due to a change in brightness of an entire screen or noise. CONSTITUTION:This moving picture encoding device is provided with a selector 3 which selectively outputs the interframe difference signal and intraframe signal of input moving picture signals, DCT circuit 4 which outputs a DCT coefficient by DCT-encoding the selected signals, quantizer 5 which quantizes the DCT coefficient, and variable-length coding circuit 13 which encodes the quantized results of the quantizer 5 into variable-length codes, and threshold circuit 17 which is positioned on the output side of the circuit 4, compares the absolute value of the DCT coefficient with a prescribed preset threshold, and, in case the absolute value of the DCT coefficient is equal to or smaller than the preset threshold when the selector 3 selects the interframe difference signal, sets its output to '0'.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moving picture coding apparatus for efficiently coding a moving picture signal using orthogonal transform coding.

[0002]

2. Description of the Related Art In recent years, moving image videophones and videoconference systems have been put into practical use due to the development of digital communication networks and the development of communication technologies. In particular, since the video conference system is extremely effective in saving the time and cost required for a business trip, its introduction is being promoted in companies. Video coding is one of the most important technologies for realizing videophones and videoconferencing systems.

FIG. 4 shows a block diagram of an example of a conventional moving picture coding apparatus. This moving picture coding apparatus selects an inter-frame difference signal and an intra-frame signal of an input moving picture signal, and codes this by DCT (discrete cosine transform) coding which is one of orthogonal transform coding. It is a device.
In FIG. 4, an image signal of a moving image obtained by picking up an image of a subject with a television camera is input to the input terminal 1 through an A / D (not shown).
It is digitized by the converter and input. This input image signal is subtracted from the image signal of the previous frame stored in the frame memory 10 by the subtractor 2 to obtain an inter-frame difference signal. The image signal of one frame before stored in the frame memory 10 is motion-compensated according to the motion vector obtained by the motion vector detection circuit 11.

The prediction determination circuit 12 determines which of the information amount of the moving image signal of the current frame (referred to as an intraframe signal) input to the input terminal 1 and the interframe difference signal from the subtractor 2 is smaller. The signal having the smaller amount of information is selected by the selector 3. The signal selected by the selector 3 is discrete cosine transformed by the DCT circuit 4 and then quantized by the quantizer 5. The discrete cosine transform is used to remove the correlation between pixels. The signal quantized by the quantizer 5 is variable-length coded by the variable-length coding circuit 13 and further speed-smoothed by the buffer memory 14, so that the signal is sent to the communication line at a constant transmission speed.

On the other hand, the signal quantized by the quantizer 5 is also sent to the inverse quantizer 6 and inversely quantized, and further the inverse DCT is performed.
After being subjected to the inverse discrete cosine transform in the (IDCT) circuit 7, it is locally decoded by being added to the image signal of one frame before in the adder 8. When the intra-frame signal is selected by the selector 3, the output of the IDCT circuit 7 becomes the local decoded signal as it is, and the selector 9 selects it.
This locally decoded signal is written in the frame memory 10.

In order to prevent the buffer memory 14 from overflowing, the quantization widths of the quantizer 5 and the inverse quantizer 6 are made coarse according to the buffer storage amount of the buffer memory 14 to reduce the amount of generated information. When the buffer storage amount becomes small, the quantization width is made fine to increase the generated information amount.

In this moving picture coding apparatus, for example, in a still area such as a background portion, the inter-frame difference signal having a smaller amount of information is DCT-encoded to improve the coding efficiency, and the moving area is also improved. In, the image quality is maintained by DCT encoding the intra-frame signal.

However, if the brightness of the entire screen changes due to flicker of illumination (especially a fluorescent lamp) or the auto iris of the camera at the time of capturing an image of a subject, or if noise generated in the camera or an analog circuit is mixed in the image signal, the still image is stopped. A large inter-frame difference signal is generated even in the area. In such a case, since a large inter-frame difference signal is DCT-encoded and transmitted in the still area, useless information is sent, and the encoding efficiency is reduced.

[0009]

As described above, in the conventional moving picture coding apparatus, the change in the brightness of the entire screen due to the flicker of the illumination, the auto iris of the camera, or the like, and the change in the brightness of the camera, the analog circuit, or the like occur. When a large inter-frame difference signal is generated in the still region due to noise, the large inter-frame difference signal is DCT-encoded even in the still region, so that there is a problem that the coding efficiency is reduced.

Therefore, according to the present invention, it is possible to prevent unnecessary information from being transmitted even if the inter-frame difference signal becomes large in a still area due to a change in the brightness of the entire screen or noise. The purpose is to provide a device.

[0011]

In order to solve the above-mentioned problems, the present invention, when an inter-frame difference signal is selected as an input of an orthogonal transform coding circuit, has an absolute value of a transform coefficient obtained by the orthogonal transform coding. If is less than or equal to a preset threshold value, a threshold value circuit that sets its output to 0 is provided on the output side of the orthogonal transform coding circuit or the quantizer.

Here, the threshold value set by the threshold value circuit is preferably such that the threshold value for the DC component of the transform coefficient obtained by orthogonal transform coding is larger than the threshold value for the AC component, and in the static region of the input moving image signal. It is desirable that the threshold value for the associated transform coefficient is larger than the threshold value for the transform coefficient belonging to the moving region. Furthermore, it is desirable that this threshold be small when the transmission bit rate is high.

[0013]

As described above, according to the present invention, the threshold value is set for the conversion coefficient of the inter-frame difference signal, and when the threshold value is less than the threshold value, the output is set to 0 to prevent the information from being transmitted, so that the luminance change of the input moving image signal and the It is possible to perform encoding that is strong against noise contamination. For example, the brightness of the entire screen may change due to flicker of fluorescent lights or auto iris of the camera,
When noise is mixed, the inter-frame difference signal becomes relatively large even in the still region, and the degree becomes more remarkable as the DC component of the conversion coefficient increases.

In such a case, in the conventional moving picture coding apparatus, although it is a still region which originally does not need to be transmitted, it is orthogonally transformed and coded and transmitted, but in the present invention, the transform coefficient in the still region is not transmitted. You can That is, when the brightness of the entire screen changes, the inter-frame difference signal becomes large even in the still region, but the degree is smaller than that in the moving region, so the conversion coefficient is transmitted only in the moving region by appropriately selecting the threshold value. The coding efficiency can be improved. in this case,
Since the increase in the inter-frame difference signal due to the change in brightness becomes larger as the DC component of the conversion coefficient increases, it is effective to increase the threshold value for the DC component.

Further, due to noise generated in the camera and the analog section, a large component is randomly generated over the entire conversion coefficient of the inter-frame difference signal. Conventionally, such a component was also transmitted, but in the present invention, a small fluctuation due to noise or the like can be prevented from being transmitted by setting a threshold value for the AC component of the conversion coefficient. Furthermore, by determining the stationary area and the moving area and increasing the threshold value of the stationary area, it is possible to more reliably prevent useless information from being transmitted in the stationary area.

Since the image quality improves as the transmission bit rate increases, the difference in brightness between the still area and the moving area becomes noticeable when the threshold value is high. Therefore, when the transmission rate is high, it is better to reduce the threshold value.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a moving picture coding apparatus according to an embodiment of the present invention. This moving picture coding apparatus includes a subtractor 2, a selector 3, a DCT (discrete cosine transform) circuit 4, a quantizer 5, an inverse quantizer 6, an IDCT (inverse discrete cosine transform) circuit 7, an adder 8, and a selector. 9, frame memory 10, motion vector detection circuit 11, prediction determination circuit 12, variable length coding circuit 13, buffer memory 1
4. A static / moving determination circuit 16 and a threshold circuit 17. It is different from the conventional moving picture coding apparatus of FIG. 4 in that a static / moving determination circuit 16 and a threshold circuit 17 are newly added.

The static / moving determination circuit 16 is an inter-frame difference signal and motion vector detection circuit 1 output from the subtractor 2.
The motion vector from 1 is used to determine whether the image signal input to the input terminal 1 is a still region or a moving region,
This circuit outputs the static / moving determination result.

FIG. 2 shows a specific configuration example of the static / moving determination circuit 16. The inter-frame difference signal from the subtractor 2 and the motion vector from the motion vector detection circuit 11 are input to the static / moving determination circuit 16. The inter-frame difference signal is
It is input to the adder 22 through the absolute value circuit 21, added to the output of the register 23, and then input to the register 23. This addition is repeated for one block of the image signal, and the register 23 outputs the sum of absolute values of the inter-frame difference signals in the block. This sum of absolute values is input to the comparator 24 and compared with the set value.

On the other hand, the motion vector is input to the 0 detection circuit 25, and it is determined whether the value is 0 or not. Comparator 24
When it is determined that the sum of absolute values of the inter-frame difference signals is larger than the set value and the 0 detection circuit 25 detects that the motion vector is not 0, the AND circuit 26 inputs the moving image input to the input terminal 1 of FIG. The corresponding area of the image signal is determined to be a moving area, and otherwise the stationary area is determined.

It should be noted that the static / moving determination can be made to some extent only by the sum of absolute values of the inter-frame difference signals or only the motion vector, but more accurate static / moving determination can be made by combining both as shown in FIG. In addition, the static / moving determination can be performed by checking the variance of the inter-frame difference signal instead of the sum of absolute values of the inter-frame difference signal.

The threshold circuit 17 receives the static / moving determination result from the static / moving determination circuit 16, the interframe / intraframe determination result from the prediction determination circuit 12, and the DCT coefficient output from the DCT circuit 4 as a DC component. It is a circuit that sets a threshold value determined by a signal indicating whether it is an AC component or not for the DCT coefficient output from the DCT circuit 4, and sets the output to 0 when the DCT coefficient is less than this threshold value.

FIG. 3 shows a specific configuration example of the threshold circuit 17. In FIG. 3, the first selector 31 has a D of FIG.
The DCT coefficient from the CT circuit 4 is input. At the control terminal of the second selector 32, the inter-frame / intra-frame determination result from the prediction determination circuit 12, the static / moving determination result from the static / moving determination circuit 16 and the DC / AC signal (whether the DCT coefficient is a DC component A signal indicating an AC component) is given, and one threshold value is selected from the three threshold values 0, DC threshold value and AC threshold value input to the signal input terminal by these three parameters. For example, the input of the DCT circuit 4 is an inter-frame difference signal, the attention area is a still area, and the DC
When the T coefficient is the DC component, the DC threshold is selected, when the input of the DCT circuit 4 is the inter-frame difference signal, and the attention area is the static area, and when the DCT coefficient is the AC component, the AC threshold is selected, and the DCT circuit is selected. When the input 4 is an intra-frame signal and the attention area is a moving area, 0 is selected as the threshold value. Then, the threshold value selected by the selector 32 and the absolute value circuit 3
The absolute value of the DCT coefficient obtained in 3 is compared by the comparator 34, and when the DCT coefficient is smaller, the selector 31 selects 0 and outputs it. Next, the operation of the moving picture coding apparatus according to this embodiment will be described.

In FIG. 1, an image signal of a moving image is input to the input terminal 1. The input image signal is digitized by an A / D converter (not shown), which is an analog image signal obtained by picking up an image of a subject such as a videophone user or a conference participant of a videoconference system by a TV camera (not shown). It is a signal.

The difference between this input image signal and the image signal of one frame before stored in the frame memory 10 by the subtracter 2, that is, the inter-frame difference signal is obtained. The image signal of one frame before stored in the frame memory 10 is motion-compensated by the motion vector obtained by the motion vector detection circuit 11. The intra-frame signal that is the input image signal and the inter-frame difference signal output from the subtracter 2 are determined by the prediction determination circuit 12 as to which information amount is smaller, and the signal with the smaller information amount is selected by the selector 3. To be selected. The selected signal is subjected to discrete cosine transform in a DCT circuit 4 which is an orthogonal transform circuit in a unit of a block composed of a plurality of pixels, for example, 8 × 8 pixels, and is subjected to DCT.
It is output as a coefficient. This discrete cosine transform is used to remove the correlation between pixels.

The DCT coefficient output from the DCT circuit 4 is compared with the threshold value set in the threshold circuit 17,
The output corresponding to the DCT coefficient equal to or less than the threshold is set to 0. The threshold value in the threshold value circuit 17 is determined by the static / moving judgment result of the static / moving judgment circuit 16, the inter-frame / intra-frame judgment result of the prediction judgment circuit 12 and the DCT coefficient as the DC component or the AC component as described above. To be done. The output of the threshold circuit 17 is quantized by the quantizer 5. The quantized signal is
The variable-length coding circuit 13 performs variable-length coding, and the buffer memory 11 smoothes the speed to send it to the communication line at a constant transmission speed.

The signal quantized by the quantizer 5 is also sent to the inverse quantizer 6 and inversely quantized, and further the inverse DCT (I
After the DCT) circuit 7 performs the inverse discrete cosine transform, the adder 8 adds the image signal of one frame before,
Locally decoded.

When the selector 3 selects the in-frame signal, the output of the IDCT circuit 7 becomes the local decoded signal as it is, and the selector 9 selects the output of the IDCT circuit 7. This locally decoded signal is written in the frame memory 10.

In the above embodiment, the threshold circuit 17 is placed before the quantizer 5, but it may be placed after the quantizer 5.
In that case, it is necessary to increase or decrease the set threshold depending on the quantization step width.

Further, the set threshold value of the threshold circuit 17 is controlled by a signal from a control unit (not shown) so that the threshold value becomes smaller as the transmission bit rate becomes higher. It is possible to make the brightness difference inconspicuous.

[0032]

As described above, according to the present invention, even if there is flicker or noise in the input moving image signal, the orthogonal transform coefficient is not transmitted in the still region such as the background portion, thereby improving the coding efficiency. Can be planned.

[Brief description of drawings]

FIG. 1 is a block diagram of a moving picture coding apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration example of a static / moving determination circuit in FIG.

FIG. 3 is a block diagram showing a configuration example of a threshold circuit in FIG.

FIG. 4 is a block diagram of a conventional moving image encoding device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Input terminal 2 ... Subtractor 3 ... Selector 4 ... DCT circuit 5 ... Quantizer 6 ... Inverse quantizer 7 ... IDCT circuit 8 ... Adder 9 ... Selector 10 ... Frame memory 11 ... Motion vector detection circuit 12 ... Prediction Judgment circuit 13 ... Variable-length coding circuit 14 ... Buffer memory 15 ... Output terminal 16 ... Threshold circuit 17 ... Static / moving judgment circuit

Claims (3)

[Claims] 1. A selecting means for selectively outputting an inter-frame difference signal and an intra-frame signal of an input moving image signal, and an orthogonal transform code for orthogonal transform coding the signal selected by this means to output transform coefficients. Quantizing means, quantizing means for quantizing the transform coefficient obtained by this means, means for variable-length coding the quantization result obtained by this means, and the orthogonal transform coding means or the quantizing means. The absolute value of the transform coefficient provided on the output side and output from the orthogonal transform coding means is compared with a preset predetermined threshold value, and when the inter-frame difference signal is selected by the selecting means, A moving picture coding apparatus, comprising: a threshold value processing means for setting an output to 0 if the absolute value of the transform coefficient is equal to or less than a preset threshold value. 2. The moving picture coding apparatus according to claim 1, wherein the threshold value set by the threshold value processing means is such that the threshold value for the DC component of the transform coefficient is larger than the threshold value for the AC component. 3. The threshold value set by the threshold value processing means is such that a threshold value for a transform coefficient belonging to a still region of the input moving image signal is larger than a threshold value for a transform coefficient belonging to a moving region. 2. The moving picture coding device according to 2.