JP3202270B2 - Video encoding device - Google Patents

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, with the development of digital communication networks and the development of communication technologies, moving picture videophones and video conference systems have been put into practical use. In particular, a video conference system is extremely effective in saving time and money required for business trips, and is therefore being introduced in companies. Video coding is one of the most important technologies for realizing a videophone or videoconferencing system.

FIG. 4 is a block diagram showing an example of a conventional moving picture coding apparatus. This video encoding device selects an inter-frame difference signal and an intra-frame signal of an input video signal, and encodes them by DCT (discrete cosine transform) encoding, which is one of orthogonal transform encoding. Device.
In FIG. 4, an input terminal 1 receives an A / D (not shown) of an image signal of a moving image obtained by imaging a subject with a television camera.
It is digitized by the converter and input. The difference between the input image signal and the image signal of the previous frame stored in the frame memory 10 by the subtracter 2 is obtained, and an inter-frame difference signal is obtained. The image signal of the previous frame stored in the frame memory 10 is subjected to motion compensation in accordance with the motion vector obtained by the motion vector detection circuit 11.

The prediction judgment circuit 12 judges which information amount is smaller between the moving picture signal of the current frame input to the input terminal 1 (this signal is called an intra-frame signal) and the inter-frame difference signal from the subtractor 2. Then, the selector 3 selects the signal having the smaller amount of information. The signal selected by the selector 3 is subjected to discrete cosine transform by the DCT circuit 4 and then quantized by the quantizer 5. Discrete cosine transform is used to remove correlation between pixels. The signal quantized by the quantizer 5 is variable-length coded by a variable-length coding circuit 13 and speed-smoothed by a buffer memory 14, so that the signal is transmitted to a communication line at a constant transmission rate.

On the other hand, the signal quantized by the quantizer 5 is also sent to an inverse quantizer 6 where it is inversely quantized,
After being subjected to the inverse discrete cosine transform by the (IDCT) circuit 7, the adder 8 adds the image signal to the image signal of one frame before to perform local decoding. 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 local decoded signal is written to the frame memory 10.

Further, in order to prevent the buffer memory 14 from overflowing, the quantization width of the quantizer 5 and the inverse quantizer 6 is coarsened in accordance with the buffer accumulation amount of the buffer memory 14 to reduce the amount of generated information. When the buffer storage amount decreases, the quantization width is reduced to increase the amount of generated information.

[0007] In this moving picture coding apparatus, for example, in a still area such as a background portion, coding efficiency is improved by DCT coding a frame difference signal having a smaller amount of information. In, the image quality is maintained by the DCT coding of the intra-frame signal.

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

[0009]

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

Accordingly, the present invention provides a moving picture coding system capable of preventing useless information from being transmitted even when the difference between the frames in a still area becomes large due to a change in brightness of the entire screen or noise. It is intended to provide a device.

[0011]

Since the present invention SUMMARY OF THE INVENTION The to solve the above problems, when the inter-frame difference signal is selected as the input of the orthogonal transform means, an absolute value of more resultant transform coefficients to the orthogonal conversion is previously Outputs 0 if it is below the set threshold
Otherwise, the threshold for outputting the transform coefficient as it is
Value processing means between the orthogonal transformation means and the quantization means, or
It is provided between the quantization means and the variable length coding means .

[0012] Here, the threshold set by the threshold processing means, larger than the threshold for the AC component of the threshold transform coefficients for the DC component of a more resultant transform coefficients to the orthogonal conversion
The value or the threshold value for the transform coefficient belonging to the still region of the input moving image signal is set to a value larger than the threshold value for the transform coefficient belonging to the moving region of the input moving image signal . Further, it is desirable that this threshold value be reduced when the transmission bit rate is high.

[0013]

As described above, according to the present invention, the threshold is set for the conversion coefficient of the inter-frame difference signal, and when the conversion coefficient is less than the threshold, the output is set to 0 and information is not transmitted. In addition, it is possible to perform encoding that is strong against noise. For example, the brightness of the entire screen changes due to flicker of fluorescent light for lighting or auto iris of camera, etc.
When noise is mixed, the inter-frame difference signal becomes relatively large even in the still region, and the degree of the difference signal becomes more remarkable as the DC component of the transform coefficient increases.

In such a case, in the conventional moving picture coding apparatus, orthogonal transform coding is performed while transmitting a still area which originally does not need to be transmitted, but in the present invention, the transform coefficient of the still area is not transmitted. Can be. That is, when the brightness of the entire screen changes, the inter-frame difference signal increases even in the still region, but the degree is smaller than that in the moving region. Therefore, by appropriately selecting the threshold, the transform coefficient is transmitted only in the moving region. Encoding efficiency can be increased. in this case,
Since the increase in the inter-frame difference signal due to the change in brightness increases as the DC component of the transform coefficient increases, it is effective to increase the threshold value for the DC component.

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

Since the image quality is improved when the transmission bit rate is increased, when the threshold value is high, the luminance difference between the still area and the moving area becomes conspicuous. Therefore, when the transmission rate is high, it is better to reduce the threshold.

[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 one embodiment of the present invention. This video encoding apparatus includes a subtracter 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, a selector 9, frame memory 10, motion vector detection circuit 11, prediction determination circuit 12, variable length encoding circuit 13, buffer memory 1
4. It is composed of a static / motion determining circuit 16 and a threshold circuit 17. The difference from the conventional moving picture coding apparatus of FIG. 4 is that a still / moving decision circuit 16 and a threshold circuit 17 are newly added.

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

FIG. 2 shows an example of a specific configuration of the static / movement determination circuit 16. The static motion determining circuit 16 receives the inter-frame difference signal from the subtractor 2 and the motion vector from the motion vector detecting circuit 11. The difference signal between frames is
The signal 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 or not the value is 0. Comparator 24
When it is determined that the sum of absolute values of the inter-frame difference signal 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, otherwise, it is determined to be a still area.

It should be noted that although the static motion can be determined to some extent only by the sum of the absolute values of the inter-frame difference signals or only the motion vector, a more accurate static motion can be determined by combining both as shown in FIG. Still / moving determination can also be performed by examining the variance of the inter-frame difference signal instead of the absolute value sum of the inter-frame difference signal.

The threshold circuit 17 calculates a static / moving determination result from the static / moving determining circuit 16, an inter-frame / intra-frame determining result from the prediction determining circuit 12, and a DCT coefficient output from the DCT circuit 4. This is a circuit in which a threshold value determined by a signal indicating whether the signal is an AC component is set for the DCT coefficient output from the DCT circuit 4, and the output is set to 0 when the DCT coefficient is equal to or less than the threshold value.

FIG. 3 shows a specific configuration example of the threshold circuit 17. In FIG. 3, the first selector 31 has D in FIG.
The DCT coefficient from the CT circuit 4 is input. The control terminal of the second selector 32 includes an inter-frame / intra-frame determination result from the prediction determination circuit 12, a static / dynamic determination result from the static / dynamic determination circuit 16, and a DC / AC signal (if the DCT coefficient is a DC component or not). A signal indicating whether the signal is an AC component) is given, and one of the three thresholds 0, DC threshold, and AC threshold input to the signal input terminal is selected based on these three parameters. For example, the input of the DCT circuit 4 is an inter-frame difference signal, the region of interest is a static region, and the
When the T coefficient is a DC component, a DC threshold is selected. When the input of the DCT circuit 4 is an inter-frame difference signal, the attention area is a stationary area, and the DCT coefficient is an AC component, the AC threshold is selected. When the input of 4 is an intra-frame signal and its attention area is a moving area, 0 is selected as a threshold value. The threshold value selected by the selector 32 and the absolute value circuit 3
The absolute value of the DCT coefficient obtained in step 3 is compared by the comparator 34. When the DCT coefficient is smaller, the selector 31 selects and outputs 0 , otherwise, the input (DCT circuit 4
Is output as it is .

In FIG. 1, an input terminal 1 receives an image signal of a moving image. This input image signal is digitized by an A / D converter (not shown) into an analog image signal obtained by capturing an image of a subject such as a user of a videophone or a conference participant of a video conference system with a television camera (not shown). Signal.

The difference between the input image signal and the image signal of the previous frame stored in the frame memory 10 by the subtracter 2, that is, an inter-frame difference signal is obtained. The image signal of the immediately preceding frame stored in the frame memory 10 has been motion compensated by the motion vector obtained by the motion vector detection circuit 11. The prediction determination circuit 12 determines which information amount is smaller between the intra-frame signal which is the input image signal and the inter-frame difference signal output from the subtractor 2, and the selector 3 selects the signal having the smaller information amount. Selected. The selected signal is subjected to a discrete cosine transform by a DCT circuit 4 which is an orthogonal transform circuit in a block unit composed of a plurality of pixels, for example, 8 × 8 pixels.
Output as coefficients. This discrete cosine transform is used to remove correlation between pixels.

The DCT coefficient output from the DCT circuit 4 is compared with a threshold set in the threshold circuit 17 and
The output corresponding to the DCT coefficient equal to or smaller than the threshold value is set to 0. As described above, the threshold value of the threshold circuit 17 is determined by the result of the static / moving determination of the static / moving determining circuit 16, the result of the inter-frame / intra-frame determination by the prediction determining circuit 12, and whether the DCT coefficient is a DC component or an AC component. Is done. The output of the threshold circuit 17 is quantized by the quantizer 5. The quantized signal is
The data is variable-length coded by the variable-length coding circuit 13 and further speed-smoothed by the buffer memory 11, so that the data is transmitted to the communication line at a constant transmission speed.

The signal quantized by the quantizer 5 is also sent to an inverse quantizer 6 where the signal is inversely quantized and further inverse DCT (I
After the inverse discrete cosine transform is performed by the DCT circuit 7, the adder 8 adds the resultant signal to the image signal of the previous frame.
It is locally decoded.

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 the output of the IDCT circuit 7. This local decoded signal is written to the frame memory 10.

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

Further, by controlling the threshold set in the threshold circuit 17 by a signal from a control unit (not shown) so as to become smaller as the transmission bit rate becomes higher, the image quality becomes higher. The step of luminance can be made inconspicuous.

[0032]

As described above, according to the present invention, even if there is flicker or noise in the input video signal, the orthogonal transform coefficients are not transmitted in a still area such as a background, thereby improving the coding efficiency. Can be planned.

[Brief description of the drawings]

FIG. 1 is a block diagram of a video encoding device according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration example of a static / motion determining circuit in FIG. 1;

FIG. 3 is a block diagram showing a configuration example of a threshold circuit in FIG. 1;

FIG. 4 is a block diagram of a conventional moving picture 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 motion judgment circuit

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-222593 (JP, A) JP-A-2-243063 (JP, A) JP-A-2-272889 (JP, A) JP-A-3- 89689 (JP, A) JP-A-3-89690 (JP, A) JP-A-3-24888 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N ^7/ 24-7 / 68 H04N 7/14-7/15 JICST file (JOIS)

Claims (4)

(57) [Claims] 1. A selection means for selectively outputting an inter-frame difference signal and an intra-frame signal of an input moving image signal, and an orthogonal transformation means for orthogonally transforming a signal output from the selection means and outputting a transform coefficient. A quantizing unit that quantizes a transform coefficient output from the orthogonal transform unit; a variable-length encoding unit that performs variable-length encoding on a quantization result obtained by the quantizing unit; and the orthogonal transform unit. Provided between the quantization means,
The absolute value of the transform coefficient output from the orthogonal transform means is compared with a predetermined threshold value, and when the inter-frame difference signal is output from the selecting means, the absolute value of the transform coefficient is determined in advance. A threshold processing means for outputting 0 if the value is equal to or less than the set threshold value, and otherwise outputting the transform coefficient output from the orthogonal transformation means as it is, wherein the threshold value set by the threshold processing means Is a moving image encoding apparatus, wherein a threshold value for a DC component of the transform coefficient is larger than a threshold value for an AC component of the transform coefficient. 2. Selection means for selectively outputting an inter-frame difference signal and an intra-frame signal of an input moving image signal, and orthogonal transformation means for orthogonally transforming a signal output from the selection means and outputting a transform coefficient. A quantizing means for quantizing a transform coefficient output from the orthogonal transform means; a variable-length coding means for performing variable-length coding on a quantization result obtained by the quantizing means; and The inter-frame difference signal is provided from the selection unit by comparing the absolute value of the transform coefficient output from the quantization unit with a predetermined threshold value, which is provided between the variable-length encoding unit. When the absolute value of the transform coefficient is less than or equal to a preset threshold value when the transform coefficient is output, 0 is output. Otherwise, a threshold processing means that directly outputs the transform coefficient output from the quantizing means. Have, before Thresholding the threshold set by means video encoding apparatus characterized by threshold for the DC component of the transform coefficients is greater than the threshold for the AC components of the conversion coefficients. 3. Selection means for selectively outputting an inter-frame difference signal and an intra-frame signal of an input moving image signal, and orthogonal transformation means for orthogonally transforming a signal output from the selection means and outputting a transform coefficient. A quantizing means for quantizing a transform coefficient output from the orthogonal transform means; a variable-length coding means for performing variable-length coding on a quantization result obtained by the quantizing means; and the input video signal For static and motion determining means for determining whether a static area or a moving area, provided between the orthogonal transform means and the quantization means,
The absolute value of the transform coefficient output from the orthogonal transform means is compared with a predetermined threshold value, and when the inter-frame difference signal is output from the selecting means, the absolute value of the transform coefficient is determined in advance. A threshold processing means for outputting 0 if the value is equal to or less than the set threshold value, and otherwise outputting the transform coefficient output from the orthogonal transformation means as it is, wherein the threshold value set by the threshold processing means Is controlled based on the determination result of the still / moving determination means so that a threshold value for a conversion coefficient belonging to a still region of the input moving image signal is larger than a threshold value for a conversion coefficient belonging to a moving region of the input moving image signal. A moving image encoding apparatus. 4. Selection means for selectively outputting an inter-frame difference signal and an intra-frame signal of an input video signal, and orthogonal transformation means for orthogonally transforming a signal output from the selection means and outputting a transform coefficient. A quantizing means for quantizing a transform coefficient output from the orthogonal transform means; a variable-length coding means for performing variable-length coding on a quantization result obtained by the quantizing means; and the input video signal A static / moving determining means for determining whether the area is a static area or a moving area; provided between the quantizing means and the variable length encoding means, and the absolute value of a transform coefficient output from the quantizing means and A comparison is made with a set predetermined threshold value, and when the inter-frame difference signal is output from the selection means, 0 is output if the absolute value of the transform coefficient is equal to or less than a preset threshold value. When the amount Threshold processing means for directly outputting the conversion coefficient output from the conversion means, wherein the threshold value set by the threshold processing means is a threshold value for a conversion coefficient belonging to a still area of the input moving image signal. A moving picture coding apparatus characterized in that the moving picture coding apparatus is controlled based on a determination result of the still / moving determining means so as to be larger than a threshold value for a transform coefficient belonging to a moving area of an image signal.