JPH07336684A - Picture signal decoding device - Google Patents

Abstract

PURPOSE:To reduce block distortion which is one of coding noises by inverse- orthogonal-transforming the data of the frequency domain of a block after adding a noise corresponding to a quantizing step to it. CONSTITUTION:An input coded signal is sent to an inverse quantizing part 2 after the coded signal having been Huffman-encoded is decoded by a variable length decoding part 1, and multiplication with a quantizing step value is executed. A block distortion judging/processing part 3 judges whether the block distortion is conspicuous or not by the encoded data of a block unit and the coding parameter of the quantizing step, etc., when that block is decoded, and adds the noise corresponding to the quantizing step to the data of the frequency domain of the block in accordance with that result, and the inverse orthogonal transformation operation is executed in an inverse orthogonal transforming part 4. The output of the inverse orthogonal transforming part 4 is outputted after it is summed with a predictive value read out of a frame memory part 7 in conformity with a predictive motion value from a predicting part 6 in an adding part 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal decoding apparatus, and more particularly to a video telephone, a video conference, and a still image transmitting apparatus for dividing an image into blocks and decoding compression-encoded data. The present invention relates to an image signal decoding device for improving image quality in an image communication system such as a video CD and an image storing / reproducing system.

[0002]

2. Description of the Related Art In recent years, ISDN (Integrated Services)
The demand for image communication services as new communication services is increasing due to the spread of Digital Network (service integrated digital network), and there are great expectations for videophones and videoconference systems. There is also a movement to realize a videophone by using a conventional analog line. Generally, in the case of transmitting image information such as a videophone or a video conference system, the amount of image information is enormous, whereas the amount of image information to be transmitted is limited in terms of the line speed and cost of the line used for transmission. It becomes necessary to perform compression coding and reduce the amount of information for transmission. Various studies have been made on encoding methods for compression, and some of them are being put to practical use in videophones, videoconference systems, and the like.

As a high-efficiency coding method for an image that is being put into practical use, the image is divided into small blocks made up of a plurality of samples, and an image signal of each small block or various predictions in each small block are performed. There is a motion-compensated inter-frame prediction orthogonal transform coding method in which an error signal is subjected to orthogonal transform such as discrete cosine transform, and its transform coefficient is quantized and coded for transmission.

In this motion-compensated inter-frame prediction orthogonal transform coding method, first, the input image signal is 1 for each image frame.
The image signal of the previous frame and the input image signal are divided into blocks of a certain size, and it is detected from which part of the image signal of the previous frame the block of the input image signal has moved. Using the detection result of this motion, the difference is obtained by using the image frame encoded one frame before as the prediction value. Then, the prediction error signal that is the difference value is subjected to orthogonal transform calculation, and the amount of information is compressed by quantizing the orthogonal transform coefficient.

FIG. 5 is a block diagram of a conventional image signal decoding apparatus. In the figure, 21 is a variable length decoding unit, 22 is an inverse quantization unit, 24 is an inverse orthogonal transform unit, 25 is an addition unit, and 26 is A prediction unit, 27 is a frame memory, and 28 is a post-filter processing unit. The input coded signal is a Huffman-coded coded signal that is decoded by the variable length decoding unit 21 and sent to the dequantization unit 22 where it is multiplied by the quantization step value. The inversely quantized coefficient is subjected to an inverse orthogonal transform operation in the inverse orthogonal transform unit 24. The output of the inverse orthogonal transform unit 24 is added to the predicted value read from the frame memory unit 27 in the addition unit 25 according to the predicted motion value from the prediction unit 26, and then output as an output signal and the frame memory. It is stored in the unit 27 and is used for decoding the next frame.

The output decoded signal has a deterioration in image quality due to a quantization error. In addition, since the quantization control is controlled by the generated code amount and transmission capacity, the quantization condition changes for each divided block, and when the encoded image is viewed on the entire screen, Distortion of a block in which a certain block is conspicuous or variation in screen roughness between blocks occurs, resulting in deterioration of image quality. Therefore, the post-filter processing unit 28 is provided to reduce the distortion, and the spatial filtering process for reducing the coding noise such as block distortion is executed.

A known document describing a conventional image signal decoding apparatus is, for example, Japanese Patent Laid-Open No. 6-54311. According to this publication, it is not necessary to unify the algorithms of both the encoder and the decoder, the block information is reduced only by the processing on the decoder side, and the encoding information is decoded in order to improve the image quality. An image signal is restored by performing inverse orthogonal transform on the transform coefficient obtained by inverse quantization, the image signal stored in the frame memory of the decoder is differentiated by the differential processing means, and the differential processing is performed by the direction extraction means. With respect to the output result of the means, the directionality of the distribution of the differential value is extracted according to the distribution state of the differential values of the pixels around the block boundary, and the block distortion detection means is based on the directionality of the differential value distribution of the direction extracting means. By detecting a pixel in which block distortion occurs, filtering processing is performed by a filtering processing means to adaptively reduce block distortion in a pixel in which block distortion occurs.
The occurrence of block distortion is detected from the restored image signal, and the block distortion is reduced by filtering the part.

[0008]

In the conventional image coding apparatus based on the orthogonal transform coding method, since the result of the orthogonal transform is quantized to reduce the amount of information, the image quality is deteriorated due to the quantization error. Occurs. In addition, since the quantization control is controlled by the generated code amount and transmission capacity, the quantization condition changes for each divided block, and when the encoded image is viewed on the entire screen, Distortion of a block in which a certain block is conspicuous or variation in screen roughness between blocks occurs, resulting in deterioration of image quality.

In order to reduce the coding noise, the conventional image signal decoding apparatus has been realized by providing a post filter section. However, the post filter for performing spatial filtering is large in circuit scale, Not only hinders downsizing and cost reduction of the decoding device, but it is difficult to perform adaptive processing according to the state of coding noise.

The present invention has been made in view of the above situation, and reduces block distortion occurring in an image using an orthogonal transform coding method without requiring additional hardware such as a spatial filter. It is an object of the present invention to provide an image signal decoding device capable of obtaining a decoded image with suppressed deterioration in image quality.

[0011]

In order to achieve the above object, the present invention provides (1) an image signal decoding apparatus for dividing an image into blocks and decoding compression-encoded data. A block distortion determining unit that adds noise in the frequency domain of the block to block distortion, which is one type of noise, is provided so that the block distortion can be reduced. Further, (2) Huffman coding is performed. Variable length decoding means for decoding the input coded signal into a coded signal, dequantization means for multiplying the signal decoded by the variable length decoding means by a quantization step value, and block unit Based on the encoded data and the encoding parameters such as the quantization step, it is determined whether the block distortion is conspicuous when the block is decoded, and the determination result determines the frequency domain data of the block. A block distortion determining means for adding noise according to the quantization step to the data, and an inverse orthogonal transforming means for performing an inverse orthogonal transform operation on the signal added with the noise by the block distortion determining means. (3) In (1) or (2) above, the block distortion determination means determines whether or not the block distortion of the inversely quantized transform coefficient is prominent, and the frequency distribution determination means. With respect to a block for which block distortion is conspicuous, a noise adding unit for adding noise according to the quantization step to the data in the frequency domain of the block is provided.

[0012]

In the image signal decoding apparatus according to the present invention, the block distortion is conspicuous when the block is decoded by the Huffman decoding of the coded data and the data and the coding parameter such as the quantization step. It is determined whether or not, as a result of the determination, for the block in which the block distortion is determined to be conspicuous, the data in the frequency domain of the block is added with noise according to the quantization step, and by inverse orthogonal transform, Block distortion, which is one of coding noises, is reduced.

[0013]

Embodiments will be described below with reference to the drawings. FIG. 1 is a block diagram for explaining an embodiment of an image signal decoding apparatus according to the present invention, in which 1 is a variable length decoding unit, 2 is a dequantization unit, and 3 is block distortion determination / processing. Department,
Reference numeral 4 is an inverse orthogonal transformation unit, 5 is an addition unit, 6 is a prediction unit, and 7 is a frame memory.

Huffman decoding is performed on the encoded data, and it is determined in block units whether the block distortion is conspicuous when the block is decoded by the data and the encoding parameter such as the quantization step. For blocks for which distortion is conspicuous, noise in accordance with the quantization step is added to the data in the frequency domain of the blocks and inverse orthogonal transform is performed, so that the image decoding unit is not subjected to a hard load, and the code is encoded. It is possible to reduce block distortion, which is one of the conversion noises.

The input coded signal is the Huffman-coded coded signal decoded by the variable length decoding unit 1 and sent to the dequantization unit 2 where it is multiplied by the quantization step value. . The block distortion determination / processing unit 3 determines whether block distortion is conspicuous when the block is decoded by the encoded data in block units and the encoding parameters such as the quantization step, and the determination result Thus, noise corresponding to the quantization step is added to the data in the frequency domain of the block, and the inverse orthogonal transform unit 4 performs the inverse orthogonal transform operation. The output of the inverse orthogonal transformation unit 4 is added to the prediction value read from the frame memory unit 7 according to the prediction motion value from the prediction unit 6 in the addition unit 5 and then output as an output signal, It is stored in the frame memory unit 7 and used for decoding the next frame.

FIG. 2 is a block diagram of the block distortion judging / processing unit in FIG. 1, in which 11 is a frequency distribution judging unit and 12 is a frequency distribution judging unit.
Is a noise adding unit. The transform coefficient inversely quantized by the inverse quantization unit 2 is determined by the frequency distribution determination unit 11 whether block distortion is conspicuous. Block distortion, in the block of the flat region with little change in brightness, the AC component is removed by quantization and only the DC component is coded, so in the decoded image the inside of the block is represented by the average value of brightness, This is a distortion in which the difference in brightness at the boundaries between blocks becomes noticeable. Therefore, block distortion can be reduced by adding an AC component to a block in a flat region where the change in luminance is small. That is, the noise adding unit 12 adds noise corresponding to the quantization step to the data in the frequency domain of the block for which the block distortion is determined to be conspicuous by the frequency distribution determining unit 11. Thereby, block distortion can be reduced.

The frequency distribution determining unit 11 determines whether the data of the block input as described above is the data of the flat area, based on the frequency distribution. 3 (a)-(d)
It is a figure which shows an example of noise addition. 8x8 as orthogonal transformation
This is an example of the case where DCT (Discrete Cosine Transform) is performed. The component on the upper right represents the DC component, the frequency increases in the horizontal direction toward the left, and the component in the vertical direction decreases toward the bottom. It represents the component whose frequency is high.

Originally, the transform coefficients having the distribution shown in FIG. 3A are quantized into those shown in FIG. 3B, and the inverse coefficients are decoded as shown in FIG. 3C. . Since the decoding coefficient as shown in FIG. 3C has only the DC component, it is determined that the block distortion becomes significant, and the noise adding unit 12 adds noise corresponding to the quantization step value Qst to FIG. 3C. As a result, as shown in FIG. 3D, this FIG.
Is sent to the inverse orthogonal transform unit 4.

FIGS. 4A and 4B are diagrams showing the positions of the coefficients used for the determination and the positions where noise is added. In the present invention, the distribution of blocks determined to add noise is, as indicated by the shaded area in the figure, direct current (0,0) component, (0,0)
It is assumed that there is a coefficient value only in the 1) component, the (1,0) component, or only the (1,1) component, that is, the coefficients other than the above components are all zero. As for the noise to be added, as shown in the dotted area in FIG. 4, a coefficient weighted with a frequency smaller than Qst is added to the coefficient having a coefficient value of 0 for the square part up to the (3,3) component. All the above processes can be processed by software, and the amount of calculation required for this process is extremely small. Therefore, block distortion can be reduced without increasing the hardware scale.

[0020]

As is apparent from the above description, according to the present invention, when Huffman decoding of encoded data is performed and the block is decoded by the data and the encoding parameter such as the quantization step in block units, It is determined whether the block distortion is conspicuous, and for the block for which the block distortion is conspicuous according to the judgment result, noise in accordance with the quantization step is added to the frequency domain data of the block, and the inverse orthogonal transform is performed. As a result, it is possible to reduce block distortion, which is one type of coding noise, without imposing a hard load on the image decoding unit, and it is possible to provide an image with better image quality to the user.

[Brief description of drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of an image signal decoding device according to the present invention.

FIG. 2 is a block diagram of a block distortion determination / processing unit in FIG.

FIG. 3 is a diagram showing an example of noise addition according to the present invention.

FIG. 4 is a diagram showing positions of coefficients used for determination and positions of adding noise in the present invention.

FIG. 5 is a block diagram of a conventional image signal decoding device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Variable length decoding part, 2 ... Inverse quantization part, 3 ... Block distortion determination / processing part, 4 ... Inverse orthogonal transformation part, 5 ... Addition part, 6 ... Prediction part, 7 ... Frame memory, 11 ... Frequency distribution Judgment part,
12 ... Noise adding section.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H03M 7/30 Z 0570-5J 7/40 0570-5J H04N 1/41 B

Claims (3)

[Claims] 1. An image signal decoding apparatus for dividing an image into blocks and decoding compression-encoded data, wherein block distortion, which is one of coding noises, is added with noise in the frequency domain of the block. An image signal decoding apparatus characterized in that it has block distortion determination means so that block distortion can be reduced. 2. A variable length decoding means for decoding a Huffman-encoded input coded signal into a coded signal, and a signal decoded by the variable length decoding means is multiplied by a quantization step value. It is determined whether or not the block distortion is conspicuous when the block is decoded by the inverse quantization means, the encoded data in block units, and the encoding parameters such as the quantization step, and the frequency of the block is determined by the determination result. It is characterized by comprising block distortion determining means for adding noise to the data in the region according to the quantization step, and inverse orthogonal transforming means for performing an inverse orthogonal transform operation on the signal added with noise by the block distortion determining means. Image signal decoding device. 3. The block distortion determination means determines whether the inversely quantized transform coefficient is prominent in block distortion, and the frequency distribution determination means determines that block distortion is conspicuous. About the block
The image signal decoding apparatus according to claim 1 or 2, further comprising: a noise adding unit that adds noise according to a quantization step to the frequency domain data of the block.