JP2776425B2 - Cell loss compensation image decoding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] In an image data transmission system for orthogonally transforming input image data, encoding coefficients in an orthogonal transform coefficient area and transmitting the cells in units of cells, the coefficient of a coefficient area when a cell is discarded occurs Among the above, regarding the cell discard compensation image decoding method for compensating for a plurality of coefficients representing low frequency components, the transmitting side compares the magnitude relations of the corresponding low frequency coefficients of the blocks of the current frame and the previous frame, and when the difference is large, By transmitting the compensation information to the receiving side, the purpose is to compensate for the coefficient having a large difference when the cell is discarded by the compensation information. , An image data transmission system having an inverse orthogonal transform unit that encodes coefficients in the orthogonal transform coefficient area, transmits the encoded cells in units of cells, and performs inverse orthogonal transform on a receiving side. In, the plurality of coefficients representing frequency components from DC to high frequency in the orthogonal transform coefficient region are grouped into a plurality of groups corresponding to a plurality of frequency bands, and the transmitting side of the image data transmission system, And a plurality of coefficients in a group of the lowest frequency in the plurality of groups corresponding to the plurality of coefficients in a block corresponding to the block in a frame one frame before the frame including the block, respectively. Information indicating whether the difference between the corresponding coefficients is large or small, and the value of the coefficient in the current block with respect to the coefficient having the large difference, to the receiving side of the image data transmission system. The transmitting means of the coefficient to be transmitted and the receiving side of the image data transmission system detect the presence / absence of cell discard, and respond to the discarded cell when the cell is discarded. According to the information indicating the magnitude relation of the coefficient difference transmitted from the coefficient magnitude relation transmitting means for the block, the coefficient having a small difference among the coefficients in the lowest frequency group is one frame more than the frame including the block. Data obtained by replacing the corresponding coefficient data in the block corresponding to the block before the frame and, for a coefficient having a large difference, the corresponding coefficient in the block with the coefficient value transmitted from the coefficient size relation transmitting means. To the inverse orthogonal transform unit.

[Industrial applications]

The present invention relates to an image data transmission system, and more particularly, to an image data transmission system that orthogonally transforms input image data, encodes coefficients in an orthogonal transformation coefficient area, and transmits the cells in units of cells. The present invention relates to a cell discard compensation image decoding method for compensating a plurality of coefficients representing low frequency components among coefficients in a region.

[Conventional technology]

Recently, digitization of image equipment and transmission methods of image digital signals have been widely developed and studied in many application fields such as videophones. Generally, image data has a very large amount of information. For example, if a current television is directly digitized, a transmission capacity of 100 Mb / s is required, and the image data has 1500 times the information amount of the audio. 1 on TV
The screen switches 30 times per second, and frames are switched in 1/30 seconds. Pixels are generally three-dimensionally arranged on one screen corresponding to one frame. Even if the level of one pixel is represented by, for example, 8 bits, the image data for one frame has a very large amount of information. More.

However, even when frames are switched in 1/30 second on a television, the picture content of each frame often does not change so much. That is, the image has many parts that are entirely stopped, such as the sky and the background, and few parts that change for each frame. Therefore, one method of compressing and encoding image data is an inter-frame encoding method.

In the inter-frame coding method, a difference in level data is obtained between two temporally consecutive frames, for example, for each corresponding pixel, and the difference is subjected to, for example, Huffman coding and transmitted on a transmission path. Assuming that the level for most pixels does not change, the difference for those pixels is zero,
The difference can be represented by one bit. A code for a pixel having a large level difference requires 8 bits or more. By encoding only these differences, the amount of information to be transmitted is greatly compressed. On the other hand, a method of directly encoding data in one frame is called an intra-frame encoding method.

Regardless of whether inter-frame coding or intra-frame coding is used, digital representation of an image signal basically requires two operations of sampling and quantization. There are two ways to sample an image, the first of which is to represent the image by level values for a discrete array of points corresponding to the pixels in the frame. The second method is to orthonormally expand a level value function (image function) defined on the XY plane corresponding to the frame plane, and use the expansion coefficient as a sample value.

FIG. 9 is an overall block diagram of an image transmission system using an orthonormal transform which is the second system. In FIG. 9, the input image is blocked by 1 and orthogonally transformed by 2, then quantized by 3, is subjected to, for example, Huffman coding by 4, and is sent to the transmission line 5 from the sending side. Then, on the receiving side, the signal from the transmission path 5 is decoded by 6, inverse orthogonally transformed by 7, deblocked by 8, and output as image data.
Here, the 1-blocking does not collectively perform two-dimensional data of K × K pixels (corresponding to one screen) and performs orthogonal transformation. Instead, blocks of about 4 × 4 to 16 × 16 pixels are regarded as one block. This is performed to reduce the conversion time by obtaining the conversion coefficient in units. The types of orthogonal transform of 2 include, for example, Hadamard transform, cosine transform, Karhunen-Loeve transform, and the like. In recent years, a discrete representation of cosine transform is used, and discrete cosine transform (DCT) described later is often used.

In the orthogonal transform coding system used in FIG. 9, attention is paid to the correlation of the small area in the screen, and the orthogonal transform is performed using the pixels of the small area as a numerical sequence. The resulting transform coefficients correspond to frequency components and represent each component from low to high frequency. In image signals, low-frequency components are generally large in power and high-frequency components are small in power. Therefore, low-frequency components should be transmitted by performing quantization by assigning a large number of bits to high-frequency components and a small number of bits to high-frequency components. The amount of information is reduced.

FIG. 10 is a diagram showing a transform coefficient area after performing orthogonal transformation on image data in one block composed of 4 × 4 pixels. In this transform coefficient area, the upper left area 9 generally represents the DC component of the signal, and the coefficients in the other areas all represent the AC component. The AC component corresponds to each frequency from a low frequency to a high frequency, and the frequency increases slightly in the lower right direction.

[Problems to be solved by the invention]

In the image data transmission system using the orthogonal transform as described above, regardless of which encoding method is used within a frame or between frames, image data is provided with, for example, a header indicating a data destination on a transmission path. Is transmitted by a fixed length cell of several bytes. In this case, if a header error or excess processing capacity of the network due to an excessive number of cells occurs, the cells are discarded in the network and may not reach the receiving side.

In such a case, conventionally, the image data is output with the coefficient data of the corresponding block in the frame immediately before the frame including the block corresponding to the discarded cell as the data of the discarded block. Was. Therefore, there is a problem that the image is deteriorated in the receiving unit when the cell is discarded. In addition, when the difference between the low-frequency coefficients among the coefficients of the corresponding block of the previous frame and the blocks of the current frame is large, the deterioration of the image quality is particularly remarkable.

According to the present invention, the transmitting side compares the magnitude relationship of the corresponding low frequency coefficients of the blocks of the current frame and the previous frame, and transmits the compensation information to the receiving side when the difference is large. It is intended to compensate for a coefficient having a large value by using compensation information.

[Means for solving the problem]

FIG. 1 is a block diagram showing the principle configuration of an image data transmission system using the cell discard compensation image decoding system of the present invention. In the figure, input image data is subjected to, for example, discrete cosine transform by an orthogonal transform unit 14, and
The data is encoded at 15 and output from the transmission unit to the transmission path 16. On the receiving side, the data input from the transmission path 16 is decoded by the decoding unit 17.
And the inverse orthogonal transform unit 12 performs, for example, inverse discrete cosine transform and outputs the image data. In the first invention and the second invention, the contents output by the coefficient magnitude relation transmitting means 11 to the transmission line 16 and the content output by the
The construction of the block diagram is completely the same except that the operation of the discard compensation means 13 provided therein is different from that of the first embodiment.

In FIG. 1, input image data is orthogonally transformed by using a plurality of pixels as blocks, and coefficients in the orthogonal transform coefficient area are encoded and transmitted in units of cells. A plurality of coefficients representing frequency components are grouped into a plurality of groups corresponding to a plurality of frequency bands.

In the first invention, the coefficient magnitude relation transmitting means 11 includes a plurality of coefficients in a group of the lowest frequency among the plurality of groups in the block currently being transmitted on the transmitting side of the image data transmission system and this block. It is determined whether the difference between the corresponding coefficients in each of the corresponding blocks in the frame one frame before the frame is larger or smaller than a predetermined threshold value for each coefficient. Determine and transmit the magnitude relation information indicating the result and the value of the coefficient in the current block for the coefficient having a large difference as compensation information, for example, a value obtained by performing coarser quantization on the coefficient in the block. It transmits to the receiving side of the system via the path 16.

On the receiving side of the image data transmission system, for example, the decoding unit 17
The discard compensating means 13 provided inside detects the presence or absence of cell discard from the contents of the cell transmitted via the transmission path 16,
When a cell is discarded, for the block corresponding to the discarded cell, the information indicating the magnitude relationship of the coefficient difference transmitted from the coefficient magnitude relationship transmitting means 11 on the transmitting side is used to perform the above-described lowest frequency grouping. Of the coefficients having a small difference, the corresponding coefficient in the corresponding block one frame before the frame including the block is transmitted, and for the coefficient having a large difference, the corresponding coefficient in the block is transmitted. The coefficient value is replaced with the coefficient value transmitted from the means 11 and output to the inverse orthogonal transform unit 12.

In the second invention, the transmission-side coefficient magnitude relation transmitting means 11 transmits information indicating whether the difference between the respective coefficients is larger or smaller than a predetermined threshold value and, for a coefficient having a large difference, the difference between the coefficients. Send the value to the receiver. In response, the discard compensating means 13 on the receiving side adds the transmitted difference and the value of the corresponding coefficient in the corresponding block one frame before for the coefficient having a large difference, and outputs the result to the inverse orthogonal transform unit 12. Except for this, the same operation as in the first invention is performed.

[Action]

In the present invention, the coefficients of the orthogonal transform coefficient area are divided into a plurality of groups corresponding to a plurality of frequency bands, and the cell discard compensation image decoding method of the present invention is applied to the lowest frequency group. Is done. As described in FIG. 10, the upper leftmost coefficient in the coefficient area indicates the DC component, and the frequency of the AC component becomes higher toward the lower right. Therefore, for example, the coefficient regions corresponding to a plurality of frequency bands are grouped by dividing the coefficient region by a broken line from the upper right to the lower left.

Then, for a plurality of coefficients in the lowest frequency group among these groups, a difference is taken between the corresponding coefficient in the current frame and the corresponding coefficient in the frame immediately before the previous frame, and the difference is calculated for each coefficient. And magnitude relationship information indicating whether the difference is larger or smaller than a predetermined threshold value, and when the difference is large, the value of the coefficient in the current block in the first invention and the value of the difference itself in the second invention Is transmitted to the receiving side, the receiving side uses the information indicating the magnitude function of each coefficient sent from the transmitting side and the value of the coefficient of the current frame when the difference is large, or the information indicating the value of the difference, and When discarding occurs, the coefficient value in the immediately preceding frame is compensated and output. As a coefficient of a group other than the lowest frequency group, for example, the value of the corresponding coefficient in the previous frame is output as it is.

As described above, in the present invention, even when coefficients of the block of the current frame are lost due to cell discarding, it is possible to output image data by reducing errors of low-frequency coefficients among those coefficients. .

[Embodiment] FIG. 2 shows an embodiment of a method of setting a frequency band in an orthogonal transform coefficient region according to the present invention. In the figure, the upper leftmost coefficient indicates a DC component, and all other coefficients indicate an AC component. The AC component has a higher frequency in the lower right region in the figure. Therefore, as shown in the figure, the frequency band can be divided by a broken line (thick line) from the upper right side to the lower left side of the coefficient area. Although the figure shows a coefficient area for 8 × 8 pixels, the number of pixels is not limited to this, and the way of dividing the frequency band by a thick broken line is not limited to this. It is.

FIG. 3 is a conceptual diagram of an embodiment of the first invention for coefficients of a group of the lowest frequency composed of three coefficients. In the figure, (a) shows coefficients in a frame one frame before the current frame, and the group of the lowest frequency includes three components of DC component L1, AC components L2 and L3. FIG. 6B shows coefficients in the current frame, and the group of the lowest frequency is composed of DC component l1 and AC components l2 and l3. Note that, among the coefficients in these frames, coefficients other than the lowest frequency group are not applied to the present invention, and thus all of the values are set to H.

The coefficients of the lowest frequency group in FIGS. 3A and 3B are compared with each other, and the difference between the coefficient in the current frame and the coefficient in the previous frame is predetermined for each coefficient. It is determined whether it is larger or smaller than the threshold value.
Here, the threshold value for each coefficient may be the same, but different values may be used because the influence on the entire image differs depending on the frequency. And for example, DC component l1
If the difference between L1 and the difference between l3 and L3 in the AC component exceeds the respective threshold value, the coefficients l1 and l3 together with information indicating the magnitude relation of the respective coefficients, for example, the normal coefficient The result is transmitted to the receiving side as (l1) and (l3) as a result of performing coarser quantization than in the quantization at the time of transmission.

FIG. 3 (c) shows the coefficients output as the current frame after normal decoding when no cell discard has occurred. On the other hand, FIG. 4D shows the coefficients after decoding when cell discarding occurs, and among the coefficients in the lowest frequency domain, those having a small difference between the current frame and the previous frame are the previous coefficients. Is output, and (l1) and (l3) sent from the transmitting side are output for coefficients having a large difference.

FIG. 4 is an overall configuration block diagram of an embodiment of an image data transmission system according to the present invention. In the figure, the transmitting side uses the output of the subtractor 22 (described later) for each coefficient in the lowest frequency group together with information indicating the magnitude relationship between the coefficient of the current frame and the previous frame and the difference between the two coefficients in the first invention. The large value of the coefficient in the current frame, and in the second invention, the value of the difference between the two coefficients, the discard compensating unit (described later) on the receiving side
A magnitude information and compensation information transmitting section 19 to be transmitted to 30, a DCT section 20 for performing discrete cosine transform (DCT), which is a kind of orthogonal transform, an output S of the DCT section 20 and 1 stored in a frame memory 21. A subtracter 22, which takes the difference from the corresponding block data S in the previous frame, a quantizer 23, which quantizes the output S of the subtracter 22, and an output of the quantizer 23, again into the format S before quantization. An inverse quantizer 24, an adder 25 for adding the output S of the inverse quantizer 24 and the block data S in the previous frame, which is an output of the frame memory 21, and a leak coefficient α to the output S of the adder 25. It comprises a leak coefficient multiplier 26 for outputting to the multiplication frame memory 21.

On the receiving side, an inverse quantizer 27 for returning the input data obtained by quantizing the signal S, which is input via the transmission line, to the original format R again, and the output R and the frame memory 28 are stored. An adder 29 for taking the sum with the block data R in the previous frame, a discard compensator 30 corresponding to the discard compensator 13 in FIG. 1, and an output R of the discard compensator 30 are subjected to inverse discrete cosine transform to obtain a pixel. An inverse DCT unit 31 that outputs image data of the area and a leak coefficient multiplier 32 that multiplies the output R of the discard compensation unit 30 by the leak coefficient α and outputs the result to the frame memory 28.

FIG. 4 shows an embodiment of the present invention using the inter-frame coding method. For example, the transmitting side calculates the difference between the block data S in the frame to be currently transmitted and the block data S in the previously transmitted frame, that is, The difference between frames is S
Is quantized, subjected to, for example, Huffman coding, and output on a transmission path. On the receiving side, the difference R is received from the transmission path, and the block data R in the previous frame is received.
To obtain the block data R in the current frame. Then, on the transmitting side, the sum of the difference S and the block data S in the previous frame is obtained by the adder 25, and after being multiplied by the leak coefficient α,
It is stored in the frame memory 21 in order to calculate an inter-frame difference with respect to the next frame. Similarly, in the receiving unit, the current block data R is multiplied by the leak coefficient α and then stored in the frame memory 28.

The leak coefficient α is for attenuating the influence of a cell discard in a short time even if it occurs. For example, the difference D ₁ is input on the receiving side, and the block data FM _{0 of the} previous frame stored in the frame memory 28 is input.
And the sum (D ₁ + FM ₀ ) is multiplied by the leak coefficient α, and the block data for the current frame represented by the following equation becomes the content FM ₁ of the frame memory 28. I do.

_{FM 1 = (D 1 + FM} 0) α = D 1 α + FM 0 α Subsequently difference D ₂ is a block data between the next frame
Is input from the transmission path, the contents of the frame memory 28 are as follows.

The contents of the _{FM 2 = (D 2 + FM} 1) α = D 2 α + D 1 α + FM 0 α 2 further when the block data D ₃ between the following frames are input frame memory 28 is given by the following equation.

_{FM 3 = (D 3 + FM} 2) α = D 3 α + D 2 α 2 + D 1 α 3 + FM 0 α 3 prediction error when Thus, for example cell discard has occurred
Effect on the overall image of the D ₁ is after input of the n frames alpha ⁿ
Attenuate. Then, by taking α smaller than 1,
The effect can be reduced to almost zero within a short time.

The discrete cosine transform (DC in FIG. 4)
T) is an example of a transformation row made from a cosine function, which performs orthogonal transformation of image data. The computation requires a multiplier, and there is a drawback in high-speed computation, but with the development of a high-speed signal processor, It has been increasingly used for transmission of quasi-moving images such as still image transmission and video conference signals.

FIG. 5 shows an embodiment of coefficient data of the lowest frequency group in the image data transmission system of FIG. FIG. 11 shows an embodiment of data according to the first aspect of the present invention in which when the difference between coefficient values is large, the coefficient value itself in the current frame is coarsely quantized and transmitted from the transmission side. In the figure, only the coefficients in the lowest frequency group are shown, and all coefficients indicating other high frequency components are omitted.

In the data on the transmission side in FIG. 5 (a), S indicates the coefficient of the lowest frequency group for six consecutive blocks in the current frame. S indicates the coefficient of the group of the lowest frequency in the immediately preceding frame, and the difference between S and S is obtained by the subtractor 22 shown in FIG. Being
Output on the transmission path.

Here, the threshold value for determining the magnitude between the data S of the current frame and the data S in the previous frame is set to 8 which is the same for each coefficient. If the difference is larger than the threshold value, the code is 1; When expressed by 0, the magnitude relationship in FIG. 4 and the compensation information output on the transmission path from the compensation information transmitting unit 19 are as follows.

The vertical bars of this compensation information indicate the divisions of six consecutive blocks (the first to third blocks from the upper left and the fourth to sixth blocks from the lower left), and the first to third blocks, For the fifth and sixth blocks, 0 indicating that the difference between the corresponding coefficients is smaller than the threshold is output for all the coefficients.

On the other hand, for the fourth block, next to the code 1 indicating that the difference 22 with respect to the DC component is larger than the threshold, auxiliary information (for example, 32) of the DC component 30 of the block in the current frame.
Is shown in parentheses, the sign 0 indicating that the difference between the coefficients on the right side of the DC component is smaller than the threshold value, and the sign indicating that the difference -9 of the coefficient below the DC component is larger than the threshold value. Subsequent to the code 1, auxiliary information (for example, 0) of one-fourth of the DC in the current frame is shown in parentheses.

FIG. 5B shows an embodiment of coefficient data on the receiving side. In the figure, the coefficient R for the previous frame is equal to the data S for the previous frame on the transmitting side.
Then, as shown in R, assuming that the cells for the fourth block and the fifth block among the cells input from the transmission path have been discarded, the above-mentioned magnitude relationship and the auxiliary information from the compensation information transmitting unit 19 are used. 4, the contents of R output to the inverse discrete cosine transform unit 31 are created.

That is, for the fifth block of R, all the values of the coefficients of R are used as they are. In contrast, the fourth
The value 32 of the auxiliary information sent from the transmitting side as a DC component for the block of, the value 0 also sent from the transmitting side as the coefficient below it, and the previous coefficient for the coefficient on the right side of the DC component Is used as the data of the current frame by using the value 13 in the data R in the frame.

FIG. 6 shows the second invention, that is, when the difference between the coefficient of the current frame and the previous frame is large, the difference is roughly quantized and transmitted to the receiving side. This is an example. In the figure, the size relationship on the transmitting side and the auxiliary information sent from the auxiliary information transmitting unit 19 to the receiving side are as follows.

That is, a value obtained by quantizing 22 as the auxiliary information of the difference 22 with respect to the DC component of the fourth block, for example, 24, and, as the auxiliary information of the difference -9 with respect to the AC component below the DC component, for example, -8, is provided to the receiving side. The DC component of the fourth block of R that is sent and input to the inverse discrete cosine transform unit 31 on the receiving side is 32, the AC component on the right is 13, and the AC component below the DC component is 2. Block data other than R on the receiving side,
R, R and block data on the transmitting side are all the same as in FIG.

FIG. 7 is an embodiment of a processing flowchart of the magnitude relationship and compensation information transmitting section 19 in the first invention. When the process starts in FIG. 12, it is first determined in step 33 whether or not the process for the lowest frequency group has been completed for all coefficients.
At 34, the difference D between the coefficient value of the current frame and the coefficient value of the previous frame is calculated. Then, in step 35, it is determined whether or not the difference D is larger than the threshold value. If the difference is small, the fact that the difference is small is transmitted to the receiving side in step 36, and the process returns to step 33.

If the difference is greater than the threshold in step 35,
At 37, the fact that the difference is large is transmitted to the receiving side, and at the same time, the coefficient value in the current frame is roughly quantized and transmitted at step 38, and the process returns to step 33. The processing from step 33 is repeated for all coefficients of the lowest frequency group,
The processing ends when the processing for all coefficients is completed.

In the second invention, when the difference is larger than the threshold value in Step 35, the fact that the difference is large is transmitted in Step 37, and in Step 38, not the coefficient value itself of the current frame but the difference is coarsely quantized. The processing is exactly the same, except that the information is transmitted.

FIG. 8 is a flowchart of a processing embodiment of the discard compensation unit 30. When the process is started in the figure, first, at step 39, it is determined whether or not the cell is discarded. If there is no discard, the discard compensation unit 30 does not perform any processing and the adder
The output from 29 is output to inverse DCT section 31 as it is.

If a cell is discarded, the coefficients in the previous frame are replaced for the low frequency having a large difference between the coefficient of the current frame and the coefficient of the previous frame, and the process ends.
Here, the replacement of the coefficient is performed by using the value obtained by quantizing the coefficient value of the current frame transmitted from the receiving side in the first invention, and using the value obtained by quantizing the difference between the coefficients in the second invention. Done.

In the above description, the coefficient having a large difference or the quantization of the difference is assumed to be performed more coarsely than the normal quantization by the quantizer 23 in FIG. 4, but this is used only when the auxiliary information is discarded. For this reason, it goes without saying that the quantization may be performed at the same level as the quantizer 23. Further, the embodiment of the present invention has been described by taking the inter-frame coding method as an example. However, the coding method is not limited to this, and the present invention can be applied to intra-frame coding.

〔The invention's effect〕

As described above in detail, according to the present invention, even if a cell is discarded, the error of the low-frequency coefficient of the current frame can be reduced, and the effect on the image quality when the cell is discarded can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a principle configuration of an image data transmission system according to the present invention, FIG. 2 is a diagram showing an embodiment of a method of setting a frequency band in a coefficient region, and FIG. 3 is a lowest frequency comprising three coefficients. FIG. 4 is a diagram showing the concept of the first embodiment of the present invention with respect to group coefficients; FIG. 4 is a block diagram showing the overall configuration of the embodiment of the image data transmission system; FIG. 5 is a coefficient of the lowest frequency group in the first invention; FIG. 6 is a diagram showing an embodiment of data, FIG. 6 is a diagram showing an embodiment of coefficient data compensated on the receiving side in the second invention, and FIG. 7 is a process of a magnitude relation and compensation information transmitting section in the first invention. FIG. 8 is a diagram showing a flowchart of an embodiment, FIG. 8 is a diagram showing a flowchart of a processing embodiment of a discard compensation unit, FIG. 9 is a block diagram showing an overall configuration of an image data transmission system using orthonormal transform, FIG. 10 is a diagram illustrating a transform coefficient domain after orthogonal transformation in the image. 12: inverse orthogonal transform section, 14: orthogonal transform section, 15: encoding section, 16: transmission section, 17: decoding section, 19: magnitude relation and compensation information transmission section, 20: discrete Cosine transform (DCT) section, 21, 28 ... frame memory, 30 ... discard compensation section, 31 ... inverse DCT section.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N ^7/ 24-7/68

Claims (2)

(57) [Claims] An orthogonal transform unit (14) orthogonally transforms input image data into a plurality of pixels as a block, encodes coefficients in the orthogonal transform coefficient area and transmits them in units of cells, and performs inverse orthogonal transform on a receiving side. In the image data transmission system having an inverse orthogonal transform unit (12), a plurality of coefficients representing frequency components from DC to high frequency in the orthogonal transform coefficient region are grouped into a plurality of groups corresponding to a plurality of frequency bands, The transmitting side of the image data transmission system transmits a plurality of coefficients in the lowest frequency group among the plurality of groups in the block currently being transmitted, and the coefficient in a frame one frame before the frame including the block. In a block corresponding to the block, between a plurality of coefficients respectively corresponding to the plurality of coefficients, information indicating whether the difference between the corresponding coefficient is large or small, A coefficient magnitude relation transmitting means (11) for transmitting a coefficient value in the current block with respect to a coefficient having a large difference to a receiving side of the image data transmission system; Is detected, and when the cell is discarded, the information indicating the magnitude relation of the coefficient difference transmitted from the coefficient magnitude relation transmitting means (11) to the block corresponding to the discarded cell,
Among the coefficients in the lowest frequency group, the coefficient with the small difference is the corresponding coefficient data in the block corresponding to the block one frame before the frame including the block, and the coefficient with the large difference is the A discard compensating means (13) for outputting data obtained by replacing a corresponding coefficient in the block with the coefficient value transmitted from the coefficient magnitude transmitting means (11) to the inverse orthogonal transform unit (12). Cell discard compensation image decoding method. 2. An orthogonal transform unit (14) orthogonally transforms input image data into a plurality of pixels as a block, encodes coefficients in the orthogonal transform coefficient area, transmits the cells in cell units, and performs inverse orthogonal transform on a receiving side. In the image data transmission system having an inverse orthogonal transform unit (12), a plurality of coefficients representing frequency components from DC to high frequency in the orthogonal transform coefficient region are grouped into a plurality of groups corresponding to a plurality of frequency bands, The transmitting side of the image data transmission system transmits a plurality of coefficients in the lowest frequency group among the plurality of groups in the block currently being transmitted, and the coefficient in a frame one frame before the frame including the block. In a block corresponding to the block, between a plurality of coefficients respectively corresponding to the plurality of coefficients, information indicating whether the difference between the corresponding coefficient is large or small, And a coefficient magnitude relation transmitting means (11) for transmitting a difference value of the coefficient with respect to the coefficient having a large difference to a receiving side of the image data transmission system. Is detected, and when a cell is discarded, information indicating the magnitude relation of the coefficient difference transmitted from the coefficient magnitude relation transmitting means (11) to the block corresponding to the discarded cell,
Among coefficients in the lowest frequency group, a coefficient having a small difference uses a corresponding coefficient in a block corresponding to the block one frame before the frame including the block, and a coefficient having a large difference is used for a coefficient having a large difference. Using the value of the difference between the coefficients transmitted from the coefficient magnitude relation transmitting means (11) and the corresponding coefficient in the block corresponding to the block one frame earlier than the frame including the block, the lowest frequency group within the lowest frequency group is used. A cell discard compensation image decoding method, comprising: a discard compensation means (13) for outputting coefficients to an inverse orthogonal transform unit (12).