Coding system and method

Abstract: An interframe coding system which eliminates higher frequency components contained in an image signal effectively and adaptively with an adaptive filter provided in a coding loop. The adaptive filter eliminates the higher frequency components with the optimal filtering intensity for an image signal specified with a filtering coefficient which is decided by a filter controller. The filtering coefficient is decided by normalization of the difference between an input image signal and a predictive signal from a frame memory by the "Activity" of the input image signal or the predictive signal. The "Activity" can be based upon the sum of the absolute or squared difference values based upon the mean value of luminance intensity of pixels of the image signal.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an image coding system, more specifically to an interframe coding system, wherein an image signal is coded on a frame-to-frame difference basis. 2. Discussion of the Related Art It is a common practice to remove redundant components in an image signal for a highly efficient coding. Especially with a dynamic image signal, an interframe coding is one of the preferable and typical arts. The interframe coding is a coding technique which codes a difference between a newly input uncoded image signal and a previously coded image signal. Fig. 19 is a block diagram of a conventional interframe coding apparatus disclosed in JP-A-63 208 382 (see also JP-A-03 136 586 and US-A-5,126,841). The interframe coding apparatus according to the figure has a frame memory 1, a motion-vector detector 2, a subtractor 3, an encoder 4, a local decoder 5, an adder 6, a filter 7 and a filter controller 8. The frame memory 1 stores an image signal of the previous frame. Operation of the conventional interframe coding system with a local decoding loop is now described with reference to the Fig. 19. The input image signal 12 is compared with the image signal of the previous frame 11 stored in the frame memory 1 by a block-matching technique in the motion vector detector 2. The motion vector detector 2 detects the quantity and direction of motion of the input image signal 12 and outputs a motion vector signal 13. The frame memory 1 outputs a motion compensation predictive signal 14 based upon the motion vector signal 13. The subtractor 3 subtracts the motion compensation predictive signal 14 from the input image signal 12 to output a predictive error signal 15 or a difference signal. The predictive error signal 15 is coded by quantization at the encoder 4, and a coded error signal 16 is output. The coded error signal 16 is decoded in the local decoder 5, and a local decoded error signal 17 is output. The local decoded error signal 17 is added to the motion compensation predictive signal 14 in the adder 6, and a local decoded signal 18 is output. The local decoded signal 18 is filtered by the filter 7 to eliminate higher frequency components in the signal. The filter smooths the local decoded signal and outputs a smoothed local decoded signal 19. Filtering is controlled by a control signal 20 output by the filter controller 8. The filter control signal 20 controls the filter based upon the motion vector signal 13. The coded error signal 16 and the motion vector signal 13 are transmitted via a transmission line to an external decoding system. Image coding is generally processed by the unit or block of 16x16 or 8x8 pixels of an image signal. With reference to other conventional coding techniques, the filter 7 can be placed after the frame memory 1 in the local decoding loop instead of after the adder 6 as shown in Fig. 19. Further, filtering can be accomplished by an intra-block filter which processes pixels within a block, and by an inter-block filter which processes pixels involving the pixels in the neighboring blocks. Furthermore, a motion detection can be achieved with a smaller unit of a pixel than a full pixel. This is designed to detect the optimal block of pixels in the previous frame which match a block of pixels in the input image signal 12. These conventional techniques can contribute to eliminate higher frequency components or redundancy in the input image signal based upon the quantity of motion. Thus, they are effective for removing noise in the signal. Consequently, coding efficiency can be improved greatly with these conventional arts. An intraframe coding with a local decoding loop is processed in the following manner according to the conventional coding with reference to Fig 19. The input image signal 12 is directly coded by quantization in the encoder 4, where the coded error signal 16 is output. The coded error signal 16 is decoded in the local decoder 5, where the local decoded error signal 17 is output. The local decoded error signal 17 is stored in the frame memory 1. The coded error signal 16 is transmitted via a transmission line to an external decoding system. The conventional interframe coding generally controls filtering based upon the motion vector, whereby an image signal can be filtered based upon the quantity of motion. In other words, a block of pixels representing motion of an image signal, or a motion block, is filtered with a low-pass filter (LPF) thereby eliminating the original definition to eliminate noise in the signal. On the other hand, a block of pixels representing no motion or a very small amount of motion, is not filtered. Fig. 20 illustrates filtering different characteristics of the motion compensation predictive signal 14 using the LPF in relation with luminance intensity (I) versus frequency (f). In the figure, 14a designates a characteristic of the image signal from the frame memory 1 without LPF filtering. 14b designates a characteristic of the image signal from the frame memory 1 after LPF filtering. In other words, a picture with no or little motion is unfiltered to keep its original definition as characteristic 14a shows. An image signal in motion is filtered with the LPF reducing its original definition to eliminate higher frequency components in the signal as characteristic 14b shows. Thus, LPF filtering eliminates higher frequency components, designated by the shaded portion in Fig. 20, in the image signal based upon the quantity of motion of the image signal. Fig. 21 illustrates LPF filtering characteristics of (a) the input image signal 12, (b) the motion compensation predictive signal 14a, and (c) the predictive error signal 15. When the motion compensation predictive signal 14a can reproduce the input image signal 12 successfully, as shown in the figure, according to the excellent motion detection performance, the difference or the predictive error signal 15 between the two signals is small. Consequently, this can contribute to a higher coding efficiency because the amount of coding information is reduced. Accordingly, when a motion block of the motion compensation predictive signal has a high quality of reproduction of the input image signal with the excellent motion detection performance and the difference between the two signals is very small, the block need not to be filtered. However, this does not apply to some cases: an image signal is filtered irrespective of the reproduction quality of the motion compensation predictive signal. When the motion compensation predictive signal 14 is filtered with the LPF, the signal loses higher frequency components as shown in Fig. 20. As a result, the difference between the input image signal and the filtered motion compensation predictive signal is the higher frequency components. In other words, a large amount of coding is needed thereby reducing the coding efficiency and resulting in a low definition. Fig. 22 illustrates the characteristics of two input signals and one output signal at the subtractor 3. Fig. 22(a) shows the characteristic of the input image signal 12, which corresponds to that of Fig. 21(a). Fig. 22(b) shows the characteristic of the filtered motion compensation predictive signal 14b in comparison to that of the unfiltered motion compensation predictive signal 14a. This shows that the higher frequency components (noise) in the signal with motion is eliminated by the LPF filtering. Fig. 22(c) shows the characteristic of the predictive error signal 15 which is the difference between the two input signals 12 and 14. The difference still contains the higher frequency components of the motion compensation predictive signal 14. This requires the encoder to have to encode bigger amounts of information leading to a lower coding efficiency. Thus, the problem is stemmed from the unnecessary filtering as Figs. 21 and 22 illustrate. The motion compensation predictive signal 14 is not to be filtered when the signal has a high quality of reproduction due to a high detection performance of motion as shown in Fig. 21. When the motion compensation predictive signal 14 is filtered unnecessarily under that condition as shown in Fig. 22, the image is damaged involving poor coding efficiency due to a higher amount of coding information or involving poor definition. The conventional interframe coding system carries another problem of quantization. Coding performance, according to the conventional system, is based upon a limited quantization which is designed optimally for a certain pattern of predictive error signal 15. In other words, the limited quantization can not deal effectively with coding signals of various patterns. When the encoder quantizes the signal with the limited quantization, the predictive error signal 15 is characterized with a poor coding efficiency and results in producing a poor coded error signal. JP-A-03 136 586 discloses a compression coder having a band limit filter for an inter-frame prediction signal. An absolute difference value between a PCM television signal and an inter-frame prediction decoded signal is calculated on a predetermined block basis, and ON/OFF control is applied to the band limit filter so as to apply band limit to the inter-frame prediction decoded signal only with a large number of pixels having a bigger amplitude difference (distortion) between the input signal and the decoded signal. This efficiency eliminates noise in an inter-frame prediction encoding loop caused by quantization through orthogonal conversion encoding. US-A-5 126 841 describes a motion compensated prediction interframe coding system which includes a first checking portion for judging whether a motion compensation of a coding block is effective and outputting a motion compensation control signal, a second checking portion for comparing the motion vector of the coding block, of which the motion compensation should be effected, with each of the motion vectors of adjacent blocks, of which the motion compensation should be effected, and judging that the intra-loop filtering processing to be performed after the motion compensation is ineffective if the number of the adjacent blocks, of which the motion vectors are identical with that of the coding block, is equal to or more than a predetermined number and that the filtering processing performed after the motion compensation is effective in another case, a motion-compensated brightness calculating portion for calculating motion-compensated brightness of pixels of the blocks, of which the motion compensation is effected, and a predictive brightness calculating portion for calculating predictive brightness of the pixels of the blocks by performing the filtering processing of the motion-compensated brightness of the pixels only if the filtering processing is effective. Further, US-A-4 047 221 shows an interframe coding apparatus in which an input signal is encoded by an interframe coder and its output code is written in a buffer memory, and is read out therefrom at a constant bit rate, and which comprises a variable characteristics comb filter at a stage preceding the interframe coder. The period of the frequency characteristics of the comb filter is selcted to be the frame period of the input signal. By a controller, the occupancy of the buffer memory is detected to control either one or both of the attenuation in the elimination band an high frequency cut-off characteristics of the comb filter detected value to prevent an overflow from the buffer memory. It is the object of this invention to provide an interframe coding system with a high coding efficiency by eliminating higher frequency components remaining in a coded signal. SUMMARY OF THE INVENTION This object according to the invention is solved with an interframe coding system comprising the features of claim 1 and with an interframe coding method comprising the features of claim 6, respectively. Preferred embodiments of the coding system according to the invention are defined in claims 2 to 5. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a block diagram illustrating a configuration of an interframe coding system according to an embodiment of the present invention; Fig. 2 shows a block diagram of a configuration of a filter controller in Fig. 1; Fig. 3 shows explanation diagrams of a block of pixels and equations of difference calculation according to the embodiment in reference to Fig. 1. Fig. 4 is an explanation diagram of the "Activity" of an image signal according to the embodiment with reference to Fig. 1; Fig. 5 is another explanation diagram of the "Activity" of an image signal according to the embodiment with reference to Fig. 1; Fig. 6 shows a flowchart of filtering control operation according to the embodiment in reference to Fig. 1. Fig. 7 shows a flowchart of another filtering control operation according to the embodiment. Fig. 8 shows a flowchart of another filtering control operation according to the embodiment. Fig. 9 is an explanation diagram of different filtering techniques of one-, two-, and three-dimensions according to the embodiment with reference to Fig. 1; Fig. 10 is an explanation diagram of intra- and inter-block filtering techniques according to the embodiment with reference to Fig. 1; Fig. 11 is an explanation diagram of an operation of an adaptive filter in Fig. 1; Fig. 12 is an explanation diagram of filtering characteristics of an image signal according to the embodiment with reference to Fig. 1; Fig. 13 shows a block diagram illustrating a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 14 shows a block diagram illustrating a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 15 shows a block diagram illustrating a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 16 shows a block diagram illustrating a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 17 shows a block diagram of a configuration of a filter controller according to the embodiment with reference to Fig. 16; Fig. 18 is an explanation diagram of an operation of an encoder according to the embodiment with reference to Fig. 16; Fig. 19 shows a block diagram of a conventional configuration of an interframe coding system; Fig. 20 shows basic characteristics of LPF filtering according to the conventional art; Fig. 21 shows a characteristic pattern of LPF filtering according to the conventional art; and Fig. 22 shows another characteristic pattern of LPF filtering according to the conventional art. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1. Fig. 1 shows an interframe coding system in accordance with an embodiment of the present invention. Fig. 1 comprises Fig. 19 of the conventional art, as modified by adding a filter controller 21 and an adaptive filter 22 and by removing the filter 7 and the filter controller 8 instead. The interframe coding system processes an image by a processing unit or block of 8x8 pixels according to this embodiment. Operation of the interframe coding system with a local decoding loop according to this embodiment is now described with reference to Fig. 1. An input image signal 12 is processed in a motion vector detector 2 with an image signal 11 of the previous frame stored in the frame memory 1 to detect a motion vector. The frame memory generates a motion compensation predictive signal 14 based upon a motion vector signal 13. The interframe coding process corresponds so far to that of the conventional art. The motion compensation predictive signal 14 is input in the filter controller 21 together with the input image signal 12. The filter controller 21 generates a filter control signal 23. Operation of the filter controller 21 according to this embodiment is now described with reference to Figs. 1 through 5. Fig. 2 shows a configuration of the filter controller 21 in Fig. 1 which includes a difference calculator 30, a decision unit 31 and a difference signal 32. The difference calculator 30 basically calculates a difference between the input image signal 12 and the motion compensation predictive signal 14 to output the difference signal 32. Specifically, the difference signal 32 is output in the following process: the difference calculator 30 calculates an absolute difference or squared difference between the signals for each pixel Then the absolute values or squared values of pixel differences are calculated for the processing unit or a block of 8x8 (64) pixels and output as the difference signal 32. Fig. 3 shows explanation diagrams and equations of difference calculation for the difference signal 32. Figs. 3(a) and 3(b) show a block or processing unit of 8x8 pixels of an image signal, respectively. Fig. 3(a) shows 64 pixels in a block of the input image signal 12 with each pixel represented by S1 through S64. Fig. 3(b) shows 64 pixels in a block of the motion compensation predictive signal 14 with each pixel represented by Y1 through Y64. Figs. 3(c) and 3(d) represent possible forms of difference calculations of a luminance intensity of the pixels between the input image signal 12 and the motion compensation predictive signal 14 for the difference signal 32. Difference signal N in Fig. 3(c) is the sum of the absolute difference between the signals, while difference signal N in Fig. 3(d) is the sum of the squared difference between the signals. The value of the difference signal 32 becomes smaller when the predictive signal accurately predicts performance of the system. The value becomes larger, on the other hand, for the following possible cases: (a) for a poor predictive signal resulting from a poor prediction performance of the system; (b) where the input image signal has varied significantly from the previous image signal; and, (c) for a poor predictive signal resulting from a poor quality of the local decoded signal held in the frame memory 1 containing a large amount of quantization errors. The decision unit 31 calculates an "Activity" of the block of pixels of the input image signal 12 responsive to the difference signal 32. The "Activity" is based upon the difference, for instance, between the maximum and minimum values of the input image signal 12 as shown in Fig. 4. Fig. 4 is an explanation diagram of the "Activity" illustrating that the "Activity" is based upon the difference between the maximum and minimum values of luminance intensity of pixels of the input image signal 12. The figure offers a brief explanation of the "Activity" by using only eight pixels instead of 64 pixels. The "Activity" of an image signal according to the figure is the difference value between the values of luminance intensity of the fourth pixel (maximum intensity) and of the fifth pixel (minimum intensity) in the eight pixels of the image signal. Fig. 5 shows another example of calculating the "Activity" of the image signal which can be either the sum of the absolute or squared difference values between the mean value of luminance intensity of the pixels and the individual values. Fig. 5(a) illustrates the "Activity" calculation of the mean value of luminance intensity of the pixels of the image signal. X1 through X8 in the diagram show the difference between the luminance intensity of the pixel and the mean value of luminance intensity of the eight pixels. Fig. 5(b) shows one of the calculation formulas of "Activity" X which is the sum of the absolute difference values of X1 through X8. Fig. 5(c) shows another calculation formula of "Activity" X which is the sum of the squared difference values of X1 through X8. The "Activity" denotes, as shown in Figs. 4 and 5, nothing like an absolute intensity but a relative value of an image signal. Specifically, "Activity" X becomes smaller with an image signal containing a large amount of lower frequency components, "Activity" X becomes larger with an image signal containing a large amount of higher frequency components. The difference signal 32 is normalized or divided by the "Activity" of the image signal in the decision unit 31 (Fig. 2). A normalized result or value decides filtering intensity for eliminating higher frequency components in the image signal. The decision unit 31 generates the filter control signal 23 specifying filtering intensity according to a normalized result. Specifically, when a normalized value is larger, indicating that the image signal contains a great amount of higher frequency components, the image signal is filtered with higher filtering intensity. When a normalized value is smaller, on the other hand, indicating that the signal contains a large amount of lower frequency components, the image signal is filtered with lower filtering intensity. The decision unit 31 provides multiple values to the adaptive filter 22 for modifying the filtering intensity of the filter. The filter control signal 23 specifies filtering intensity of the image signal using one of the multiple values according to a normalized result. The decision unit 31 according to this embodiment provides two values, for a brief explanation, indicating ON and OFF for controlling the filter with the filter control signal 23. In this case, a threshold value is to be set as a threshold for deciding whether the image signal is to be filtered or not. When a normalized value is larger than the threshold, the signal is to be filtered for eliminating higher frequency components and consequently reducing the amount of coding information. When a normalized value is smaller, on the other hand, than the threshold, the image signal is unfiltered in order to keep its original definition. Fig. 6 shows a flowchart of filtering control in the filter controller 21 controlling the filtering operation (ON/OFF) with the motion compensation predictive signal. The filter controller 21 controls filtering based upon data from the input image signal 12 and the motion compensation predictive signal 14. The following is the operating sequences of filtering control with an image signal: ST1. The adaptive filter is set OFF as an initial value. ST2. The difference calculator 30 calculates difference signal N between the input image signal 12 and the motion compensation predictive signal 14. ST3. The decision unit 31 calculates "Activity" X of the input image signal 12. ST6. The decision unit 31 normalizes or divides difference signal N by "Activity" X of the input image signal. When a result of ST6 is smaller than a threshold value Th NX , indicating a successful prediction of an image signal, the motion compensation predictive signal is not to be filtered and the filter remains OFF. This indicates that difference signal N is smaller or "Activity" X is larger. In other words, the motion compensation predictive signal with a smaller value of difference signal N has reproduced the image successfully and therefore does not need to be filtered. On the other hand, the motion compensation predictive signal with a larger value of "Activity" X can indicate that the motion compensation predictive signal 14 contains a large amount of higher frequency components due to the original input image signal containing a large amount of higher frequency components. The motion compensation predictive signal, in this case, is not to be filtered. When a result of ST6 is larger than a threshold value Th NX , indicating a poor prediction of the image, the motion compensation predictive signal is to be filtered and the filter is turned ON. This indicates that the difference signal N is larger or that "Activity" X is smaller. In other words, the motion compensation predictive signal with a larger value of the difference signal N due to a poor prediction, thus containing a large amount of higher frequency components. Therefore the motion compensation predictive signal, in this case, should be filtered in order to eliminate the higher frequency components for a highly efficient coding. On the other hand, the motion compensation predictive signal with a smaller value of "Activity" X is also to be filtered. However, there is a situation when the motion compensation predictive signal with a larger value of "Activity" X is not to be filtered. It is often the case when "Activity" X is larger, the motion compensation predictive signal 14 contains a large amount of higher frequency components. In this case the motion compensation predictive signal is not to be filtered keeping the higher frequency components with it, which can achieve a higher coding efficiency. Fig. 7 shows a flowchart of another filtering control method accomplished in the filter controller 21. As ST4 in the figure shows, when difference signal N is larger than a first threshold value Th N , the filter is turned ON. This means that the motion compensation predictive signal contains a large amount of higher frequency components and therefore is to be filtered in order to eliminate the higher frequency components to improve the coding efficiency. As ST5 in the figure shows, when "Activity" X is smaller than a second threshold value Th x , the filter is turned ON. This indicates that the input image signal 12 contains almost no higher frequency components. In other words, the motion compensation predictive signal with less or no higher frequency components cannot be influenced by filtering in terms of the elimination of higher frequency components and can still have more effective result with filtering rather than without filtering in terms of the elimination of noise. On the other hand, when "Activity" X is larger than a threshold value Th x , the filter is not used (OFF). This indicates that the input image signal 12 contains a large amount of higher frequency components and, consequently, the motion compensation predictive signal 14 also contains a large amount of higher frequency components. In this case the motion compensation predictive signal 14 remains unfiltered keeping higher frequency components with it. Fig. 8 shows a flowchart of another filtering control method which can be executed in the filter controller 21. Fig. 8 is a combination of Figs. 6 and 7. The combination brings about a more adaptive control of filtering the image signal with many choices of threshold values available. With further reference to the embodiment, filtering control in accordance with the invention can be operated based on the comparison between the "Activity" of the input image signal and a threshold value Th x . In other words, the flowchart in Fig. 7 can be altered by omitting the difference calculation step ST2 and the comparison step ST4 between N and Th x . When a division is operated by zero, the quotient is generally supposed to be infinite. This theory, however, does not apply to normalization here, for the calculation is generally with a finite length word. In other words, when a difference results from the difference calculator 30 is normalized by zero at ST6 in Fig. 6, a value of the "Activity", the quotient or normalized result is considered the maximum value of the finite word used. Accordingly, when normalization uses an eight-bit operation, the quotient is "FF", which is the maximum value of an eight-bit word, which is the maximum normalized value. The adaptive filter 22 filters the motion compensation predictive signal 14 according to the filter control signal 23 for eliminating higher frequency components in the signal. As shown in Fig. 9, the adaptive filter 22 can provide a multi-dimensional filtering including one-, two- and three-dimensional filters. The three-dimensional filter can change the filtering coefficient based upon the motion in an image signal as the frame varies with time. Filtering intensity of the adaptive filter can be varied according to the filtering coefficient specified by the filter control signal 23. Fig. 9 illustrates multi-dimensional low-pass filtering of the adaptive filter 22. Fig. 9(a) shows a one dimensional filtering of a line of pixels having a processing pixel as shown by a black dot in the processing unit or block of pixels. Fig. 9(b) shows horizontal and vertical lines of pixels in the block, and illustrates a two-dimensional filtering. Fig. 9(c) shows horizontal and vertical lines of pixels in blocks of time-varying frames t1, t2 and t3, thereby illustrating a three-dimensional filtering. Black dots in Fig. 9 indicate processing pixels. A processing pixel can be filtered with the influence of the neighboring pixels according to any of the one-, two-, and three-dimensional filtering techniques. An image signal can be filtered either by an intra-block filtering, or by using pixels within the processing unit or block of 8x8 pixels exclusively, or by an inter-block filtering, or by using pixels in the neighboring blocks comprehensively as Fig. 10 shows. Fig. 10 illustrates examples of the inter-block filtering with the processing pixel in a black dot. F1 and F2 in the figure show, respectively, the inter-block filtering of the processing pixel by using pixels in the neighboring blocks. Fig. 11 is an explanation diagram illustrating a one-dimensional filtering. Fig. 11(a) shows five pixels, S1 through S5, for a one-dimensional filter, including a black dot, S3, as the processing pixel. K1 through K5 in the figure indicate filtering coefficients for each corresponding pixels S1 through S5 in the figure. The sum of the coefficients is 1.0 as shown in Fig. 11(b). Filtering intensity for the processing pixel is specified or decided by the result of the following calculation: the luminance intensity of each pixel of the processing pixel and the neighboring pixels S1 through S5 is multiplied by a group of corresponding filtering coefficients kl through K5. Each result of the multiplication is summed up. Figs. 11(c) and 11(d) show different patterns of filtering coefficients K1 through K5 for the multiplication under the condition that the sum of the coefficients is 1.0. When the value of filtering coefficient of the processing pixel is not 1.0 (K3=0.6) or those of the neighboring pixels are non-zero (K1=K2=K4=K5=0.1) as shown in Fig. 11(c), the luminance intensity of the processing pixel is influenced by those of the neighboring pixels to be normalized. When the value of filtering coefficient of the processing pixel is 1.0 (K3=1.0) or those of the neighboring pixels are zero (K1,K2,K4,K5=0.0) as shown in Fig. 11(d), the luminance intensity of the processing pixel has no influence by those of the neighboring pixels keeping its original luminance intensity. This indicates that the processing pixel is not to be filtered or the low-pass filter is to be OFF. Fig. 12 is an explanation diagram illustrating different characteristics of the predictive signal 24 by different degrees of filtering intensity with different filtering coefficients. The filter control signal 23 specifies the optimal filtering intensity for the motion compensation predictive signal 14 to eliminate higher frequency components in the signal in a multiphase manner with multiple choices of the pattern of filtering coefficients. The four characteristic of the prediction signal, 24a through 24b, in the figure indicate different filtering results by multiphase elimination of higher frequency components in the signal with different patterns of filtering coefficients. Characteristic 24a shows that the motion compensation predictive signal 14 is not filtered containing a great amount of higher frequency components. Characteristic 24d shows that the motion compensation predictive signal 14 is filtered containing a large amount of lower frequency components with very small portion of or no higher frequency components. The characteristics of the signal can be changed with different filtering coefficients from 24a to 24d as an arrow A indicates if the predictive signal contains a large amount of higher frequency components. As an arrow B indicates, the characteristic can be changed from 24d to 24a with different filtering coefficient if the signal requires higher definition. Generally, an image signal can keep its original resolution, its highest definition, without filtering. When filtered the predictive signal 24 is subtracted from the input image signal 12 in a subtractor 3, where a predictive error signal 15 is output. The predictive error signal 15 is coded by quantization in an encoder 4, where a coded error signal 16 is output. The coded error signal 16 is decoded in a local decoder 5, where a local decoded error signal 17 is output. The local decoded error signal 17 is added to the predictive signal 24 in an adder 6, where a local decoded signal 18 is output. The local decoded signal is stored or written in the frame memory 1. Embodiment 2. With further reference to Fig. 1, the interframe coding system in accordance with the invention can omit the motion vector detector 2 in terms of simplification of the system as shown in Fig. 13. In this case, the quantity of motion of an image signal is to be considered zero. Embodiment 3. With further reference to Fig. 1, the interframe coding system in accordance with the invention can provide the adaptive filter 22 after the adder 6 in the local decoding loop instead of being after the frame memory 1, as is shown in Fig. 14. Embodiment 4. With further reference to Fig. 1, the interframe coding system in accordance with the invention can combine the previous two embodiments. The interframe coding system omits the motion vector detector 2 and provides the adaptive filter 22 after the adder 6 in the local decoding loop instead of being after the frame memory 1, as is shown in Fig. 15. In Fig. 15 the motion vector detector 2 is not provided. Therefore, the interframe coding system with this configuration operates under the condition that the quantity of motion of an image signal is zero. Embodiment 5. Fig. 16 shows another configuration of the interframe coding system in accordance with the invention. The filter controller 21 outputs a pixel difference signal 25 or a control signal to the encoder 4 as well as the filter control signal 23 to the adaptive filter 22. The pixel difference signal 25 is the difference per pixel which is calculated by dividing the value of the difference signal 32 by the number of pixels in a block of pixels (e.g. 8x8=64) in the filter controller 21. Fig. 17 shows a configuration of the filter controller 21 according to this embodiment. Fig. 17 comprises Fig. 2, as modified by adding a pixel difference calculator 33 for calculating the pixel difference signal 25. The difference calculator 30 outputs the difference signal 32 by the following way stated hereinbefore in reference to Fig. 2. The absolute or squared difference values of luminance intensity of pixels between the input image signal 12 and the motion compensation predictive signal 14 are summated within a block of pixels (8x8=64). The value of the difference signal 32 is divided in the pixel difference calculator 33 by the number of pixels in a block of pixels (8x8=64) to produce the average value of the difference signal 32 in order to output the pixel difference signal 25. The pixel difference signal 25 is the threshold value of quantization which is output to the encoder 4. The encoder 4 decides the optimal quantization step size for coding the image signal under the condition that qantization error does not exceed the value of the pixel difference signal 25. Fig. 18 shows the pixel difference of an image signal illustrating the operation of deciding the quantization step size in the encoder 4 in accordance with this embodiment. In Fig. 18(a), D1 designates the pixel difference of a block containing eight pixels, D2 designates the pixel difference of another block of eight pixels, and D3 designates the pixel difference of another block of eight pixels. Those pixel differences are supposed to be given the following condition in the filter controller 21 as shown in Fig. 18(b): D1>D2>D3. When the pixel difference signal 25 is input into the encoder 4, the encoder 4 selects the optimal quantization step size for an input image signal based upon the pixel difference signal 25, so that the value of quantization error should be less than the value of the pixel difference signal 25. With D1 as the pixel difference signal 25, for instance, the encoder 4 selects a quantization step size SS1 (SS1<D1), with D2 as the signal a step size SS2 (SS2<D2) is selected; and a step size SS3 (SS3<D3) is selected for D3. Under that condition, the encoder 4 quantizes an input image signal by selecting the optimal quantization step sizes based upon the value on the pixel difference signal 25 in the following relations as shown in Fig. 18(d): SS1>SS2>SS3. Thus, the quantization step size becomes larger when the difference between the input image signal 12 and the motion compensation predictive signal 14 is large. On the other hand, the quantization step size becomes smaller when the difference between the two signals is smaller. Consequently this leads to a highly efficient coding performance minimizing the quantization error to have a high quality decoded image. The pixel difference signal 25 is not the only signal used to decided the quantization step size in the encoder 4. The quantization step size is also controlled or limited by the size of an output buffer which temporarily stores coded signals to be output from the encoder 4. The size of the buffer can limit the quantity of coding signals. In this case, the quantization step size is limited by the size of the output buffer. Embodiment 6. With further reference to the foregoing embodiments, the "Activity" of an image signal in accordance with the invention can be represented in a variety of manners as a characteristic of the image signal. The foregoing embodiments provide the "Activity", a characteristic, of an image signal by a difference between image signals or by the sum of the absolute or squared difference values of an image signal vs. the mean value. There are other possible forms of the "Activity" of an image signal using other difference signals or the variance technique. Embodiment 7. With further reference to the foregoing embodiments, the processing unit of image signal in accordance with the invention can be a block of 16x16, 32x32, or 8x16 pixels for example, instead of a block of 8x8 pixels. Otherwise, the processing unit can possibly be other forms, instead of a block of pixels. Embodiment 8. With further reference to the foregoing embodiments, a coding signal or coding data in accordance with the invention can be based upon radar signal or sound signal, instead of image signal. As described hereinbefore, one of the distinctive features of the interframe coding system of the present invention is that the filter controller normalizes a difference between the input image signal and the signal from the frame memory by either the input image signal or the signal from the frame memory. The filter controller generates the filter control signal based upon the normalization to control filtering intensity for eliminating higher frequency components in the signal. The filter controller also generates the filter control signal based upon comparison between the "Activity" of the input image signal or of difference and a threshold value. Accordingly, filtering can be operated adaptively to each image signal, so that an image signal can be provided free from higher frequency components in the encoder for coding. This can contribute to a highly efficient coding leading to a high definition coded image even with motion. As stated hereinbefore, the interframe coding system and method of the present invention have the following advantageous effects. One of the advantageous features is that the filter controller controls filtering intensity for eliminating higher frequency components in an image signal by using the difference between the input image signal and the image signal from the frame memory, whereby an image signal can be provided free from higher frequency components in the encoder leading to a highly efficient coding. Another advantageous feature is that filtering operation is designed to deal with any image signals comprehensively including pictures with no or little motion and pictures with motion. Therefore pictures with motion can keep the original definition, while pictures with no or little motion can acquire high definition coded image by eliminating higher frequency components in the signal. Another advantageous feature is that filtering can produce a highly coding efficient image signal by eliminating higher frequency components in the signal in the event that the predictive signal still contains a large amount of quantization error. Another advantageous feature is that filtering intensity can be altered in various degrees adapted to each image signal for a proper elimination of higher frequency components in the signal. This can provide a high quality predictive image for a highly efficient coding with a smaller coding loop gain. Another advantageous feature is that a pixel difference is used to control coding error, so that a decoded image can carry less coded error. This can contribute to a highly efficient prediction for the forthcoming frames of an image signal. Another advantageous feature is that filtering control is designed to deal with any image signals comprehensively including pictures with no or little motion and pictures with motion. Therefore pictures with motion can keep the original definition, ^Awhile pictures with no or little motion can acquire high definition coded image by eliminating higher frequency components in the signal. Another advantageous feature is that filtering can produce a highly coding efficient image signal by eliminating higher frequency components in the signal in the event that the predictive signal still contains a large amount of quantization error. The decision unit may include means for comparing the difference signal and a predefined value, and issuing the control signal based on a comparison result.

Claims: An interframe coding system for frame-to-frame coding of an input signal (12), having a local decoding loop which includes a filter (22) for filtering a predictive signal (14), comprising a filter controller (21) for receiving the input signal (12) and the predictive signal (14) for generating a control signal (23) to control the filter (22), wherein the filter controller (21) comprises: a difference calculator (30) for receiving the input signal (12) and the predictive signal (14), calculating a difference between the input signal (12) and the predictive signal (14) and outputting a difference signal (32); and a decision unit (31) for receiving the difference signal (32) and the input signal (12) or the predictive signal (14) instead of the input signal, for generating the control signal (23) from the difference signal (32) and the input signal (12) or the predictive signal (14) instead of the input signal, and for outputting the control signal (23), the decision unit (31) including means for calculating an activity value from the input signal or from the predictive signal instead of the input signal and means for normalizing the difference signal (32) by the activity value and for outputting the control signal (23); wherein the filter (22) includes a plurality of patterns of filtering coefficients for processing a pixel and its neighboring pixels and a means for selecting a pattern of filtering coefficients based on the control signal (23). the interframe coding system of claim 1, wherein the local decoding loop includes decoding means (5) for producing a previous input singal from a previous frame; a motion vector detector (2) which receives the input signal (12) and the previous input signal (11), and outputs a motion vector (13); and a frame memory (1) for receiving the motion vector (13) and the previous input signal and outputting a motion compensation predictive signal to the filter controller (21) as the predictive signal (14). The interframe coding system of claim 1, wherein the filter controller (21) includes means for generating one of multiple values as the control signal (23) to designate the pattern of filtering coefficients of the filter (22). The interframe coding system of claim 3, wherein the filter (22) is any one of a one dimensional filtering means, a two dimensional filtering means or a three dimensional filtering means which includes time axis as the third dimension. The interframe coding system of claim 4, wherein the filter (22) is placed either before or after the frame memory (1). An interframe coding method for frame-to-frame coding of an input signal, wherein a coding system includes a local decoding loop comprising a frame memory (1) for receiving a decoded signal and outputting a predictive signal, and a filter (22) for receiving and filtering the predictive signal, characterized by the steps of: (a) calculating a difference between the input signal and the predictive signal; (b) calculating an activity value from the input signal; (c) selecting a pattern of filtering coefficients of the filter (22) from a plurality of patterns of filtering coefficents based on the difference and the activity value; and (d) filtering the predictive signal based on the selected pattern of filtering coefficients.

Inventors: KATO YOSHIAKI, MURAKAMI TOKUMICHI, SHIMADA TOSHIAKI, KATO, YOSHIAKI, MURAKAMI, TOKUMICHI, SHIMADA, TOSHIAKI, KATO, YOSHIAKI, C/O MITSUBISHI DENKI K.K., MURAKAMI, TOKUMICHI, C/O MITSUBISHI DENKI K.K., SHIMADA, TOSHIAKI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H04N7
IPC codes: G06T9, H04N7