Coding system

Abstract: A coding system which includes an encoder (4) for coding an input signal (12) into a coded signal (16) and a frame memory (1) for outputting a predictive (14, 15) signal. The system further comprises a coding controller (41) responsive to the input signal and the predictive signal, for calculating a characteristic of the input signal and for generating a coding control signal (42) for controlling the encoder based on the characteristic.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a coding system according to the pre-characterising part of claim 1. 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. 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 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. The conventional interframe coding system carries a 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. Helgeson et al: "Impact of Recent Compression/Decompression Technologies in Video Distribution Systems", 11th Digital Avionics Systems Conference, 5 October 1992, pages 568 - 572, XP 000 366 739 Seattle, refers to the International consultative Committee for Telephone and Telegraph (CCITT) H. 261 document which specifies a method of communication for visual telephony. CCITT is based on transform coding and temporal predictive coding with motion compensation. The CCITT H. 261 hybrid video coding algorithm utilises both intraframe and interframe coding schemes. The intraframe mode realises compression in the spatial dimension within a single frame which compression in the temporal mode is disabled. In addition to compression within a single frame, the interframe mode provides compression between consecutive frames in time. The CCITT encoder algorithm requires ad decoder to reconstruct and motion compensate the previous (N-1) frame and utilises this reconstructed image as a prediction for the current frame (N). Ohtsuka et al: : "Development of 135 Mbit/s HDTV codec", Signal processing image Communication, vol. 4, no. 4/5, pages 379 - 387, XP 000 293 755 Amsterdam NL, discloses a coding system which includes an encoder for coding an input signal into a coded signal and a frame memory for outputting a predictive signal, which system comprises a coding controller responsive to the input signal and the predictive signal for calculating a characteristic (signal energy) of the input signal and for generating a coding control signal for controlling the encoder based on the characteristic. A comparison is made between characteristics of the interframe signal and the intraframe signal for determining the coding mode, namely interframe or intraframe coding. Bellifemine et al: "Image Coding with Block DCT and Motion Compensation. Two Different Approaches", European Transaction on Telecommunications and Related Technologies, vol. 2, no. 3, 1 May 1991, pages 305 - 312, XP 000 236 963, describes two approaches for the purpose of image data compression having the common features: segmentation of picture in 8 x 8 blocks to perform adaptivity; use of temporal prediction and motion compensation to remove temporal correlation; use of two dimensional DCT to remove spatial correlation. According to one approach a selection is made on a block by block basis to make the coding scheme more effective in the energy reduction sense. A differential motion compensated block can be rejected if there is no energy reduction. Usually, this rejection occurs if the related block in the previous frame is less correlated to the actual block so that the prediction technique works less effectively. The energy of the compensated block (differential signal) is compared with that of the uncompensated block (original signal), and the block with the lowest energy is chosen for transmission, while the other one is rejected. This means that the best coding mode between inter and intra is chosen for each block according to the local characteristics of the image. It is the object of the present invention to provide a coding controller which allows the encoder to code efficiently with coding signals of various patterns. This can lead to the overall coding efficiency of the coding system. According to the invention, this object is solved by coding system having the features of claim 1. Preferred embodiments of the invention are defined in the subclaims. SUMMARY OF THE INVENTION The invention, therefore, relates to a coding system which includes an encoder for coding an input signal including quantization into a coded signal and a frame memory for outputting a predictive signal, comprising a coding controller responsive to the input signal and the predictive signal, for generating a coding control signal for controlling the encoder, wherein the coding controller includes an activity calculator for calculating an activity value of each of the input signal and a predictive error signal formed by subtracting the input signal from the predictive signal, the activity value becoming smaller when the corresponding signal contains a great amount of lower frequency components, and becoming lager when the corresponding signal contains a great amount of higher frequency components, and an activity comparative selector for comparing the activity values and for generating the coding control signal, which provides a quantization step size, on the basis of said activity values. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a block diagram of 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 coding controller according to the embodiment with reference to Fig. 1; Fig. 3 shows a block diagram of a configuration of a "Activity" comparative selector according to the embodiment with reference to Fig. 1; Fig. 4 shows a block diagram of a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 5 is an explanation diagram of weighting operated in a weighting circuit according to the embodiment with reference to Fig. 4; Fig. 6 shows a flowchart of the basis operation in the weighting circuit according to the embodiment; Fig. 7 shows a flowchart of an example of weighting operation in the weighting circuit according to the embodiment; Fig. 8 shows a flowchart of the basic operation in the comparator according to the embodiment; Fig. 9 shows a flowchart of the basic operation in the control signal generator according to the embodiment; Fig. 10 shows a memory table of the control signal generator according to the embodiment; Fig. 11 schows a block diagram of a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 12 shows a block diagram of a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 13 shows a block diagram of a configuration of a coding controller according to the embodiment; Fig. 14 shows a block diagram of a configuration of an "Activity" comparative selector according to the embodiment. Fig. 15 shows a block diagram of a configuration of an interframe coding system according to another embodiment of the present invention; Fig. 16 shows a block diagram of 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 an interframe coding system according to another embodiment of the present invention; Fig. 18 shows a block diagram of a configuration of an interframe coding system according to another embodiment of the present invention; and Fig. 19 shows a block diagram of a conventional configuration of an interframe coding system; Embodiment 1. Fig. 1 shows a configuration of interframe coding system according to an embodiment of the present invention. Fig. 1 comprises a coding controller 41, a coding control signal 42 and a select signal 150. Operation with the local decoding loop of the interframe coding system according to this embodiment is now described with reference to Fig. 1. The image signal 11 of the previous frame stored in the frame memory 1 or the local decoded image signal is used as a predictive signal. The subtractor 3 subtracts the input image signal 12 from the predictive signal 11 to output the predictive error signal 15. The coding controller 41 calculates the "Activities" of the input image signal 12 and the predictive error signal 15 to output the coding control signal 42 and the select signal 150, respectively, to the encoder 4. The encoder quantizes the select signal 150 to output the coded error signal 16. The local decoder 5 decodes the coded error signal 16 to output the local decoded error signal 17. The adder 6 combines the local decoded error signal 17 with the image signal 11 to output the local decoded signal 18. The local decoded signal 18 is stored in the frame memory 1. The coded error signal 16 is, on the other hand, also transmitted through a transmission line. Operation of the coding controller 41 in accordance with this embodiment is now described with reference to Fig. 2. The coding controller 41 has an "Activity" calculator 45, an "Activity" comparative selector 46, an "Activity" signal 12a based upon the input image signal 12, an "Activity" signal 15a based upon the predictive error signal 15, the coding control signal 42 and the select signal 150. In the coding controller 41, the "Activity" calculator 45 calculates each "Activity" of the input image signal 12 and the predictive error signal 15 to output their "Activities" to the "Activity" comparative selector 46. The "Activity" of an image signal can be the difference between the maximum and minimum values of luminance intensity of pixels of a signal or the sum of the absolute or squared values of the difference between the mean value and the values of luminance intensity of a block of pixels in an image signal. The "Activity" does not refer to the absolute luminance intensity of each pixel in an image signal. It involves a block of pixels of an image signal. When a block of pixels of an image signal contain a great amount of lower frequency components its "Activity" becomes smaller, while when they contain a great amount of higher frequency components its "Activity" becomes larger. In the "Activity" comparative selector 46, the "Activities" 12a and 15a are compared to select an optimal mode signal under a given priority based upon the user's or system's needs. The priority could be: (1) to prioritize coding efficiency; (2) to prioritize picture quality; or, (3) to put significance both on the coding efficiency and the picture quality and prioritize either of them if one has more significance depending on a given situation. There are more possible significant items can be added to the list of condition stated above, apart from coding efficiency and picture quality. Condition can be limited depending on the purpose or capacity of the system or the user's needs. When coding efficiency is prioritized, for instance, a mode signal of the image signal with smaller "Activity" by the comparison is output by the "Activity" comparison selector 46 as stated hereinbefore. On the other hand, when picture quality is prioritized, the mode signal of the input image signal 12 is output by the "Activity" comparison selector 46. When both of coding efficiency and picture quality are prioritized, the "Activity" comparative selector 46 outputs the mode signal of either the input image signal 12 or the predictive error signal 15 based upon the comparison result of their "Activities" under a certain consideration. The "Activity" comparative selector 46 outputs the coding control signal 42 providing coding parameters. The coding parameters include the quantization step size based upon the condition and the "Activities" of the image signals. The "Activity" comparison selector 46 also outputs the image signal selected by the mode signal as the select signal 15o. Operation of the "Activity" comparative selector 46 according to this embodiment is now described with reference to Fig. 3. The "Activity" comparative selector 46 has a weighting circuit 46a, a comparator 46b and a control signal generator 46c. The weighting circuit 46a weights the "Activity" of the input signal 12a and/or the "Activity" of the predictive error signal 15a. The comparator 46b compares the "Activity" of the input image signal 12a with the "Activity" of the predictive error signal 15a output from the weighting circuit 46a. The control signal generator 46c selects either the input image signal 12 or the predictive error signal 15 as the select signal 15o and also outputs the coding control signal 42 by multiplexing the quantization step size, coding coefficients, etc. The weighting circuit 46a will be left to the later discussion for a brief description of the "Activity" comparative selector 46 in this embodiment. Accordingly, the comparator 46b inputs the "Activity" of the input image signal 12a and the "Activity" of the predictive error signal 15a. The comparator 46b compares those "Activities" and outputs a mode signal 46m based upon the comparison result to the control signal generator 46c. The mode signal 46m actuates a switch to input either the input image signal 12 or the predictive error signal 15 in the control signal generator 46c. The mode signal 46m is output as a part of the coding control signal 42 as one of the coding parameters included in the multiplexed signal together with the quantization step size, discrete cosine transform (DCT) coefficients, etc. to the encoder 4. The encoder 4 demultiplexes and checks the multiplexed coding control signal 42, whereby the encoder 4 is informed, for instance with the mode signal, which signal is selected as the select signal 15o of the input image signal 12 and the predictive error signal 15. When the input image signal 12 is selected as the select signal 15o, the encoder 4 performs "intraframe" coding, while the predictive error signal 15 is selected, the encoder 4 performs "interframe" coding. Interframe and intraframe codings are controlled by quantization step sizes and the DCT coefficients. Embodiment 2. With further reference to the previous embodiment referring to Fig. 1, the interframe coding system in accordance with the invention can have a filter controller, an adaptive filter and a motion vector detector as shown in Fig. 4. Accordingly, Fig. 4 comprises Fig. 1, as modified by adding the motion vector detector 2, the motion vector signal 13, the motion compensation predictive signal 14, the filter controller 21, the adaptive filter 22, the filter control signal 23 and the predictive signal 24. With further reference to Fig. 1, the predictive error signal 15 in accordance with the invention can be based partly upon the filtered predictive signal 24 through the filter controller 21 and the adaptive filter 22 as shown in Fig. 4 instead of the image signal of the previous frame 11 unfiltered in Fig. 1. In other words, the predictive error signal 15 is the subtracted result of the input image signal 12 by the filtered predictive signal 24 in the subtractor 3. Consequently, the coding controller 41 according to this embodiment compares the "Activity" of the input image signal 12 and the "Activity" of the filtered predictive error signal 15 to output the coding control signal 42. The filter controller 21 according to this embodiment calculates the difference between the input image signal 12 and the image signal from the frame memory 1 or the motion compensation predictive signal 14. The filter controller 21 normalizes the difference by the "Activity" of either the input image signal 12 or the motion compensation predictive signal 14. Accordingly, the optimal filtering intensity for the motion compensation predictive signal 14 is decided based upon the normalized result. This can contribute to a desirable elimination of higher frequency components in the image signal leaving no higher frequency components in the prediction error signal, which leads to a high coding efficiency. The coding control signal 42 according to this embodiment can be output by the coding controller 41 based upon the mode signal. The mode signal is to select the image signal having smaller "Activity" of the two: the "Activities" of the input image signal 12a and the "Activity" of the optimally filtered prediction error signal 15a. This also contributes to a high coding efficiency. Embodiment 3. With further reference to the previous two embodiments, the "Activity" comparative selector 46 in the coding controller 41 in accordance with the invention can select the mode signal 46m based only upon the input signal resulted in the smallest "Activity" in the "Activity" comparison. Accordingly, the "Activity" comparative selector 46 outputs the mode signal based upon the smallest "Activity" in the comparator and outputs the coding control signal 42 of coding parameter, including the quantization step size, etc. based upon the mode signal and the "Activity". An image signal with smaller "Activity" indicates that the image signal can be coded with higher coding efficiency in the encoder. Embodiment 4. With further reference to the previous two embodiments or Figs. 1 and 4, the "Activity" comparative selector 46 in accordance with the invention can weight "Activities" of the input signals, at least one of them, for comparison. The significance and operation of weighting "Activity" operated in the weighting circuit 46a in the "Activity" comparative selector 46 in Fig. 3 is now described with reference to Figs. 3 and 5. Fig. 5 shows the relation between the "Activities" of input signals and weighting. Fig. 5(a) shows the original "Activity" of the input image signal 12a and the original "Activity" of the predictive error signal with no weight added in the weighting circuit 46a. Fig. 5(b) shows the original "Activity" of the input image signal 12a with no weight added and the "Activity" of the predictive error signal with twice as much weight added as the original "Activity". Under the condition that the "Activity" comparative selector 46 is to select the mode of an input signal with the smallest "Activity", the "Activity" comparative selector 46 selects the predictive error signal 15 as the select signal 15o according to Fig. 5(a). The figure shows that the "Activity" of the predictive error signal 15a is constantly smaller than the "Activity" of the input image signal 12a. Under that condition, the "Activity" comparative selector 46, on the other hand, selects either of the signals alternatively of the input image signal 12 or the predictive error signal 15 added a twofold weight on its "Activity" 15b when it has smaller "Activity" than the other as the select signal 15o according to Fig. 5(b). The figure shows that the predictive error signal 15 is selected during the time period T0 through T1 as the select signal 15o. The input image signal 12 is selected during the time period T1 through T2 as the select signal 15o due to the weighting on the predictive error signal. Fig. 6 shows a flowchart of the basic operation in the weighting circuit 46a according to the embodiment in reference to Fig. 3. When the weighting circuit 46a inputs the "Activity" of the input image signal 12a and the "Activity" of the predictive error signal (difference signal between the input image signal 12 and the predictive signal from the frame memory) 15a, the circuit reads out the previously assigned weighting coefficients W1 and W2, respectively, for the signals. The input "Activities" of the signals 12a and 15a are multiplied by the weighting coefficients W1 and W2, respectively, and a weighted "Activity" of the input image signal 12b and a weighted "Activity" of the predictive error signal 15b is output. With further reference to this embodiment, the weighting circuit 46a in accordance with the invention can weight only one of the "Activities" of the input signals instead of both of them. With further reference to the embodiment, the weighting circuit 46a in accordance with the invention can process three or more input signals, instead of two as described in this embodiment. An example of using three input signals in that condition will be described later in reference to Fig. 12. Fig. 7 shows a flowchart of an example of weighting operation in the weighting circuit 46a according to the embodiment in reference to Fig. 5. In this example, the "Activity" of the input image signal 12a is not weighted and output as "Activity" 12b after the weighting circuit. The "Activity" of the predictive error signal 15a is doubled by the weight and output as its weighted "Activity" 15b. Fig. 8 shows a flowchart of the basic operation in the comparator 46b according to the embodiment in reference to Fig. 3. The comparator 46b compares the input "Activities" of the input image signal 12b and of the predictive error signal 15b from the weighting circuit 46a to decide a coding mode. When "Activity" 12b is smaller than "Activity" 15b, the comparator 46b selects INTRA mode (for an intraframe coding) while the comparator selects INTER mode (for an interframe coding) when "Activity" 12b is larger than "Activity" 15b. When the comparator selects the mode, it outputs mode signal 46m to the control signal generator 46c. With further reference to the embodiment, the weighting circuit 46a in accordance with the invention can process three or more input signals, instead of two as described in this embodiment. An example of using three input signals in that situation will be described later in reference to Fig. 12. Fig. 9 shows a flowchart of the basic operation in the control signal generator 46c according to the embodiment in reference to Fig. 3. The control signal generator 46c inputs the input image signal 12, the predictive error signal 15, the weighted "Activity" of the input image signal 12b, the weighted "Activity" of the predictive error signal 15b and the mode signal 46m. The control signal generator 46c outputs the control signal 42 and one of the input signals as the select signal 15o based upon the mode signal 46m. The control signal generator 46c is informed of the selection of either the INTER or INTRA mode with the mode signal 46m. When the INTER mode is selected, the control signal generator outputs the predictive error signal 15 as the select signal 15o. When the INTRA mode is selected, the control signal generator outputs the input image signal 12 as the select signal 15o. The control signal generator 46c reads out the optimal quantization step size (QUANT) and the number of the DCT coefficients in the memory table in Fig. 10 according to the weighted "Activity" of the selected signal 12b or 15b. The readout quantization step size (QUANT) and DCT coefficients as well as the mode signal 46m are multiplexed to be output as the control signal 42. With further reference to the embodiment, the control signal generator 46c in accordance with the invention can input the "Activity" of the input image signal 12a and the "Activity" of the predictive error signal 15a, instead of the weighted "Activities" of those signals 12b and 15b. In this case, the threshold values or the contents of the memory table in Fig. 10 have to be altered. With further reference to the embodiment, the number of threshold value in Fig. 10 in accordance with the invention can be varied according to the assigned QUANT and DCT coefficients. The values of the QUANT and DCT coefficients can be also assigned based upon a given condition or priority, such as the picture quality or coding efficiency. Thus, selective, intentional control of an output signal, or the select signal 150 can be realized by the weighting technique. In other words, weighting can control the-selection probability of input image signals in the "Activity" comparative selector 46. The select signal 150 can be controlled by weighting technique which is provided with previously set weighting coefficients in the weighting circuit according to the user's or system's needs, or according to the picture quality desired. Embodiment 5. With further reference to Fig. 4, the coding controller 41 in accordance with the invention can compare the "Activity" of the filtered predictive error signal with the "Activity" of the unfiltered predictive error signal to output the coding control signal 42 based upon the compared result, specifically, the signal with smaller "Activity". Fig. 11 shows a configuration of interframe coding system according to this embodiment providing a predictive signal 24a filtered in the adaptive filter 22 and a predictive signal 24b unfiltered or the motion compensation predictive signal 14. Those predictive signals 24a and 24b are subtracted, respectively, from the input image signal 12 in the subtractors 3a and 3b. The differences resulted from the subtraction are output as the predictive error signals respectively. The predictive error signals are compared in the coding controller 41 to output the coding control signal 42 in the same manner described hereinbefore. Embodiment 6. With further reference to Fig. 11 or the previous embodiment, the coding controller 41 in accordance with the invention can add weight on either or both of the "Activities" of the predictive error signals, filtered in the adaptive filter 22 or/and unfiltered, for comparison to output the coding control signal 42. Embodiment 7. With further reference to Fig. 11, the coding control signal 42 in accordance with the invention can be based upon the compared result among the "Activities" of three signals, the filtered predictive error signal and the unfiltered predictive error signal, and the input image signal 12, specifically selecting the mode of the signal with the smallest "Activity". Fig. 12 shows a configuration of interframe coding system according to this embodiment. Fig. 12 comprises Fig. 11, as modified by adding the input image signal 12 input in the coding controller 41. The coding controller 41 uses three input signals of the input image signal 12, the filtered predictive error signal 15x and the unfiltered predictive error signal 15y for the "Activity" comparison to output the coding control signal 42 by selecting the mode of the signal with the smallest "Activity". Fig. 13 shows a block diagram of a configuration of the coding controller 41 according to the embodiment. Fig. 13 comprises Fig. 2, as modified by adding a filtered predictive error signal 15x and an unfiltered predictive error signal 15y, instead of the predictive error signal 15 and also adding an "Activity" of the filtered predictive error signal 15a and an "Activity" of the unfiltered predictive error signal 15c. Therefore the coding controller 41 according to this embodiment inputs three signals, instead of two as described hereinbefore. The "Activity" calculator 45 calculates the "Activities" of the three signals and outputs the calculated result to the "Activity" comparative selector 46. Fig. 14 shows a block diagram of a configuration of the "Activity" comparative selector 46 according to the embodiment. Fig. 14 comprises Fig. 3, as modified by adding the filtered predictive error signal 15x, the unfiltered predictive error signal 15y, the "Activity" of the predictive error signal 15c, and a weighted "Activity" of the predictive error signal 15d. The three input "Activities" of the signals in the "Activity" comparative selector 46 are added weight in the weighting circuit 46a. Weighting coefficients W1, W2, and W3 is assigned in the weighting circuit in advance. The weighting circuit adds weight by multiplying the "Activity" of input image signal 12a by a weighting coefficient W1 to output a weighted "Activity" of the input image signal 12b to the comparator 46b. In the same manner, the "Activity" 15a is multiplied by W2 to have a weighted "Activity" 15b and the "Activity" 15c is multiplied by W3 to have a weighted "Activity" 15d. The comparator 46b compares the three weighted "Activities" to generate a mode signal 46m based upon the comparison result. The control signal generator 46c selects one of the three signals as the output select signal 15o, of the input image signal 12, the filtered predictive error signal 15x, and the unfiltered predictive error signal 15y based upon the mode signal 46m. When the input image signal 12 is selected for the select signal 15o, the control signal generator 46c reads out a quantization step size and a DCT coefficient previously stored in the memory table according to the value of the weighted "Activity" of the input image signal 12b. When the filtered predictive error signal 15x is selected for the select signal 15o, the control signal generator 46c reads out a quantization step size and a DCT coefficient in that manner as described hereinbefore based upon the value of the weighted "Activity" of the filtered predictive error signal 15b. When the unfiltered predictive error signal 15y is selected for the select signal 15o, the control signal generator 46c reads out a quantization step size and a DCT coefficient in that manner as described hereinbefore based upon the value of the weighted "Activity" of the unfiltered predictive error signal 15d. The readout quantization step size and DCT coefficient as well as the mode signal 46m are multiplexed to be output as the control signal 42. Embodiment 8. With further reference to Fig. 12 or the previous embodiment, the coding controller 41 in accordance with the invention can add weight on at least one of the "Activities" of the three signals, the filtered predictive signal 15a, the unfiltered predictive error signal 15, and the input image signal 12 for the "Activity" comparison to output the coding control signal 42 based upon the signal with the smallest "Activity". Embodiment 9. With further reference to the previous embodiment, the coding control signal 42 in accordance with the invention can be output based upon a mode signal derived from an image signal with smaller "Activity" compared between those of the filtered predictive error signal 15a and the input image signal 12a. Embodiment 10. With further reference to the previous embodiment, the coding controller 41 in accordance with the invention can add weight on either or both of the "Activities" of the filtered predictive error signal through the processing units or/and the input image signal 12 for comparison to output the coding control signal 42. Fig. 15 shows a configuration of interframe coding system according to this embodiment. Fig. 15 comprises Fig. 4, as modified by adding another input of the filter control signal 23 from the filter controller 21 into the coding controller 41. The filter control signal 23 is for a weighting control. The filter control signal informs the coding controller 41 of the condition of the predictive error signal 15 whether it is filtered or not in the adaptive filter 22. The control signal 23 is analyzed in the weighting circuit 46a in the coding controller 41 to inform whether the adaptive filter 22 is ON or OFF. When the weighting circuit 46a knows that the predictive error signal 15 is based upon the predictive signal 24 filtered in the adaptive filter 22, for example, it adds weight on the predictive error signal 15. When the weighting circuit 46a knows that the predictive error signal 15 is based upon the predictive error signal 24 unfiltered in the adaptive filter 22, on the other hand, it does not add weight on the predictive error signal. Dynamic weighting control can be realized with a variety of modes or values in the filter control signal 23 providing more than two modes of ON and OFF. Embodiment 11. With further reference to Fig. 4, the filter control signal 23 in accordance with the invention can be specified by a difference itself between the input image signal 12 and the image signal from the frame memory 1 without the normalization action process in the filter controller 21. Embodiment 12. With further reference to Fig. 4, the filter control signal 23 in accordance with the invention can be specified based upon a difference between the input image signal 12 and the image signal from the frame memory 1. Embodiment 13. With further reference to Fig. 4, the filter controller 21 in accordance with the invention can output the filter control signal 23 by normalizing the difference between the input image signal 12 and an image signal from the frame memory 1 by the image signal from the frame memory 13. Embodiment 14. With further reference to Fig. 4, the adaptive filter 16 in accordance with the invention can be provided after the adder 6 in the local decoding loop as shown in Fig. 16. The adaptive filter 22 filters the local decoded signal 18 based upon the filter control signal 23 from the filter controller 21. As a result, either the filtered or unfiltered local decoded signal is stored in the frame memory 1. The filtered, for instance, image signal 14 or predictive signal 24 from the frame memory 1 is subtracted from the input image signal 12 in the subtractor 3 to output the predictive error signal 15 for interframe coding. Embodiment 15. The frame memory 1 in accordance with the invention can store an image signal by the field instead of the frame. The frame memory can also store a multiple number of time-varying frames or fields instead one frame or one field. Embodiment 16. Fig. 17 shows a configuration of interframe coding system in accordance with the invention. Fig. 17 comprises Fig. 4, as modified by adding a coding-filtering controller 50 instead of providing separately the filter controller 21 and the coding controller 41. Both of the filter controller and coding controller calculate the "Activity" of an image signal in the same manner to control a signal and therefore it may be desirable to integrate them in terms of the concentration of the similar unit including the same function. Thus, the integrated controller of filtering and coding can provide more control efficient coding system and contribute to downsizing or minimization of the overall system. Embodiment 17. With further reference to Fig. 4, the motion vector detector 2 in accordance with the invention can be removed from the system when the system prioritizes simplification. With this configuration, the quantity of motion of an image signal is always considered zero. Embodiment 18. With further reference to Figs. 1 through 17, coding control system in accordance with the invention can provide a feedback loop based upon the coded data from the encoder 4 or the local error signal 16, adding to the coding controller 41 or the filtering-coding controller 50. Fig. 18 shows a configuration of the interframe coding system according to this embodiment. Fig. 18 comprises Fig. 1, as modified by adding a transmission buffer 100, a transmission signal 160 and a feedback signal 101. The transmission buffer 100 stores coded data from the encoder 4 based upon the coded error signal 16. The transmission signal 160 of the coded error signal 16 is transmitted onto a transmission line from the transmission buffer 100. The feedback signal 101 is fed back to the coding controller 41 from the transmission buffer 100. Accordingly, the coding controller 41 controls coding with the coding control signal 42 by the feedback signal 101. The feedback signal 101 indicates, for instance, occupied ratio of coded error signals 16 in the transmission buffer 100. In reception of the occupied ratio, the coding controller 41 controls coding with the coding control signal 42 to reduce the amount coding in the encoder 4, for instance, when the coded error signal 16 occupies the transmission buffer 100 in larger proportion. On the contrary, the coding controller 41 controls coding to increase the amount of coding in the encoder 4 with the coding control signal 42, for instance, when the coded error signal 16 occupies the transmission buffer 100 in smaller proportion. Embodiment 19. 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 20. 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 21. 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 stated hereinbefore, the interframe coding system of the present invention has the following advantageous effects. One of the advantageous features is that the coding controller outputs the coding control signal based upon the "Activities" of the input image signal and the predictive error signal. This can improve coding efficiency. Another advantageous feature is that the coding controller adds weight on the "Activity" of an input image signal. The coding controller can control the output ratio of an image signal as the select signal from the coding controller in accordance with a user's or system's needs or the condition of the processing image signal. Another advantageous feature is that filtering control and coding control are 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. Further, the coding controller outputs the coding control signal based upon the signal with the smallest "Activity" among others, so that the coding efficiency can be improved. Another advantageous feature is that the coding controller outputs the coding control signal based upon the mode signal by selecting the image signal with the smallest "Activity" among others. This can improve the coding efficiency by providing a highly efficient prediction with less coding loop gain.

Claims: A coding system which includes an encoder (4) for coding an input signal (12) including quantization into a coded signal (16) and a frame memory (1) for outputting a predictive signal (11), comprising: a coding controller (41) responsive to the input signal (12) and the predictive signal (11), for generating a coding control signal (42) for controlling the encoder (4) characterized in that the coding controller (41) includes an activity calculator (45) for calculating an activity value of each of the input signal (12) and a predictive error signal (15) formed by subtracting the input signal (12) from the predictive signal (11), the activity value becoming smaller when the corresponding signal contains a great amount of lower frequency components, and becoming larger when the corresponding signal contains a great amount of higher frequency components, and an activity comparative selector (46) for comparing the activity values and for generating the coding control signal (42) which provides a quantization step size on the basis of said activity values. The coding system of claim 1, characterized in that the activity comparative selector (46) is also provided for selecting either the input signal (12) or the predictive error signal (15) for encoding based on the comparison result. The coding system of claim 1 or 2, characterised in that the activity comparative selector (46) includes means (46a) for weighting at least one of characteristics. The coding system of one of claims 1 to 3, characterised by a filter (22) for receiving and filtering the predictive signal (14), and outputting a filtered signal (24); and a filter controller (21) responsive to the input signal (12) and the predictive signal (14) for generating a filter control signal (23) to control the filter (22). The coding system of claim 4, characterised in that coding controller (41) comprises: means for interframe coding which produces a first interframe signal based on the filtered signal and a second interframe signal based on the unfiltered predictive signal; means for intraframe coding which produces an intraframe signal; means (45) for determining characteristics of the first and second interframe signals and the intraframe signal; means (56b) for comparing any two of the characteristic of the first interframe signal, the characteristic of the second interframe signal, and the characteristic of the intraframe signal; and means (46c) for generating the coding control signal based on the comparison result. The coding systems of claim 5, characterised in that comparing means (46a) includes means for weighting at least one of characteristics. The coding system of claim 4, characterised in that coding controller (41) comprises: means for interframe coding which produces an interframe signal based on the filtered signal; means for intraframe coding which produces an intraframe signal based on the filtered signal; means for intraframe coding which produces an intraframe signal; means for determining a characteristic of the interframe signal and a characteristic of the intraframe signal; means for comparing the two characteristics; and means for generating the coding control signal based on the comparison result. The coding system of claim 7, characterised in that the comparing means includes means for weighting at least one of characteristics. The coding system of claim 1, characterised in that coding controller (41) includes means for generating the coding control signal based on the coded signal, forming a feed back loop. The coding system of claim 2, 5 or 7, characterised in that the coding controller (41) includes means (45) for calculating the characteristic based on a difference between the maximum and minimum values of the signal, on the sum of the absolute difference values between the individual difference values between the individual values and the mean value of the individual values, or on the sum of the squared difference values between the individual values and the means value of the individual values.

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: H04N7, G06T9