Encoding/decoding apparatus

Abstract: There is provided an encoding apparatus provided with a quantization unit (7) for quantizing image information, a generation unit (15) for generating coefficient information for determining the quantizing steps of the quantization unit, an encoding unit (9) for effecting variable length encoding on the data quantized by the quantization unit, a dividing unit for dividing the image information of a picture into plural blocks, and a transmission unit for effecting transmission by adding the coefficient information to each block divided by the dividing unit. Also there is provided a decoding apparatus for reproducing image information from a data train transmitted in the unit of a data frame containing a code obtained by quantizing image information and encoding thus quantized data, and coefficient information relating to the steps of quantization and determined for each picture, comprising a decoding unit for decoding the code, a detection unit for detecting an error in the coefficient information, and a replacement unit for replacing the information in the data frame with information of another data frame, based on the output of the detection unit.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an encoding apparatus for quantizing image information and encoding thus quantized data, and a decoding apparatus for decoding thus encoded data. Related Background Art For the purpose of encoding color image signal, the ADCT (adaptive discrete cosine transformation) encoding is gaining popularity in recent years. This method is based on encoding by decomposing the image into its spatial frequency components, and is one of orthogonal transformation encoding methods. The DCT encoding is also adopted by the JPEG (Joint Photographic Expert Group) established as an international standard organization for such encoding methods, and the DCT is gaining popularity as the orthogonal transformation suitable for image information. The encoding and decoding by DCT are in general conducted in the following manner. The transformation of two-dimensional frequency components is executed in a unit block of 8 x 8 pixels, since such transformation in the entire picture requires an excessively long time. The DCT is conducted for each block to obtain DCT coefficients in an 8 x 8 matrix, in which the coefficients at the right side, lower side and lower right area respectively signify the high frequency components in the horizontal, vertical and diagonal directions, and the coefficient at the upper left corner indicates the DC component. Then said DCT coefficients are quantized, rougher in the higher frequency side and finer in the lower frequency side, with weighting on the quantizing levels for the DCT coefficients based on the human visual characteristics. Then the thus quantized DCT coefficients are encoded. Since the components are concentrated in the lower frequency side, the encoding is conducted by zigzag scanning between the high and low frequency components. The significant coefficients with non-zero quantized outputs are grouped according to their values, and the Huffman encoding is executed on a set of an identification number for said group, an immediately preceding significant coefficient, and a number positioned therebetween and indicating the run length of insignificant coefficients with zero quantized outputs, followed by the addition of a same-length code identifying the value in the group. Since the diagonal high frequency components have generally low probability in the images, the latter portion after zigzag scanning is expected to become all zero in most cases. Consequently, a very high compression rate can be expected in the variable length code obtained in this manner, and an image reproduction with scarce deterioration in image quality can be achieved if a compression rate of several to one is anticipated in average. However, the transmission channel generally has a fixed transmission capacity per unit time, and, if a picture has to be transmitted within a predetermined period as in the case of transmitting the moving image, it is desirable that the obtained code has a fixed number of bits (transfer rate) for a picture or for a unit pixel block. The number of bits per picture is determined by the control coefficient C, which determines the coarseness of quantization. Thus, the control coefficient C is an important parameter, determining the transfer rate of image and also the quality thereof. Various methods have been proposed for determining said control coefficient C. For each frame of image data compressed as explained above, the control coefficient C employed in the quantization of the image data is attached, and the inverse quantization is conducted with said control coefficient C at the reproduction of the compressed image data whereby the image is reproduced through a process exactly inverse to that at encoding. Thus, in the conventional art, the information for said control coefficient C for quantisation is attached to the image information of each frame. Said coefficient information, being an extremely important parameter determining the quantisation characteristics of the image and thus influencing the image quality, may destruct the quantisation properties of an image frame and provide a visually unpleasant image if an error is involved in said information for example from the transmission channel. US-A-4982282 discloses an encoding apparatus which compresses a signal in which coefficient information is transmitted once per frame, and a corresponding decoding apparatus which expands the signal. SUMMARY OF THE INVENTION In consideration of the foregoing, the object of the present invention is to provide an encoding apparatus and a decoding apparatus capable of improving the reliability of image reproduction. A first aspect of the present invention provides an encoding apparatus as set out in claim 1. A second aspect of the present invention provides a decoding apparatus as set out in claim 4. A third aspect of the present invention provides a method of encoding as set out in claim 1. A fourth aspect of the present invention provides a method of decoding as set out in claim 8. Other objects of the present invention, and the advantages thereof, will become fully apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of an first example of an image encoding apparatus; Fig. 2 is a block diagram of a first example of a image decoding apparatus; Fig. 3 is a view showing the format of the transmitted signal; Figs. 4A to 4D are views showing the encoding process; Fig. 5 is a chart showing the method of setting the control coefficient for determining the image transfer rate; Fig. 6 is a block diagram of a second example of an image decoding apparatus; Fig. 7 is a block diagram of an image encoding apparatus constituting an first embodiment of the present invention; Fig. 8 is a block diagram of an image decoding apparatus constituting an embodiment of the present invneiton; Fig. 9 is a view showing the structure of a frame in the embodiment of the present invention; and Fig. 10 is a view showing the format of transmitted signal in the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following there will be explained a first explanatory example of an image transmission system, with reference to Figs. 1 to 5. At first reference is made to Fig. 1, for explaining the configuration and function of an image encoding apparatus in the image transmission system. An analog image signal, entered from an input terminal 1, is subjected to 8-bit digitization by an A/D converter 2. The digitized image signal is supplied to a two-dimensional spatial filter (SPF) 3, for eliminating high frequency components that cannot be processed in the DCT (discrete cosine transformation) to be explained later. Thus the filter 3 alleviates the visual block distortion. Subsequently the digital image signal is supplied to a block dividing circuit 4, divided therein two-dimensionally into pixel blocks each consisting of 8 x 8 pixels, and is supplied to the subsequent process in the unit of the pixel block. The image signal from said block dividing circuit 4 is supplied to a DCT circuit 5, which generates an 8 x 8 data matrix indicating the frequency components. More specifically, a pixel block shown in Fig. 4A and consisting of image data D 11 - D 88 is converted, by the DCT circuit 5, into a data matrix shown in Fig. 4B and consisting of data X 11 - X 88 . The coefficient X 11 indicates the DC component in the horizontal and vertical directions in the pixel block, or, namely, the average value of this 8 x 8 pixel block. The coefficients X 11 - X 88 , or generally represented as X ij , indicate components of higher frequency in the vertical direction for a larger value of i, and components of higher frequency in the horizontal direction for a larger value of j. The data matrix generated by the DCT circuit 5 is supplied to a frame memory 6 and a coefficient generation circuit 15. A quantization matrix generating circuit 14 generates a quantization matrix W 11 - W 88 (cf. Fig. 4C) indicating the weights of quantization step sizes for the DCT coefficients X 11 - X 88 , and the coefficient generation circuit 15 generates a control coefficient C to be explained later in more detail. The quantization matrix W 11 - W 88 and the control coefficient C are supplied to a multiplier 16. The multiplier 16 effects calculations W ij x C = Q ij , and the outputs Q 11 - Q 88 are supplied to a linear quantization circuit 7 for determinig the quantization steps thereof. The linear quantization circuit 7 calculates X ij /Q ij to generate outputs G 11 - G 88 . The quantized data G 11 - G 88 are released in succession from the DC component, by means of a zigzag scanning circuit 8. More specifically said zigzag scanning circuit 8 sends said outputs in the order of G 11 , G 12 , G 21 , G 31 , G 22 , G 13 , G 14 , G 23 , G 32 , G 41 , ..., G 85 , G 86 , G 77 , G 68 , G 78 , G 87 and G 88 to a variable length encoding circuit (VLC) 9. The variable length encoding circuit 9 effects Huffman encoding by calculating a predicted value, for example on the DC component G 11 , among the nearby pixel blocks and encoding the error to said predicted value. However, the transmission channel generally has a fixed transmission capacity per unit time, and it is desirable that the obtained code has a fixed number of bits per each picture or per each pixel block in case a picture is to be transmitted at every predetermined period as in the case of transmitting a moving image. The number of bits is determined by the control coefficient C generated by the coefficient generation circuit 15. More specifically, if the control coefficient C is selected larger, the probability of G ij = 0 becomes larger, so that the total number of bits NB (transfer rate) of the encoded data decreases. The relationship between the control coefficient C and the total number of bits NB, though variable depending on the image, is a simply decreasing function in any case, and is already known to assume the form of a logarithmic curve as shown in Fig. 4D for average images. Therefore, the coefficient C 0 for obtaining a desired total number of bits NB 0 can be predicted, by effecting the encoding with a coefficient C 1 , determining the total number of bits NB 1 of thus obtained code, and calculating the predicted value of C 0 based on NB 1 and C 1 , utilizing a fact that the logarithmic curve shown in Fig. 4D passes through a point (C 1 , NB 1 ). The coefficient C 0 is predicted in the coefficient generation circuit 15 as will be explained in the following in relation to Fig. 5. At first certain control coefficients are selected. The data compressions are simultaneously executed, respectively with the coefficients, and the total number of bits per frame is calculated for each coefficient. Then a range of coefficients providing the desired total number of bits is looked for (C 2 and C 3 in case of Fig. 5), and the coefficient C 0 providing the desired total number of bits is determined by linear approximation from the two points of said range. Though the coefficient F 0 determined in this manner is somewhat different from the true coefficient C 0 , but is optimum in that the obtained number of bits never exceeds the desired number of bits since the logarithmic curve is convex downwards. The coefficient F 0 thus determined and the variable length encoded image signal from the VLC 9 are supplied to a frame multiplexer 10, which attaches the information of the control coefficient F 0 to the image signal. The format of the transmission signal released from the frame multiplexer 10 is shown in Fig. 3. In Fig. 3 there are shown vertical synchronization codes 40, 43; control coefficient information 41, 44 of frames, containing the information of the coefficient F 0 in the present embodiment; and variable length encoded image information 42, 45. The transmission signal from the frame multiplexer 10 is error correction encoded in an error correction encoding (ECC) circuit 11, then modulated in a modulation circuit 12 according to the characteristics of the transmission channel, and is supplied thereto from an output terminal 13. Now reference is made to Fig. 2, for explaining the configuration and function of an image decoding apparatus of the image transmission system constituting the 1st embodiment. The transmission signal (data train) from the transmission channel is entered from an input terminal 16, thus demodulated in a demodulation circuit 18 and supplied to an error correction code (ECC) decoding circuit 19. The ECC decoding circuit 19 corrects the code errors in the transmitted data train, according to a predetermined algorithm. Also in case there are generated data for which the error correction is identified as being impossible an error flag released for each error correction code word is set at "1", and the error flag is supplied to a coefficient error discrimination circuit 28. The coefficient error discrimination circuit 28 discriminates whether the code word with the error flag "1" contains the coefficient information, and, if such coefficient information is contained, sets a coefficient error signal, released for each frame of the image information, at "1". Said coefficient error signal is supplied to an error process circuit 29. The transmission signal, subjected to the error correction in said ECC decoding circuit 19, is supplied to a coefficient decoder 20, and is separated therein into the image information and the coefficient information. The coefficient information, separated in said coefficient decoder 20, is supplied to the error process circuit 29, which transmits the coefficient information to a multiplier 30 in case the coefficient error signal supplied from the coefficient error discrimination circuit 28 is not "1". The image information separated in the coefficient decoder 20 is supplied to an IVLC 21 for variable length decoding. A zigzag scanning circuit 22, a linear inverse quantization circuit 23, an inverse quantization matrix generation circuit 31 and an IDCT (inverse discrete cosine transformation) circuit 24 effect a process exactly inverse to the process executed by the zigzag scanning circuit 8, the linear quantization circuit 7, the quantization matrix generation circuit 14 and the DCT circuit 5 in the encoding apparatus shown in Fig. 1. The image information subjected to the above-explained process is buffered in a frame memory 25, then converted into an analog signal in a D/A converter 26, and released from an output terminal 27, for supply to image display means such as a monitor unit. On the other hand, in case said coefficient error signal, supplied from the coefficient error discrimination circuit 28 to the error process circuit 29, is "1", the error process circuit 29 decodes the image information by replacing the coefficient information, attached to all the frames, with the coefficient information of the current frame. In the following there will be explained, with reference to Fig. 3, the process in case the coefficient information contains an error. It is assumed that an error is generated in the transmission signal shown in Fig. 3 on the transmission channel and that the ECC decoding circuit 19 shown in Fig. 2 sets the error flag as "1" in a section 1 shown in Fig. 3. The error flag is set at "1" for a code word for which the error correction is not possible, and the error flag is supplied to the coefficient error discrimination circuit 28, which discriminates whether the code word with the error flag "1" contains the coefficient information. Since the section 1 contains the coefficient information 44, the coefficient error discrimination circuit 28 supplies the error process circuit 29 with a coefficient error signal "1" for the image information of a frame, corresponding to said coefficient information 44. In response to the coefficient error signal "1" from the coefficient error discrimination circuit 28, the error process circuit 29 disregards the coefficient information 44, and effects the decoding of the image information 45 by supplying the multiplier 30 with the coefficient information 41 used in the decoding of a preceding frame. In the following there will be given a detailed explanation of a second explanatory example of an image transmission system. In the second example, the configuration and function of the image encoding apparatus are same as those in the 1st example, and will not, therefore, be explained. Now reference is made to Fig. 6 for explaining the configuration and function of the second example of an image decoding apparatus. In Fig. 6, components same as or equivalent to those in Fig. 2 are represented by same numbers and will not be explained further in the following. Also the functions of the apparatus are same as those in the 1st embodiment, except for the process in case an error is generated in the coefficient information. Such process will be explained in the following. In case the coefficient error signal, supplied from the coefficient error discrimination circuit 28 to an error process circuit 32, is "1", the image of the preceding frame is frozen. More specifically, the error process circuit 32 sends a freeze signal to the frame memory 33, which in response releases again the image information of the preceding frame. More specifically, with reference to Fig. 3, in case the coefficient error signal is "1", the error process circuit 32 send a freeze signal to the frame memory 33, for disregarding the coefficient information 44 and the image information 45 and releasing again the image of the preceding frame, namely the image regenerated from the information 42. In response the frame memory 33 sends the image of the preceding frame again to the D/A converter for output from the output terminal 27. In the following there will be given a detailed explanation of an embodiment of the image transmission system of the present invention, with reference to Figs. 7 to 10. At first reference is made to Fig. 7 for explaining the configuration and function of an image encoding apparatus, constituting the embodiment of the image transmission system. In Fig. 7, components same as or equivalent to those in Fig. 1 are represented by same numbers and will not be explained further. The embodiment is same as the 1st example, except for the process of attaching the coefficient F 0 , released from the coefficient generation circuit 15, to the image signal from the VLC 9. Said attaching of the coefficient F 0 to the image signal is conducted in the following manner. The coefficient F 0 from the coefficient generation circuit 15 and the variable length encoded image signal, from the VLC 9, are supplied to a resynchronization multiplexer 36, which divides a frame into plural areas and attaches information of the coefficient F 0 to the image signal of each of said divided areas. The divided area will hereinafter be called a re-sync block. The relation of re-sync blocks in a frame will be explained in the following, with reference to Fig. 9. In Fig. 9, if the picture is composed of 1280 pixels in the horizonal direction and 1088 pixels in the vertical direction, and if each pixel is A/D converted with 8 bits, the data amount per picture is : 1280 x 1088 x 8 = 11,141,120 bits. Since each DCT block is composed of 8 pixels in the horizontal and vertical directions with a predetermined amount of image data, a re-sync block is selected as composed of 40 DCT blocks. Consequently a picture is composed of 544 re-sync blocks, which are arranged 4 in the horizontal direction and 136 in the vertical direction. The data amount in each re-sync block is: 40 x 8 x 8 x 8 = 20,480 bits. The format of the transmission signal released from the re-sync multiplexer 36 is shown in Fig. 10. There are shown a vertical synchronization code 50; re-sync codes 31, 34, 37, 40, 43; coefficient information codes 32, 35, 38, 41, 44 for the coefficient F 0 released from the coefficient generation circuit 15; and variable length encoded image data 33, 36, 39, 42, 45. Except for the above-explained process, the processes of the embodiment are same as those of the first example. Now reference is made to Fig. 8 for explaining the configuration and function of an image decoding apparatus in the embodiment, wherein components same as or equivalent to those in Fig. 2 or 6 are represented by same numbers. The transmission signal (data train) from the transmission channel is entered from the input terminal 17, demodulated by the demodulation circuit 18 and supplied to the ECC decoding circuit 19. The ECC decoding circuit 19 corrects the code errors in the transmitted data train, according to a predetermined algorithm. In case there are generated data for which the error correction is identified impossible, an error flag released for each error correction code word is set at "1", and the error flag is supplied to a re-sync block error discrimination circuit 34. The re-sync error discrimination circuit 34 discriminates a block containing said coefficient information, of which code words has said error flag "1", and, if such block containing the coefficient information is present, sets a re-sync block error signal, released for each re-sync block of the image information, at "1". The re-sync block error signal is supplied to a coefficient selector 35. The transmission signal, subjected to the error correction in said ECC decoding circuit 19, is supplied to the coefficient decoder 20, and is separated therein into the image information signal and the coefficient information signal. The coefficient information, separated in said coefficient decoder 20, is supplied to the coefficient selector 35, which transmits said coefficient information to the multiplier 30 in case the re-sync block error signal "1" is not supplied from the re-sync block error discrimination circuit 34. The image information separated in the coefficient decoder 20 is supplied to the IVLC 21 for variable length decoding. The zigzag scanning circuit 22, the linear inverse quantization circuit 23, the inverse quantization matrix generation circuit 31 and the IDCT (inverse discrete cosine transformation) circuit 24 effect a process exactly inverse to the process executed by the zigzag scanning circuit 8, the linear quantization circuit 7, the quantization matrix generation circuit 14 and the DCT circuit 5 in the encoding apparatus shown in Fig. 1. The image information subjected to the above-explained process is buffered in the frame memory 25, then converted into an analog signal in the D/A converter 26, and released from the output terminal 27, for supply to image display means such as a monitor unit. On the other hand, in case the re-sync block error signal "1" is supplied to the coefficient selector 35, the selector 35 disregards the coefficient information signal attached to the incorrectible re-sync block, and replaces the information signal with coefficient information attached to a re-sync block preceding or succeeding to the incorrectible re-sync block. Also in case a frame contains three or more incorrectible re-sync blocks, the frame currently processed is disregarded, and preceding or succeeding frame is frozen. More specifically, the coefficient selector 35 is provided therein with a re-sync block counter, and sends a freeze signal to the frame memory 33 in case the number of error re-sync blocks within a frame exceeds a certain threshold value, and, in response the frame memory 33 does not release the image information of the current frame but that of a preceding or succeeding frame, thereby replacing the image information of the current frame. Now reference is made to Fig. 10 for explaining the process in case the coefficient information, attached to each re-sync block, contains an error. It is assumed that an error is generated, on the transmission channel, in the transmission signal shown in Fig. 10, whereby the ECC decoding circuit 19 shown in Fig. 8 sets the error flag at "1" in a section 1 shown in Fig. 10. The error flag is set at "1" for a code word containing an incorrectible error. The error flag is supplied to the re-sync block error discrimination circuit 34, which discriminates whether the code word with the error flag "1" contains the coefficient information. In the section 1, for the image information of the re-sync block corresponding to the coefficient information 58 and 61, a re-sync block error signal "1" is entered into the coefficient selector 35. In response to said re-sync block error signal, the coefficient selector 35 disregards the coefficient information 58, 61 in the section 1, and supplies the multiplier 30 with the coefficient information attached to the preceding re-sync block, for example the coefficient information 55. Also as explained above, the coefficient selector 35 is provided with a re-sync block error counter, and, in case the number of error re-sync blocks exceeds a certain threshold value, the image of the frame is considered unreliable and is replaced by the image of the preceding frame. In the present embodiment, the threshold value is selected as two. More specifically with reference to Fig. 10, if an error flag is set in the section 2, the number of error re-sync block is considered as three, since the section 2 contains the coefficient information 54, 57, 61. Thus the coefficient selector 35 sends a freeze signal to the frame memory 33, thereby freezing the image of the preceding frame. As explained detailedly in the foregoing, in the image signal encoding apparatus and the decoding apparatus of the present invention, if an error is generated, for example on the transmission channel, in the control coefficient information which is an extremely important parameter affecting the image quality, the image reproduction is conducted by disregarding such coefficient information involving error and replacing it with effective coefficient information. It is therefore rendered possible to minimize the influence of the error in the coefficient information, and to improve the reliability of image reproduction. Also the present invention is subject to various modifications within the scope thereof. For example, in the foregoing embodiment, the coefficient information or the image information is replaced by the previously processed information, but it may naturally be replaced by the subsequent coefficient or image information. In other words, the foregoing description of embodiments has been given for illustrative purposes only and not to be construed as imposing any limitation in every respect. The scope of the invention is, therefore, to be determined solely by the following claims and not limited by the text of the specification and alterations made within a scope equivalent to the scope of the claims fall within the scope of the invention.

Claims: An encoding apparatus comprising: a) block forming means (4) for forming encoding blocks of sampled values of image data representing an input signal representative of successive image frames; b) determining means (15) for determining control information (F 0 ) for controlling quantization steps for each successive plurality of said encoding blocks of sampled values representing a predetermined amount of the image data; c) quantization means (7) for quantizing each of said encoding blocks of sampled values on the basis of the control information (F 0 ) determined by said determining means (15) to provide respective blocks of quantized data; d) coding means (9) for coding each of said blocks of quantized data provided by the quantization means (7); the apparatus being characterised by: e) means for combining a plurality of the blocks of coded data to form respective transmission blocks, a plurality of the transmission blocks corresponding to said predetermined amount of the image data; and f) means (36) for adding the control information (F 0 ) determined for each said predetermined amount of the image data to each transmission block of coded data corresponding to the respective predetermined amount of image data to produce a data train such that if a decoder detects any errors occurring in the control information (F 0 ) in a transmission block it is possible to replace the control information (F 0 ) in said transmission block with the quantizing step information of another transmission block. An apparatus according to claim 1, further comprising orthogonal conversion means (5) for orthogonally-converting the blocks divided from the input signal, and wherein the quantization means (7) is arranged to quantize the input signal orthogonally-converted by the orthogonal conversion means (5). An apparatus according to claim 2, wherein the orthogonal conversion means is a discrete cosine transform means (5). A decoding apparatus for decoding image data from a data train transmitted in units of transmission blocks (40,41,42;50-65) each containing a code (42;53,56,59,62,65) obtained by quantizing the image data and encoding the thus quantized image data, and quantizing step information (41,52,55,58,61,64) representing the steps of the quantization of the image data, the apparatus comprising: decoding means (19) for decoding the code (42); the apparatus being characterised in that: each transmission block of said data train includes quantizing step information (41,52,58,61,64) representative of the quantization steps determined for a unit of a predetermined amount of image data corresponding to a plurality of transmission blocks including said transmission block; and in that the apparatus comprises: detection means (28) for detecting any errors occurring in the quantizing step information (52,55,58,61,64) in each transmission block; and replacement means (29) for replacing quantizing step information (52,55,58,61,64) in any transmission block where an error is detected by the detection means (28) with the quantizing step information of another transmission block. An apparatus according to claim 4, including means for determining whether more than a threshold number of transmission blocks corresponding to one of said units of image data contains an error in the quantizing step information (52,55,58,61,64), and if so replacing the data (53,56,59,62,65) in the transmission blocks corresponding to said unit of image data by the data in the transmission blocks corresponding to another of said units of image data. An apparatus according to any one of the preceding claims in which the image data is converted into frequency components. A method of encoding an input signal representative of successive image frames comprising the steps of: a) forming encoding blocks of sampled values of image data representing the input signal; b) determining control information (F 0 ) for controlling quantization steps for each successive plurality of encoding blocks of sampled values representing a predetermined amount of image data; c) quantizing each of said encoding blocks of sampled values on the basis of the determined control information (F 0 ) to provide a respective block of quantized data; d) coding each of said blocks of quantized data; the method being characterised by the steps of: e) combining a plurality of the blocks of coded data to form respective transmission blocks, a plurality of the transmission blocks corresponding to said predetermined amount of the image data; and f) adding the control information (F 0 ) determined for each said predetermined amount of the image data to each transmission block of coded data to produce a data train such that if in a decoding step any errors are detected occurring in the quantization step information in a transmission block, it is possible to replace the quantizing step information in said transmission block with the quantizing step information of another transmission block. A method of decoding image data from a data train transmitted in units of transmission blocks (40,41,42;50-65) each containing a code (42;53,56,59,62,65) obtained by quantizing the image data and encoding the thus quantized image data, and quantizing step information (41,52,55,58,61,64) representing the steps of the quantization of the image data, the method comprising the step of: decoding the code (42); the method being characterised in that: each transmission block of said data train includes quantizing step information (41,52,58,61,64) representative of the quantization steps determined for a unit of a predetermined amount of image data corresponding to the plurality of transmission blocks including said transmission block; and the method includes the steps of: detecting any errors occurring in the quantizing step information (52,55,58,61,64) in each transmission block; and replacing quantizing step information (52,55,58,61,64) in any transmission block where an error is detected by the detection means (28) with the quantizing step information of another transmission block. A method according to claim 8, including the step of determining whether more than a threshold number of transmission blocks corresponding to one of said units of image data contains an error in the quantizing step information (52,55,58,61,64), and if so replacing the data (53,56,59,62,65) in the transmission blocks corresponding to said unit of image data by the data in the transmission blocks corresponding to another of said units of image data. A method according to any one of claims 7 to 9, in which the image data is converted into frequency components.

Inventors: IWABUCHI YOSHITSUGU, NAGASAWA KENICHI, IWABUCHI,YOSHITSUGU, NAGASAWA,KENICHI, Iwabuchi,Yoshitsugu , c/o Canon Kabushiki Kaisha, Nagasawa,Kenichi , c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N7
IPC codes: H04N7