Method of correcting errors in digital VCR

Abstract: A method of correcting errors in a digital VCR in which the error correcting capability is maximized and probabilities that no error is detected and data decoding fails are minimized, by using a relatively small length of redundancy. The method comprises the first step of detecting errors from vertical and horizontal sections of a code of data played back from a tape and setting cross-points of the detected errors to flags, the second step of detecting errors from the horizontal code sections of the played back data, correcting the detected errors and erasing only the errors positioned at the cross-points set to the flags at the first step, in ones of the horizontal code sections from which detectable but non-correctable errors are detected, and the third step of correcting the errors erased at the second in a vertical direction.

Description: The present invention relates in general to digital video cassette recorders, and more particularly to a method of correcting errors in a digital video cassette recorder (VCR) in which the error correcting capability is maximized and probabilities that no error is detected and data decoding fails are minimized. It is common that an error correcting ccder is used in a digital VCR to correct errors occurring in compressing data to record it or decoding the recorded data. In the error correcting coder, there have generally been used block codes or convolutional codes. In particular, the block codes have more widely been used in the digital VCR than the convolutional codes. In a reed solomon (RS) code as one of the block codes, there are present a part designating input data and a part designating a redundancy. Here, the capability of detecting and correcting the errors is dependent on the redundancy. In order to maximize the capability of correcting the errors of the data recorded on a tape, it is preferred to use the reed solomon code with a two-dimensional structure as shown in Fig. 1 rather than a one-dimensional structure of a block unit. Referring to Fig. 2, there is shown a block diagram of a conventional reed solomon coder with the two-dimensional structure. As shown in this drawing, the conventional reed solomon coder comprises a vertical encoder 1 for encoding input data Din in a vertical direction and storing the encoded data in a first memory 2, a horizontal encoder 3 for encoding the data stored in the first memory 2 in a horizontal direction and recording the encoded data on a tape 4, and a horizontal decoder 5 for decoding data played back from the tape 4 in the horizontal direction and storing the decoded data in a second memory 6. The horizontal decoder 5 is adapted to detect errors from the data played back from the tape 4 and correct the detected errors. Also, the horizontal decoder 5 serves to erase all horizontal code sections of the data from which detectable but non-correctable errors are detected. The conventional reed solomon coder also comprises a second memory 6 for storing output data from the horizontal decoder, and a vertical decoder 7 for decoding the data stored in the second memory 6 in the vertical direction and correcting the data code sections erased in the horizontal direction by the horizontal decoder 5 in the vertical direction. Fig. 3 is a flowchart illustrating a conventional method of detecting and correcting the errors of the data recorded on the tape in Fig. 2. The operation of the conventional reed solomon coder with the above-mentioned construction will hereinafter be described with reference to Fig. 3. In operation, the input data Din is encoded in the vertical direction by the vertical encoder 1 and then stored in the first memory 2. Then, the data stored in the first memory 2 is encoded in the horizontal direction by the horizontal encoder 3 and then recorded on the tape 4. Here, the errors occurring in the data which is recorded on the tape in the above manner are detected and corrected by the horizontal decoder 5 and the vertical decoder 7 as shown in Fig. 3. First, the horizontal decoder 5 detects the horizontally occurring errors from the data recorded on the tape 4 and corrects the detected errors. Also, the horizontal decoder 5 erases all the horizontal code sections of the data from which the detectable but non-correctable errors are detected. Thereafter, the vertical decoder 7 corrects the data code sections erased in the horizontal direction by the horizontal decoder 5 in the vertical direction. In the reed solomon code (n1,k1) of the two-dimensional structure, provided that the total length of a code word is nl, a length of a redundancy constituting a portion of the code word is n1-k1, the number of the errors to be corrected is Ec and the number of the errors to be detected is Ed, the error detection and correction can performed within the range of the following equation: Ec + Ed < n-k Namely, the correction is made possible for the errors of the number smaller than or equal to Ec, but impossible for the errors of the number greater than Ec. The detection is made possible for the errors of the number smaller than or equal to Ed, but impossible for the errors of the number greater than Ec. Also, the horizontal code sections of the data from which the detectable but non-correctable errors are detected are all erased. In the reed solomon code (n2,k2) of the two-dimensional structure, wnere n2 is the total length of a code word and n2-k2 is a length of a redundancy constituting a portion of the code word, the data code sections erased in the horizontal direction are corrected in the vertical direction by the vertical decoder 7. Namely, assuming that the number of the vertically existing errors of the data recorded on the tape 4 is t1 and the number of the data sections erased by the horizontal decoder 5 is t2, the error detection and correction can performed within the range of the following equation: t1 + 2t2 < n2-k2 Noticeably, the data sections erased by the horizontal decoder 5 are given the correction capability twice as much as the errors with no position information. However, the conventional reed solomon coder has a disadvantage in that the horizontal code sections of the data from which the detectable but non-correctable errors are detected are all erased and then corrected in the vertical direction. Namely, when no error is detected from the horizontal code sections of the data, the error correction is made impossible and the non-corrected errors exert a bad influence on a picture quality. Also, because the horizontal code sections of the data from which the detectable but non-correctable errors are detected are all erased, even data sections with no error are erased, resulting in a loss in the information. The information loss means that the errors to be corrected by the vertical decoder is increased in number. Further, the errors which cannot be corrected by the vertical decoder are applied to an error concealment unit, which corrects the surrounding values into estimated values. In this case, an increase in the number of the errors being applied to the error concealment unit results in a degradation in the picture quality. A method and apparatus for decoding error correction code in two-dimensional form are described in EP-A-0 167 627; a paper in IEEE Transactions on Consumer Electronics, vol. 35, No. 4, Nov. 1989, pages 907-916, describes error codes for digital recording systems in which Reed-Solomon codes are used and in which errors during the storage/retrieval process are detected and Reed-Solomon code is used to correct the errors. Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method of correcting errors in a digital VCR in which the error correcting capability is maximized and probabilities that no error is detected and data decoding fails are minimized, by using a relatively small length of redundancy. In accordance with the present invention, there is provided a method of correcting errors in a digital VCR according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view illustrating a general reed solomon code with a two-dimensional structure; Fig. 2 is a block diagram of a conventional reed solomon coder with the two-dimensional structure; Fig. 3 is a flowchart illustrating a conventional method of correcting errors in the conventional reed solomon coder in Fig. 2; Fig. 4 is a flowchart illustrating a method of correcting errors in a reed solomon coder with a two-dimensional structure in accordance with the present invention; Fig. 5 is a view illustrating the first step of detecting errors in the method in Fig. 4; and Fig. 6 is a graph comparing the present invention with the prior art with respect to probabilities that no error is detected and data decoding fails. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. Fig. 4, there is shown a flowchart illustrating a method of correcting errors in a reed solomon coder with a two-dimensional structure in accordance with the present invention. As shown in this drawing, the error correcting method of the present invention comprises the first step (S1-S3) of detecting errors from vertical and horizontal sections of a code of data recorded on a tape and setting cross-points of the detected errors to flags, the second step (S4 and S5) of, through a horizontal decoder, detecting errors from the horizontal code sections of the data, correcting the detected errors and erasing only the errors positioned at the cross-points set to the flags at the first step, in ones of the horizontal code sections from which detectable but non-correctable errors are detected and the third step (S6) of correcting the errors erased at the second step in a vertical direction through a vertical decoder. The error correcting method which is performed in the above-mentioned manner in accordance with the present invention will hereinafter be described in detail with reference to Figs. 4 to 6. As shown in Fig. 4, at the step S1, errors are detected from the vertical code sections of the data recorded on the tape. Also, errors are detected from the horizontal code sections of the data recorded on the tape at the step S2. Then, at the step S3, cross-points of the errors detected at the steps S1 and S2 are set to the flags as shown in Fig. 5. Namely, when the number of the errors detected at the step S1 is smaller than or equal to n2-k2 in the code (n2,k2), the errors are detected from the vertical code sections. Also, when the number of the errors detected at the step S2 is smaller than or equal to n1-k1 in the code (n1,k1), the errors are detected from the horizontal code sections. Noticeably, the redundancy constituting a portion of the data code word may be used as a parity for the error detection and correction. Here, since the parities for the error detection and correction are all used only to detect the errors, the probability Pud of detecting the errors are high as shown in Fig. 6. On the other hand, at the step S4, errors are detected from the horizontal code sections of the data and then corrected. At the step S5, in ones of the horizontal code sections from which the detectable but non-correctable errors are detected, there are erased only the errors positioned at the cross-points set to the flags at the step S3. The errors erased at the step S5 are corrected in the vertical direction through the vertical decoder at the step S6. The errors which cannot be corrected at the step S6 are applied to an error concealment unit, which corrects the surrounding values into estimated values. As mentioned above, because only the errors of the horizontal code sections erased at the step S5 are corrected by the vertical decoder, the number of the errors to be corrected by the vertical decoder and an amount of the data to be decoded by the vertical decoder are small. Therefore, the probability that the data decoding fails is low. Also, since the errors to be corrected by the vertical decoder are small in number, the errors being applied to the error concealment unit are small in number. This results in improvement in the picture quality. Fig. 6 is a graph comparing the present invention with the prior art with respect to the probabilities Pud and Pdf that no error is detected and the data decoding fails. It can be seen from this drawing that the error correcting method of the present invention provides the probabilities Pud and Pdf lower than those in the prior art. As hereinbefore described, according to the present invention, the errors are detected from the vertical and horizontal code sections of the data played back from the tape before the data is decoded vertically and horizontally, the cross-points of the detected errors are set to the flags and only the errors positioned at the cross-points are corrected. This has the effect of preventing a loss in the information with no error and reducing the number of the errors to be corrected by the vertical decoder. Therefore, the probabilities that no error is detected and the data decoding fails become low.

Claims: A method of correcting errors in a digital VCR, comprising the steps of: (a) detecting errors from vertical and horizontal sections of a code of data played back from a tape and setting cross-points of the detected errors to flags (steps S1-S3); (b) detecting errors from the horizontal code sections of the played back data, correcting the detected errors from the vertical and horizontal code sections and erasing only the errors positioned at the cross-points set to the flags at the step (a), in ones of the horizontal code sections from which detectable but non-correctable errors are detected (steps S4-S5); and (c) correcting the errors erased at step (b) in a vertical direction (step S6). A method of correcting errors in a digital VCR, as set forth in Claim 1, wherein said step (b) includes the steps of: detecting the errors from the horizontal code sections of the played back data (S4); and erasing only the errors positioned at the cross-points set to the flags at said step (a), in ones of the horizontal code sections from which the errors are detected (S5).

Inventors: KIM SANG WU, KIM SEUNG HO, KIM, SANG WU, KIM, SEUNG HO
Applicants: LG ELECTRONICS INC, LG ELECTRONICS INC.
Application year: 1993
Main classification: H04N5
IPC codes: G11B20, H04N5