MXPA00005315A - A codeword for use in digital optical media and a method of generation thereof - Google Patents

Abstract

A codeword (102) for use in error correction of digital optical media, the codeword (102) having a plurality of data symbols and a plurality of parity symbols (PA1, PA2, PB3, PB4), and includes an augmented channel word (DAB) which can be read as either a first value (DA) or a second alternate value (DB). The augmented channel word is one of the plurality of data and parity symbols, wherein the augmented channel word retains its value irrespective of any error correction performed.

Description

A WORD CODE THAT IS USED IN DIGITAL OPTICAL MEDIA AND A METHOD FOR THE GENERATION OF THE SAME FIELD OF THE INVENTION The present invention relates to the authenticity verification of optical means, in general, and in particular with the generation of bistable symbols.

BACKGROUND OF THE INVENTION Digital data written on Compact Discs (in abbreviated form CD) and Digital Video Optical Discs (in abbreviated form DVD) pass a well-defined series of processing steps. , including, interleaving, Reed-Solomon coding and Eight-to-Fourteen Modulation Coding and the abbreviation EFM - in the case of CDs or eight-to-sixteen encoding (known as EFM-Plus) for DVDs. The international standards applicable to CDs are defined in a series of CD Colored Books (Red Book, Yellow Book and Green Paper, for example) and are also available as ISO / IEC standards. The DVD standard is defined by a series of books owned by the DEVD Consortium (now known as the DVD Forum) available through Toshiba Co. from Japan. In particular, reference should be made to the CD publication of ISO / IEC - Information technology Data exchange on optical data discs only read 120 mm (CD-ROM), ISO / IEC 10149, 2nd edition, 1995 and specifications DVD for Read Only Discs, Part 1, Physical Specifications, Version 1.0 of August 1996, respectively. Other data processing information on optical media can be found in Chapters 3 and 4 of The Compac Disc Handbook, second edition of Ken C. Pholmann, published by AR Editions Inc., Madison, Wisc, 1992 and in Chapters 5 and 9 of the book Principal or Digital Audio, 3rd edition of Ken C. Pohlman published by McGraw-Hill Inc., New York, 1995. In corresponding patent applications, specifically the US application No. 08 / 869,209 and U.S. Provisional Application No. 60 / 038,080, of the Applicant herein incorporated by reference, describe methods for producing and reading bistable (ambiguous) data on recordable and compressed optical media. It is difficult to faithfully reproduce the locations and values of the bistable data without accessing the specially modified recording or master recording equipment. Therefore, bistable data on a disk can not be easily reproduced on illegal copies of the disk. The software that verifies the presence of bistable data can determine if a particular disk is genuine (it has bistable data of the correct values in the correct locations of the disk) or if it is an illegal copy (there is no bistable data or the bistable data are in locations erroneous or have erroneous values). In addition, the pattern of bistable data locations and values can represent a key to be used in decrypting the content of a genuine disk. This key can not be extracted from a disk that is an illegal copy.

Definitions The following definitions are used throughout the application: "symbol" - a data byte or a word channel, depending on the context "data symbol" - 8 bit data that is represented on a disk as a word channel "value of data "- a data symbol value that is read from a disk (after demodulation and Reed-Solomon error detection)" bistable symbol "- a symbol that, if read more than once, returns more than a data value "word channel" - the 14-bit (EFM) or 16-bit (EFM Plus) binary representation of a data symbol "comparison symbols" - a pair of data symbols whose respective channel words differ only by a displacement of one only bit "1" for a bit position "main symbol" - the first accompanying symbol "alternative symbol" - the second accompanying symbol "increased symbol" - a channel word with a "1" that moves to the left in half position bit and corresponds to a data symbol and moves to the right in half bit position and corresponds to the accompanying symbol of the first representation of the symbol. Special hardware and software are required to produce an enhanced symbol. "displaced symbol" - a channel word that does not violate the run length rules but has no corresponding data symbol.

Essentially, a word channel comprises one of nine unused EFM symbols "replaced symbols" - a word channel that has been replaced by another word channel representing a different data symbol, as represented by the original word "erasure" - a potentially wrong symbol whose location is known as "error" - an error symbol whose location is not known. Reference will now be made to Figure 1 wherein a high level block diagram for the method of data detection and decoding is illustrated, as implemented for the storage of optical data on a CD-ROM (reference number 10 ). In the case of a CD (Figure 1), the data 12 being read is detected and demodulated (step 14) using EFM tables. The demodulated data 12a then passes through two stages of the Reed-Solomon error detection, known as decoding and correction Cl and C2 (steps 16 and 18, respectively). Reed-Solomon error detection can correct up to two errors. That is, in each error correction step the demodulated data 12a is corrected (with reference 12b and 12c). Finally, the CD-ROM reader firmware makes the corrected data 12c accessible to the computer program in the form of 12d data. Based on the number and type of errors, if any, that are reported by the C2 decoder and in compliance with possible Error Recovery Mode specified by the access software, the firmware of the controller unit (drive) may or may not really allow data to leave the controlling unit. The output during each data processing step (steps 14 to 20) includes data (12a, 12b, 12c and 12d) plus flags, referred to as 22a, 22b, 22c and 22d, indicating the status (e.g., error condition of the data 12. The method of data detection and decoding for DVD-ROM 30, illustrated in Figure 2, is similar to that described for a CD-ROM 10 (Figure 1). In summary, the data 32 is detected and demodulated (step 34) using EFM plus tables The demodulated data 32a then passes through two stages of Reed-Solomon error detection, known as decoding and correction Pl and PO (steps 36 and 38, respectively), before being read by the DVD reader firmware (step 40) allowing the 32d data to be accessible to the computer program. The object of the present invention and the aforementioned inventions is to write one or more symbols increased on a disk that will be read by common readers in the form of bistable symbols. This requires that each of the accompanying symbols derived from an augmented symbol pass intact through the Reed-Solomon error correction. Reference is now made to the Figure 3 schematically illustrates a single bistable symbol, designated DAB, and its associated code words Cl and C2, with references 42 and 44, which are shown horizontally and vertically, respectively. The bistable symbol DAB represents the data read from the augmented symbol previously written. The DAB bistable symbol can be read either as its two accompanying symbols, DA (main) and DB (substitute). Each Cl code word contains 28 data symbols and 4 parity symbols and each code word C2 contains 24 data symbols and 4 parity symbols. For clarity purposes, only the DAB bistable symbol and the relevant parity symbols are displayed. According to the Reed-Solomon transverse interleaving coding (CIRC) any given data symbol is contained exactly in a code word Cl and exactly in a code word C2. The code words Cl and C2 containing DAB (hereinafter referred to as primary codewords) 42 and 44 respectively, are intercepted in the DAB bistable symbol. For the purposes of the example, four parity symbols, with references PA?, PA, PA3 and PA of the primary code word Cl 42 and four parity symbols referred to as QAI # QA2, QA3 and QA4, of the primary code word C2, 44 are erased as shown. These erasures are effectively generated by shifting the EFM symbols from the respective parity symbols. The four parity symbols (QA ?, QA2 QA3 and QA4) of the code word C2, 44 are also contained in four additional code words Cl, designated 45, 46, 47 and 48, which are referred to as secondary code words C2. In order to prevent the primary code word C2 parity symbols, deleted (Qñ ?, QA2 QA3 and QA) from being corrected by their accompanying secondary code words Cl (45, 46, 47 and 48), the four parity symbols, in general designated PAH-PAI, PA2I-PA2, PAS-PA34, and PA-PA 4, of each of the four secondary codewords Cl, are also erased by the aforementioned displacement method. In this way, there are two primary code words 40 and 42 respectively, associated with the bistable data symbol DAB, which has erased parity symbols, and four secondary codewords Cl (45, 46, 47 and 48) which also has symbols of parity deleted. As described above, when a CD-ROM disc is read, for example, the data is detected by an optical detector, the Cl code words are demodulated from 14 bits to 8 bits (step 14) and assembled and decoded (step 16). ), the code words C2 are assembled and decoded (step 18), the sector data is assembled (step 20) and the data 12d is made available to the outside world. Errors detected at any stage are fixed, if possible. If the error is not fixed, the data in error is marked with a flag (22a-22d) and passed to the next step. Reference is now made to Figure 4 which schematically illustrates the processing steps that are carried out when a sector containing a bistable symbol is read, and which can be summarized as follows: a) the optical detector generates one of the accompanying symbols from the word increased channel (step 60). b) The EFM demodulator decodes the value of the corresponding 8-bit symbol (step 62). An error is not generated since the two bistable symbol values are legitimate. c) The demodulator tries to decode the displaced EFM symbols, already mentioned (step 64). An error flag is generated for each non-decodable symbol (erasure) and passed over the decoder Cl (step 66). d) Since the primary code word Cl has four erasures, the decoder Cl can not correct any of the various symbols in the code word and, in particular, will not alter the value of the bistable symbol. The bistable symbol can, therefore, pass through the decoder Cl, intact. The decoder Cl can flag the components of the code word Cl before passing them to the decoder C2 (step 68). e) As the secondary code words Cl also have four erasures, the decoder Cl can not correct the erasures in the parity symbols Q of the primary codewords C2. The decoder Cl can flag the components of the secondary code words Cl before passing them to the decoder C2 (step 70). f) The word primary code C2 has four uncorrected erasures plus a bistable symbol with a flag. Therefore, the decoder C2 can not correct the bistable symbol. Therefore, the decoder C2 can put a flag in the code word C2, to mark that it is non-correctable (step 72). g) the firmware of the controller unit reads flags C2 for a sector (step 74). If there are non-correctable C2 errors (known as E32 errors), the firmware of the controller unit may refuse to pass the sector data to the guest software. In this case, it is impossible to detect bistable symbols. Even if the data is read, different controller units behave differently if E32 errors are generated. For example, many controller units slow down when trying to correct errors and others return to corrupted data. In summary, the firmware behavior of the controller unit in response to E32 errors is usually unpredictable.

SUMMARY OF THE INVENTION An objective of this invention is to provide an improved method for generating ambiguities that overcome the limitations and disadvantages of existing methods. A further object of this invention is to write one or more augmented symbols on a disk that will be read by common readers of digital optical media, as if they were bistable symbols. This requires that each of the accompanying symbols derived from an augmented symbol pass intact through the Reed-Solomon error correction. Therefore, a further objective of this invention is to provide a code word containing a bistable symbol that remains intact even if the error correction is carried out. Therefore, according to a preferred embodiment of this invention, a code word is provided which is used in error correction of digital optical media. The code word, which has a plurality of data and parity symbols, includes an increased channel word that can be read either as a first value or as a second alternative value, the word channel increased is one of the plurality of data and symbols of parity. The word increased channel retains its value regardless of any error correction that is made. Furthermore, according to a preferred embodiment of this invention, the plurality of parity symbols includes at least one parity symbol generated for the first value of the increased channel word and at least one parity symbol generated for the second alternative value. of the word channel increased. The error correction is a Reed-Solomon error correction. In addition, according to a preferred embodiment of the present invention, the digital optical media is any type of optical medium, including Compact Discs (CD), Compact Disc Read Only Memory (CD-ROM) and Discs of Digital Video (DVD). Further, according to a preferred embodiment of the present invention, the code word may be either a codeword Cl and / or a codeword C2 if the optical medium is a CD-ROM. Alternatively, if the medium is a DVD ROM, the code word can be either Pl and / or PO. Additionally, in accordance with a preferred embodiment of this invention, a method for generating a code word that is used in error correction of digital optical media is provided, the code word has a plurality of data and parity symbols. The method includes the steps of: a) generating an increased channel word from two companion bytes having a first value and a second alternative value; and b) writing the word "augmented channel" in the digital optical medium in the form of a plurality of data and parity symbols of the code word. In addition, according to a preferred embodiment of this invention, the word "augmented channel" retains its value regardless of whether an error correction is made. The word augmented channel includes a word channel that has a bit of channel '1' that when moving to the left in bit position corresponds to a data symbol that has the first value and that when moving to the right in% bit position corresponds to a data symbol that has the second alternative value. Further, a method is provided in accordance with a preferred embodiment of this invention to ensure that a bistable data symbol, which can be read either as a first value or as a second alternative value, is not affected by the correction of error of digital optical media. In the case of digital optical media that includes Compact Discs (CD) and Compact Disc Read Only Memory (CD-ROM) the method includes the steps of: a) generating a composite primary word C2 that contains the bistable data symbol; b) generate a composite primary code word Cl containing the bistable data symbol; c) calculate secondary code words Cl for the bistable data symbol from the compound primary word C2; d) writing the bistable data symbol as an augmented symbol, within the primary composite code words Cl and C2, in the digital optical medium; and e) intercalating and writing the primary composite codewords Cl and C2, excluding the bistable data symbol, in the digital optical medium. Furthermore, according to a preferred embodiment of the present invention, the step of generating a composite code word C2 includes the steps of: a) generating a first code word C2 from the first value of the bistable symbol; b) generating a second code word C2 from the alternative value of the bistable symbol; and c) merge the first and second code words C2. Furthermore, according to a preferred embodiment of this invention, the step of merging the first and second code words C2 includes the steps of: a) calculating the main parity symbols of the main code word C2 for the first value of the bistable symbol, the code word C2 has a plurality of parity and data values; b) calculating the alternative parity symbols of the main code word C2 for the second alternative value of the bistable symbol; and c) replacing at least one of the principal parity symbols with one of the alternative parity symbols. Further, in accordance with a preferred embodiment of the present invention, the step of generating a composite code word Cl includes the steps of: a) generating a first code word Cl from the first value of the bistable symbol; b) generating a second code word Cl from the alternative value of the bistable symbol; and c) merge the first and second code words Cl. The step of merging the first and second code words Cl includes the steps of: a) calculating the major parity symbols of the main code word Cl for the first value of the bistable symbol, the main code word Cl having a plurality of parity values and data; b) calculating the alternative parity symbols of the main code word Cl for the second alternative value of the bistable symbol; and c) replacing at least one of the principal parity symbols with one of the alternative parity symbols. The distance between the first and second valid codewords C2 and the first and second codewords Cl is equal to the number of parity symbols plus one. Furthermore, according to a preferred embodiment of this invention, there is provided a method for using Digital Video Discs (DVDs) to ensure that the bistable data symbols, which can be read either as a first value or as a second alternative value, are not affected by the error correction. The method includes the steps of: a) generating a composite primary word code PO that contains the bistable data symbols; b) generate a composite primary code word Pl that contains the bistable data symbols; c) calculating the secondary code words Pl for the bistable data symbols from the compound primary code word PO; d) writing the bistable data symbol as an augmented symbol within the primary composite code words Pl and PO in the digital optical medium; and e) intercalating and writing the primary composite code words Pl and PO, excluding bistable data symbols, in the digital optical medium.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully understood and appreciated from the following detailed description taken in conjunction with the accompanying drawings, wherein: Figure 1 is a high-level block diagram illustrating the detection method of data and decoding, according to its implementation for the storage of optical data in CD ROM; Figure 2 is an illustration of a high-level block diagram for the data detection and decoding method as implemented for the storage of optical data for a DVD-ROM; Figure 3 schematically illustrates a single unresolved bistable symbol for the CD-ROM and its associated code words Cl and C2; Figure 4 is a high-level block diagram illustration of the processing steps performed when a sector containing a bistable symbol is read; Figure 5 is a schematic illustration of a single bistable CD-ROM data symbol and its associated code words Cl and C2, in accordance with a preferred embodiment of the present invention; Figure 6 is a flow diagram illustration of the generation of the bistable symbol of Figure 5, according to a preferred embodiment of the present invention; Figures 7A-7C are a schematic illustration of the main and alternative code words C2, for the bistable symbol of Figure 5; Figures 8A-8B and 9A-9B illustrate the results of the error correction performed on the main and alternative values, respectively, of the bistable symbol of Figure 5; Figures 10A-10C are schematic illustrations of the main and alternative code words Cl for the bistable symbol of Figure 5; Figures 11A-11B and 12A-12B illustrate the results of the error correction performed for the main and alternative values, respectively, of the bistable symbol of Figure 5; Figure 13 is a schematic illustration of a single bistable parity symbol generating a bistable data symbol according to a preferred embodiment of the present invention; and Figure 14 is a schematic illustration of a single bistable data symbol DVD-ROM and its associated code words Pl and PO, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION As already described, by overloading the primary code words Cl and C2 with erasures, the error correction of the code words can be prevented, thus allowing a bistable symbol to pass through the correction of Cl and C2 error, intact. However, a consequence of this method is the possible production of E32 errors. The Applicant has realized that by selectively modifying the parity symbols (or data) within the main code word Cl for a bistable symbol, the value of the bistable symbol may remain intact, even if an error correction is made. Reference will now be made to Figure 5 where a single bistable data symbol, designated DAB, and its associated code words Cl and C2 are schematically illustrated, with references 102 and 104, respectively, in accordance with a preferred embodiment of the present invention. . The bistable symbol DAB is similar to the bistable symbol DAB described above in relation to Figure 3 and has two possible values; DA (main) and DB (alternative). Each Cl code word contains 28 data symbols and 4 parity symbols and each code word C2 contains 24 data symbols and 4 parity symbols. To give more clarity only the symbols that relate to this description are those that are shown. According to the Reed-Solomon Transversal Interleaved Coding (CIRC), the code words Cl and C2 containing DAB, referenced 102 and 104 respectively, intersect in the DAB flip-flop symbol. The word primary code Cl, 102, contains four parity symbols, with reference identifications PAi, PA2, PB3 and PB, and the primary code word C2, 104, contains four parity symbols, with reference. Reference is now made to the Figures 6 to 12 illustrate the generation of DAB bistable symbols and the use of composite code words C2, AB, to allow the DAB flip-flop to pass intact, despite some error correction Cl and C2 being carried out , as will be described below in greater detail. Figure 6 is an illustration of a flow chart of the generation of the DAB flip-flop symbol according to a preferred embodiment of the present invention. Figures 7A-7C illustrate the main and alternative code words C2, referred to as A and B, respectively, and the merged code word C2 which is generally denoted AB, formed by the combination of the main code words C2 and alternative, that is, A and WB, respectively. Figures 8A and 8B as well as 9A and 9B illustrate the results of the error correction performed on the main and alternative values of the DAB bistable symbol, respectively. Similarly, Figures 10A to 10C illustrate the primary and alternate Cl code words, which bear the reference VA and VB, respectively, and the merged code word Cl, which is generally designated as VAB (formed by combining the main code words and alternative C2, ie VA and VB, respectively). Figures HA and 11B as well as Figures 12A and 12B illustrate the results of the error correction performed on the main and alternative values of the DAB bistable symbol, respectively. With particular reference to Figures 6 and 7A to 7C, the code word C2, WA, which includes the data symbol DA, is generated in (step 110). A is a valid code word C2 comprising 24 data symbols (only DA is shown for clarity) and four parity symbols QAi-QA. The symbol DB is replaced by DA (step 112) (the remaining data symbols 23 remain unchanged) and the parity symbols (QB? -QB4) are recalculated (step 114). The resulting code word B is also valid. Due to the nature of Reed-Solomon coding rules, the minimum distance between valid code words A and WB is five, that is, the code words differ in five positions. In general, the distance between valid code words is equal to the number of parity symbols plus one. Therefore, the distance between the code words Cl and C2, valid, for a CD-ROM, each of which contains four parity symbols, is equal to five. The distance between the valid code words PI and PO for a DVD, which contains 10 to 16 parity symbols, respectively, are respectively 11 and 17. The composite code word C2, AB is generated (step 116) by replacing two of the symbols of parity (say QA3 • and QA4) calculated for the symbol A, the parity symbols QB3 and QB4 are calculated for the symbol B. The component symbols of the composite code word C2 AB, except for DAB, are interleaved and written to the disk, in the typical manner (step 118). DB is written as an augmented symbol, in the way already described above. AB is an invalid code word that contains a DAB bistable symbol • When the disk is read, either DA or DB will be returned. The C2 decoder will try to correct the errors. As shown in Figures 8A-8B, if the main DA value is returned (Figure 8A) and as only two errors (QB3 and QB4) / the decoder will correct them to QA3 and QA # in order to return the code word A ( Figure 8B) from the code word AB • As the error is correctable, the C2 decoder does not report any uncorrectable error. Similarly, as shown in the Figures 9A-9B, if the alternative DB value is returned (Figure 9A) the decoder will correct two errors, QA? and QA2 / to give QB1 and QB2 / in order to return the code word B (Figure 9B) without reporting any errors. With reference to Figures 10A-10C, the composite word Cl VAB, which also includes the data symbol DA, is generated (steps 120-128 of Figure 6) in a manner similar to the generation of the composite code word C2, WAB , which was described above with reference to Figures 6A-6C. That is, steps 110-118 are repeated for the code words Cl, VA and VB. VA is a valid code word Cl comprising 28 data symbols (only DA is displayed) and four PA parity symbols? - PA4 • The symbol DB is replaced by DA (step 122) (the remaining data symbols 27 remain unchanged) and the parity symbols (PBI-PB4) are recalculated (step 124). The resulting code word VB is also valid according to the Reed-Solomon coding rules, and differs from VA in five parts. The composite code word Cl, VAB is formed at this time (step 126) by replacing two of the parity symbols (say PA3 and A4) calculated for the VA symbol, with the parity symbols PB3 and PB4 calculated for the symbol VB. The components of the composite code word Cl, VAB / except for DAB / are interleaved and written to the disk in the typical manner (step 128). DAB is written as an augmented symbol as previously described. VAB is an invalid code word that contains a DAB bistable symbol. When the disk is read, either DA or DB is returned. The Cl decoder will try to correct the errors. As shown in Figures HA and 11B, if the main DA value is returned (Figure HA) and since there are only two errors (PB3 and PB4), the decoder will correct them in PA3 and PA4, reconstructing the VA code word (Figure 11B ) from the word code VAB. Since the error is correctable, the Cl decoder does not report any uncorrectable errors.

Similarly, as shown in Figures 12A-12B, if the alternative value DB is returned (Figure 12A) the decoder will correct the two errors (PA? And PA2) in PB? and PB2 in order to return the code word VB (Figure 12B) without any error being reported. The results of the modifications in the code words Cl and C2 can be visualized more easily in relation to Figure 5, which shows the bistable byte and its associated code words Cl and C2. The code word C2 104 is displayed in the vertical direction and the code word Cl 102 is shown horizontally. The code words Cl and C2, which contain the flip-flop symbol DAB are the primary codewords Cl and C2, respectively. If the raised symbol DAB is read as DA, the decoder Cl will correct the two error symbols PB3 and PB4, and the decoder C2 will correct the two error symbols QB3 and QB4 • If the increased symbol DAB is read as DB, the decoder C1 will correct the two error symbols PA1 and PA2 and the decoder C2 will correct the two error symbols QAI and QA2 • In either case, any of the accompanying bytes DA and DB can be read without causing non-correctable C2 code words. If the encoder Cl is unable to correct the two erroneous parity symbols (for example, it is known that for performance reasons some chip manufacturers implement suboptimal C2 encoders), the C2 decoder can still correct its own two errors. In order to avoid the Reed-Solomon correction of QAI, QA2 / QB3 / and QB4 in the parity bytes, the parity bytes of the secondary code words Cl, with reference 105 to 108, ie the Cl code words containing one of QAI_QA2 / QB3-QB4 / are adjusted according to the values of QA ?, QA2, QB3 and QB4 respectively (step 132). Therefore, the secondary codewords Cl, 105 and 106, contain parity bytes PAII-PAI4 and PA2I-PA24 respectively, while the secondary codewords Cl 107 and 108 contain parity bytes PB3I-B34 and PB4I_PB44; respectively. An alternative implementation of this invention is illustrated in Figure 13 to which reference will be made below. Figure 13 schematically illustrates a single bistable parity symbol, designated PAB3 with reference 200, which generates a bistable data symbol DB, generally designated 202 according to a preferred embodiment of this invention. The primary code words Cl and C2 associated with the bistable data symbols DB have the references 204 and 206, respectively. As shown, the bistable symbol PAB3 is one of the 204 parity symbols of the primary code word Cl. The primary code word Cl has two parity symbols calculated according to the data symbol DA (PAI and PA2) # a symbol of parity calculated according to data symbol DB (PB4) and a bistable parity symbol (PAB3) that can be read either as PA3 or PB3. The data symbols actually written on the disk are DB. The primary code word C2, 206 is similar to the primary code word C2, 104 which was described above in relation to the mode of Figure 5. In addition, the secondary code words Cl, referenced 105-108, are the same as the which were described above in relation to the embodiment of Figure 5 and are therefore designated with a similar reference. In operation, when the primary code word Cl, 202, is read, if the symbol of a third party is read as PA3 then there are two erroneous symbols in the code word: DB and PB4. These symbols are corrected by Reed-Solomon to the DA and PA4 values. • If the third parity symbol is read as PB3, then there are two erroneous symbols in the code word, ie PA? and PA2 • These are corrected by Reed-Solomon to values PB1 and PB2. In this case, the data symbol remains as DB. Therefore, the data symbol 202 is bistable, capable of being read either as DA or as DB.

It will be appreciated that the present invention is not limited to CD-ROM discs, but is also applicable to digital video discs (DVD). DVD discs are characterized by a modified form of error correction. On DVDs, the sectors are arranged in ECC blocks, where each contains 16 data sectors and the parity Pl and PO is analogous to parity Cl and C2. The code words PI have 172 data symbols and 10 parity symbols (allowing up to five (5) errors to be corrected), while the PO code words have 192 data symbols and 16 parity symbols (allowing for correction to eight (8) errors). The data is written to the disc in EFM Plus (modulation from eight to sixteen (8 to 16)) as described in the DVD documentation. As already described above in relation to Figure 2, the error correction is performed on DVD by first detecting the data and carrying out the EFM Plus demodulation (step 34). The decoding Pl followed by the decoding PO and the correction (respectively steps 36 and 38) are performed. Flags can be produced at any stage if errors are present. Referring now to Figure 14, which is a schematic illustration of a bistable simple data symbol, DVD-ROM, DAB, designated with the number 300 and its associated primary words PI and PO, designated 302 and 304, respectively, according to to a preferred embodiment of the present invention. The secondary code words PO, with references 305 to 310, are also illustrated. In a manner similar to the method described above with respect to the CD-ROM mode (Figures 5 to 12), the flip-flop symbol DAB, 300, is generated by selecting two accompanying symbols represented by an increased channel word EFM Plus. In this way, when the word increased channel EFM Plus is read from the disk, it will be decoded in one of its two accompanying symbols, be it the first (main) symbol DA or the second symbol (alternative) DB. If the sector containing the augmented symbol is read several times, then either will be detected between the main symbol or the alternative symbol. In contrast to a CD-ROM, for which there is a one-to-one relationship between a symbol and its word EFM channel, the word channel depends on the state of the modulator, due to a byte encoded on DVD. According to the "Main Conversion Table and Substitution Table" as described in the DVD Specifications series, there are four (4) possible states of the modulator. In order to ensure correct coding, the EFM Plus channel words representing each of the accompanying bytes must be in the same state and must have the same text status, as provided in the DVD Tables. The data is written on a DVD as synchronization frames, comprising a synchronization code followed by 91 EFM-Plus coded symbols. According to the specification, the state of the modulator for a code word after a synchronization code is in State 1. Therefore, it is possible to select companion bytes whose coded EFM Plus channel words, main and alternative, are in the State 1 if the bistable symbol is selected as the symbol that continues to a synchronization code. Once the accompanying data bytes are selected as above, the parity symbols of the corresponding primary code words, Pl and PO, are calculated. As shown in Figure 14, half (5) of the parity symbols Pl are calculated according to the bistable value DA, and half (5) of the parity symbols Pl are calculated according to the bistable value DB. Similarly, half (8) of the parity symbols PO are calculated according to the bistable value DA and half (8) of the parity symbols PO are calculated according to the bistable value DB. When an example of the bistable symbol is read, the Pl decoder will correct the 5 wrong parity symbols, allowing the bistable symbol to pass intact to the PO decoder. Similarly, the PO decoder will correct the 8 erroneous parity symbols, leaving the bistable symbol intact. Therefore, when the data sectors are reassembled from the ECC block, the bistable symbol will be read without having gone through any correction. It will be appreciated by those skilled in the art that the invention is not limited to what has been shown and described here in particular. On the contrary, the scope of the invention will be defined by the following claims:

Claims (27)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property; 1. A code word to be used in error correction of digital optical media, the code word has a plurality of data symbols and a plurality of parity symbols, the code word comprises an augmented channel word which can be read either as a first value or as a second alternative value; the word "augmented channel" is a symbol of the plurality of parity symbols and data symbols; where the word augmented channel retains its value regardless of whether an error correction has been made. A code word according to claim 1, wherein the plurality of parity symbols comprises: at least one parity symbol generated for the first value of the increased channel word; and at least one parity symbol generated for the second alternative value of the word channel increased. 3. A code word according to claim 1, wherein the error correction is a Reed-Solomon error correction. 4. A code word according to any one of claims 1 to 3, wherein the digital optical means are any type of optical means, among which the following are included: Compact Disc (CD), Compact Disc Read Only Memory (CD-ROM) and Digital Video Discs (DVD). 5. A code word according to any of claims 1 to 3, wherein the code word is a code word Cl and / or a code word C2. 6. A code word according to any of claims 1 to 3, wherein the code word is a code word Pl and / or a code word PO. 7. A method for generating a code word to be used in error correction of digital optical media, the code word has a plurality of data symbols and parity symbols, the method consists of the steps of: generating an increased channel word from two companion bytes that have a first value and a second alternative value; and writing the word channel increased in the digital optical medium, in the form of a plurality of data symbols and parity symbols of the code word. A method according to claim 7 and wherein the word "augmented channel" retains its value regardless of whether an error correction has been made. A method according to any of claims 7 to 8, wherein the word "augmented channel" comprises a channel word having a channel bit '1' which when shifted to the left in% bit position corresponds to a data symbol which has the first value and when it moves to the right in% bit position it corresponds to a data symbol that has the first value and when it moves to the right in% bit position it corresponds to a data symbol that has a second value alternative. A method according to any of claims 7 to 9, wherein the digital optical means corresponds to a type of optical medium included in the following list: Compact Disc (CD), Compact Disc Read Only Memory (CD-ROM) and Digital Video Discs (DVD). 11. A method according to any of claims 7 to 10, wherein the error correction is a Reed-Solomon error correction. 12. A method to ensure that a bistable data symbol, which can be read either as a first value or as a second alternative value, is not affected by the error correction of the digital optical means, the method comprises the steps of: generating a composite primary word C2 that contains the bistable data symbol; generate a primary composite code word Cl containing the bistable data symbol; calculating the secondary code words Cl for the bistable data symbols from the composite primary word C2; write the bistable data symbol as an augmented symbol within the primary composite code words Cl and C2, in the digital optical environment; and interleaving and writing the primary composite code words Cl and C2, excluding the bistable data symbol, in the digital optical medium. A method according to claim 12, wherein the step of generating a composite code word C2 comprises the steps of: generating a first code word C2 from the first value of the bistable symbol; generating a second code word C2 from the first alternative value of the bistable symbol; and merge the first and second words code C2. A method according to claim 12, wherein the distance between the first and second code words C2 is equal to the number of parity symbols plus one. 15. A method according to claim 12, wherein the step of merging comprises the steps of: calculating the main parity symbols of the main code word C2 for the first value of the bistable symbol; the code word C2 has a plurality of parity and data values; calculating the alternative parity symbols of the main code word C2 for the second alternative value of the bistable symbol; and replace at least one of the major parity symbols with one of the alternative parity symbols. 16. A method according to claim 12, wherein the step of generating a composite code word Cl comprises the steps of: generating a first code word Cl from the first value of the bistable symbol; generating a second codeword Cl from the first alternative value of the bistable symbol; and merge the first and second Cl code words. A method according to claim 16, wherein the step of merging comprises the steps of: calculating the main parity symbols of the main code word Cl for the first value of the bistable symbol, the main code word Cl has a plurality of values of parity and data; calculating the alternative parity symbols of the main code word Cl for the second alternative value of the bistable symbol; and replace at least one of the main parity symbols with one of the alternative parity symbols. 18. A method according to claim 16, wherein the distance between the first and second code words Cl is equal to the number of parity symbols plus one. 19. A method according to any of claims 12 to 18, wherein the digital optical means correspond to a type of optical media included in the following list: Compact Disc (CD) and Compact Disc Read Only Memory (CD-ROM) . 20. A method to ensure that a bistable data symbol, which has been read either as a first value or as a second alternative value, is not affected by the error correction of Digital Video Discs (DVD), the method it comprises the steps of: generating a compound primary code word PO containing the bistable data symbol; generate a composite primary code word Pl that contains the bistable data symbol; calculate the secondary codewords Pl for the bistable data symbol from the word compound primary code PO; write the bistable data symbol as an augmented symbol within the primary composite code words Pl and PO, in the digital optical medium; and interleaving and writing the primary composite code words Pl and PO, excluding the bistable data symbol, in the digital optical medium. 21. A method according to claim 20, wherein the step of generating a composite code word PO comprises the steps of: generating a first word code PO from the first value of the bistable symbol; generating a second word code PO from the alternative value of the bistable symbol; and merge the first and second words PO code. 22. A method according to claim 21, wherein the distance between the first and second code words PO is equal to the number of parity symbols plus one. A method according to claim 21, wherein the step of merging comprises the steps of: calculating the main parity symbols of the main code word PO for the first value of the bistable symbol, the code word PO has a plurality of values of data and parity values; calculating the alternative parity symbols of the main code word PO for the second alternative value of the bistable symbol; and replace at least one of the major parity symbols with one of the alternative parity symbols. 24. A method according to claim 20, wherein the step of generating a composite code word Pl comprises the steps of: generating a first code word Pl from the first value of the bistable symbol; generating a second code word Pl from the alternative value of the bistable symbol; and merge the first and second words code Pl. 25. A method according to claim 24, wherein the step of merging comprises the steps of: calculating the main parity symbols of the main code word Pl for the first value of the bistable symbol, the main code word Pl has a plurality of values of parity and data; calculating the alternative parity symbols of the main code word Pl for the second alternative value of the bistable symbol; and replace at least one of the major parity symbols with one of the alternative parity symbols. 26. A method according to claim 24, wherein the distance between the first and second code words Pl is equal to the number of parity symbols plus one. 27. A method according to any of claims 20 to 26, wherein the error correction is a Reed-Solomon error correction.