Copy protection of optical discs
View Patent ↗Subversive DSV (SDSV) sequences of data symbols having a large absolute value of DSV are extremely valuable in the copy protection of optical discs as they can induce uncorrectable read errors. However, very few SDSV sequences of data symbols can be found in multimodal codes such as Eight-to-Sixteen Modulation (ESM) utilised in DVDs. It is required to select data symbols, for encoding using a multimodal code, which are capable of forcing an encoder to produce at least one subversive sequence of code words. A possible code word for a data symbol is selected if the code word has a large absolute value of DSV and there are no alternative code words, or all alternative code words are equivalent, or all alternatives except one are ruled out by RLL rules.
1. A computer implemented method of selecting data symbols for incorporation within user data to be encoded by an encoder using a multimodal code employing one or more multimodal conversion tables stored in a memory of a computer so as to force the encoder to produce at least one subversive sequence of code words, the method comprising:
(a) selecting a first code word from the one or more multimodal conversion tables by accessing the one or more multimodal conversion tables in the memory, which has a digital sum value (DSV) whose absolute value is large enough to cause data reading errors;
(b) determining a first data symbol and first state which when encoded may result in the first code word according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory;
(c) determining a second data symbol and second state compatible with the first state according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory, determining a second code word which may result by encoding the second data symbol and second state according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory, and pre-pending the second code word to the first code word to form a sequence of code words;
(d) pre-pending the second data symbol to the first data symbol to form a sequence of data symbols when an absolute DSV of the sequence of code words is large enough to cause reading errors and also when one of the following conditions applies according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory: there is no alternative first code word which may result by encoding the first data symbol and first state, there is an alternative first code word which may result by encoding the first data symbol and first state but the first code word and a first prior state associated with the first code word are equivalent to the alternative first code word and a first alternative prior state associated with the alternative first code word, or the second code word pre-pended to the alternative first code word is ruled out by run length limiting (RLL) rules; otherwise, removing the second code word from the sequence of code words, determining an alternative first data symbol and first state which when encoded may result in the first code word, and repeating (c) and (d) for the alternative first data symbol.
2. The method according to claim 1 , wherein the first code word and a first prior state associated with the first code word are determined to be equivalent to the alternative first code word and a first alternative prior state associated with the alternative first code word by the following conditions being met: a product of a DSV for the first code word and a DSV for the alternative first code word is greater than or equal to zero; transitions of the first code word and transitions of the alternative first code word have a same parity; the first prior state is equal to the first alternative prior state; and the absolute DSV of the first code word is within a selected value of the absolute DSV of the alternative first code word.
3. The method according to claim 1 , further comprising:
(e) determining whether the sequence of data symbols is a subversive sequence when one of the following conditions is met according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory: encoding of the first data symbol and first state may not result in an alternative second code word and alternative second state; or when encoding of the first data symbol and first state may result in an alternative second code word and alternative second state but the second code word and first state are equivalent to the alternative second code word and an alternative first state associated with the alternative second code word; otherwise, removing the second code word from the sequence of code words, determining an alternative first data symbol and first state which when encoded may result in the first code word, and repeating (c) and (d) for the alternative first data symbol.
4. The method according to claim 3 , wherein the sequence of data symbols is determined to be a subversive sequence by looking at the corresponding sequence of code words and determining when at least the following conditions are met according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory: a number of transitions of the sequence of code words is even; and a next state for the sequence of data symbols is the first state.
5. The method according to claim 3 , further comprising:
(f) adding the sequence of data symbols to a list of subversive sequence data patterns when the sequence of data symbols is determined to be a subversive sequence; otherwise, determining a third data symbol and third state compatible with the second state according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory and processing the third data symbol and third state in the same fashion as the second data symbol and second state unless the sequence of data symbols has reached a maximum limit, in which case the method stops processing for the first code word.
6. A method of selecting data symbols for incorporation within user data to be encoded by an encoder using a multimodal code employing one or more multimodal conversion tables stored in a memory of a computer so as to force the encoder to produce at least one subversive sequence of code words, the method comprising:
(a) selecting a first code word from the one or more multimodal conversion tables by accessing the one or more multimodal conversion tables in the memory, which has a digital sum value (DSV) whose absolute value is large enough to cause data reading errors;
(b) determining a first data symbol and first state which when encoded may result in the first code word according to the one or more multimodal conversion tables as determined by accessing the one or more multimodal conversion tables in the memory;
(c) systematically proceeding backward through the one or more multimodal conversion tables starting with the first data symbol and first state to generate a sequence of data symbols which when encoded forces the encoder to produce the at least one subversive sequence of code words; and
(d) repeating (a) through (c) to generate a list of subversive digital sum value (SDSV) sequences.