A digital data storage (DDS) system for reading DDS tapes employs a partial response maximum likelihood detection system which utilises redundancy in the 8-10 DC free modulation encoding to reduce low frequency noise. The system incorporates a time-varying trellis decoder which embodies some of the
A digital data storage (DDS) system for reading DDS tapes employs a partial response maximum likelihood detection system which utilises redundancy in the 8-10 DC free modulation encoding to reduce low frequency noise. The system incorporates a time-varying trellis decoder which embodies some of the PR1 rules together with the rules regarding the charge state or the digital sum variation (DSV) implicit in 8-10 modulation coding. The decoder operates to reject low frequency noise such as that caused by crosstalk noise between adjacent tracks on the tape. The trellis topography has been considerably simplified by adopting a two step six state trellis which operates on bit pairs and in which the states relate to the current DSV value, and sign of the previous bit.
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1. A method of retrieving data from a partial response channel having implicit rules, said data having been read from a helical scan magnetic tape storage medium that stores said data in tracks having alternate azimuth magnetizations, the data read from the tracks having alternate azimuth magnetizat
1. A method of retrieving data from a partial response channel having implicit rules, said data having been read from a helical scan magnetic tape storage medium that stores said data in tracks having alternate azimuth magnetizations, the data read from the tracks having alternate azimuth magnetizations having a predominate crosstalk noise component at a relatively low frequency compared to the frequency of the read data, said stored and read data comprising a series of codewords encoded from input data using a DC-free code, each of said codewords having a digital sum variation (DSV) bounded within a first preset range at the codeword boundary, said method comprising:equalizing said data to a partial response target having a substantial DC response; andreducing by a substantial amount the predominate low frequency noise component from the equalized data by applying a set of equalized data to a time-varying trellis decoder that rejects candidate paths that violate (a) the DSV bounds and (b) the partial response rules.2. The method of claim 1, further including determining the most likely sequence of data in the equalized data with the reduced low frequency noise component by responding to an output of the decoder, the decoder output including the equalized data with The reduced low frequency noise component.3. The method of claim 1, further including reading said data from said tape storage medium by using a read head that scans the tape helically and reads said data from pairs of tracks having alternate azimuth magnetizations.4. A method according to claim 1, wherein said code is an 8-10 DC free modulation code.5. A method according to claim 1, wherein said code is an 8-10 DC free modulation code as set out in standard ECMA 236.6. A method according to claim 1, wherein said partial response target has a polynomial function with a (1+D) factor.7. A method according to claim 6, wherein said polynomial function does not have a (1-D) factor.8. A method according to claim 1, wherein said partial response target is a partial response class 1 (PR1) target whose transfer function is of the form (1+D).9. A method according to claim 1, wherein said partial response target is a partial response class 2 (PR2) target whose transfer function is of the form (1+D2).10. A method according to claim 1, wherein said code words are encoded according to a single rate code.11. A method according to claim 1, wherein the DSV is constrained to be either +1 or ?1 at the end of each of said code words.12. A method according to claim 1, wherein said trellis decoder is a two step decoder operating on bit pairs.13. A method according to claim 1, wherein the nodes of the trellis decoder represent the DSV, and the sign of the previous bit.14. A method according to claim 1, wherein said DC free modulation code is that specified in ECMA 236, and said trellis decoder has four states at the codeword boundaries and six states for each bit pair intermediate the codeword boundaries.15. A method according to claim 1, including a path memory for storing candidate paths of data, wherein, at the completion of each codeword, the respective candidate paths are stored in the path memory and assigned a respective address, and the respective address is used during decoding intermediate the codeword boundaries, and the path memory is updated at the completion of each new codeword.16. Apparatus for retrieving data from a partial response channel having implicit rules, said data having been read from a helical scan magnetic tape storage medium that stores said data in tracks having alternate azimuth magnetizations, the data read from the tracks having alternate azimuth magnetizations having a predominate crosstalk noise component at a relatively low frequency compared to the frequency of the read data, said stored and read data comprising a series of codewords encoded from input data using a DC-free code, each of said codewords having a digital sum variation (DSV) bounded within a first preset range at the codeword boundary, the apparatus comprising:an equalizer for equalizing said data to a partial response target having a substantial DC response;a time-varying trellis decoder for receiving a set of equalized data and for rejecting candidate paths which violate (a) the DSV bounds and (b) the partial response rules, thereby substantially reducing a low frequency noise component in said data; anda path length memory for receiving the data with the reduced low frequency noise component, as derived by the decoder, for outputting the most likely data sequence.17. The apparatus of claim 16, further including a transducer for reading data from the helical scan magnetic tape storage tracks having alternate azimuth magnetizations.18. The apparatus of claim 16, wherein said trellis decoder is a two-step decoder arranged to operate on bit pairs.19. The apparatus of claim 16, wherein said path memory is adapted to store candidate paths of data, the path memory being arranged to store, at the completion of each codeword, the respective candidate paths at assigned addresses for the candidate paths, the path memory being arranged so (a) the addresses are used during decoding intermediate the codeword boundaries, and (b) the path memory is updated at the completion of each new codeword.20. A trellis decoding arrangement for decoding a codeword comprising:a multi-state time varying trellis arranged to have a reduced number of states toward the completion of each codeword, the trellis including a path memory for storing candidate paths and data, wherein the trellis is arranged such that:(a) at the completion of each codeword the respective candidate paths are stored in the path memory at respective assigned addresses for the candidate paths,(b) the respective address is used during decoding of the codeword intermediate the codeword boundaries, and(c) the path memory is updated at the completion of each new codeword.21. The trellis decoding arrangement of claim 20, wherein the codeword is derived from data retrieved from a partial response channel having implicit rules, said data having been read from a helical scan magnetic tape storage medium that stores said data in tracks having alternate azimuth magnetizations, the data read from the tracks having alternate azimuth magnetizations having a predominate crosstalk noise component at a relatively low frequency compared to the frequency of the read data, said stored and read data comprising a series of codewords encoded from input data using a DC-free code, each of said codewords having a digital sum variation (DSV) bounded within a first preset range at the codewords boundary.22. The trellis decoding arrangement of claim 21, further including a transducer for reading data from the helical scan magnetic tape storage tracks having alternate azimuth magnetizations.23. A method of decoding a codeword by using a multi-state time varying trellis having a path memory, the method comprising:causing the trellis to have a reduced number of states toward the completion of each codeword;at the completion of each codeword storing the respective candidate paths in the path memory at respective assigned addresses for the candidate paths;using the respective address during decoding of the codeword intermediate the codeword boundaries; andupdating the path memory at the completion of each new codeword.24. The method of claim 23, further including retrieving data from a partial response channel having implicit rules, said data having been read from a helical scan magnetic tape storage medium that stores said data in tracks having alternate azimuth magnetizations, the data read from the tracks having alternate azimuth magnetizations having a predominate crosstalk noise component at a relatively low frequency compared to the frequency of the read data, said stored and read data comprising a series of codewords encoded from input data using a DC-free code, each of said codewords having a digital sum variation (DSV) bounded within a first preset range at the codewords boundary.25. The method of claim 24, further including reading said data from said tape storage medium by using a read head that scans the tape helically and reads said data from pairs of tracks having alternate azimuth magnetizations.26. Apparatus for performing the steps of claim 23.
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