Parallel spectral equalization channels with low density parity check codes

The present invention combines low density parity checking with parallel equalization channels to enhance data retrieval from tape. Viterbi analysis is done as a precursor to the use of at least one low density parity decoder. A signal decoder may include a plurality of viterbi processors and at least one low density parity check decoder. The decoder may receive a plurality of pulse-shaped signals and produces decoded output signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to signal processing for enhancing the retrieval of data from magnetic tape.

2. Background Art

Data is stored on a magnetic tape as variations in the magnetic field produced by particles in the tape. This data is typically read by sensing these fields with a tape head. As the tape travels past the tape head, variations in the magnetic field induce a time-varying voltage in the head. The analog signals generated represent the data stored on the tape. The typical tape reader also incorporates pre-amplifiers, analog processors, analog-to-digital converters, and the like to process tape head output signals.

Increases in tape data storage capacity necessitate an increase in signal data density. However, increases in signal data density may decrease performance due to reduced signal-to-noise ratios. In addition, partial response channel systems depend heavily on inter-symbol interference to function properly. Such systems achieve this interference by passing the received signal through a pulse shaping filter matching the signal spectral response to channel requirements. However, variations in the head-to-tape positioning alter the signal spectral components. The resulting mis-equalization causes increasing signal degradation.

Low density parity check decoding is an evolving technology for retrieving data from magnetic storage media through partial response channels. Low density parity check decoding uses an iterative approach based on multiple passes of a signal containing data through a decoder. The information learned in each decoder pass is used in subsequent decoder passes. Iterations continue until the data is fully decoded from the signal or the design maximum number of decoder passes is reached. Low density parity check decoding can theoretically achieve an effective increase in signal strength of 5 dB, thus compensating for reduced signal amplitudes associated with higher signal data density recording.

SUMMARY OF THE INVENTION

It is an object of this invention to achieve reduced degradation due to lower signal-to-noise ratios and less sensitivity to variations in the head-to-tape separation due to increases in signal data density.

The present invention uses parallel spectral equalization channels together with a decoder based on low density parity checking to compensate for performance degradation due to increases in signal data density. The decoder incorporates viterbi analysis as a precursor to the use of at least one low density parity decoder. This provides a computationally efficient system and method of incorporating low density parity check decoding with parallel equalization channels to compensate for increased performance degradation due to increases in signal data density with variations in head-to-tape separation.

In accordance with the present invention, a system for use in a tape drive is provided. A tape head is provided to access magnetic tape. The tape head generates read signals based in part on a spatial relationship between the tape head and the tape. Each of a plurality of pulse shaping filters receives the read signals and produces pulse-shaped signals. Each pulse shaping filter has at least one filter parameter based on a possible tape head-to-tape spatial relationship unique to that filter. A signal decoder receives the pulse-shaped signals and produces decoded output signals. The signal decoder includes a plurality of viterbi processors, each accepting pulse-shaped signals from one of the plurality of pulse shaping filters. The signal decoder also includes at least one low density parity check decoder producing the decoded output signals based on output from one of the viterbi processors.

In an embodiment of the present invention, each viterbi processor includes a soft output viterbi processor.

In another embodiment of the present invention, the signal decoder includes one low density parity check decoder. Decoder logic selects one of the viterbi processor outputs as the low density parity check decoder input.

In still another embodiment of the present invention, the signal decoder includes a plurality of low density parity check decoders. Decoder logic selects output from one of the plurality of low density parity check decoders as the decoded output signals.

In yet another embodiment of the present invention, each viterbi processor generates a series of probabilities. Control logic may select output from one of the viterbi processors based on one or more of a distribution of the probabilities, a standard deviation of the probabilities, at least one number of indeterminate values, at least one viterbi metric, and the like.

A method of retrieving data from tape is also provided. The tape is read with a tape head to generate a read signal. The tape head has a spatial relationship with the tape as the tape passes the tape head described by at least one variable spatial parameter. The read signal is filtered with a set of parallel filters. Each filter receives the read signal and produces a filtered signal. Each filter is based on at least one unique value for the spatial parameters. Each filtered signal is processed with a viterbi algorithm. A decoded output signal is based on selecting and parity checking one of the viterbi processed filtered signals.

The above features, and other features and advantages of the present invention, are readily apparent from the following detailed descriptions thereof when taken in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1depicts an embodiment of the system using multiple low density parity check decoders. In this embodiment, tape10is located relative to head12by a tape spacing distance14within a range of possible tape spacing distances. Head12is operative to generate read signals16based on the tape spacing distance14. A pre-processor18uses amplifiers, analog processors, analog-to-digital converters, and other well known circuitry to pre-process the read signals16. A plurality of pulse shaping filters22receive the pre-processed read signals20and produce a plurality of pulse-shaped signals24. Each pulse shaping filter22has at least one filter parameter based on a particular tape-to-head spacing parameter. Preferably, each filter22compensates for a different tape-to-head spacing relationship. The tape-to-head spacing is typically a function of the distance between the tape10and head12in a direction normal to the surface of the tape10. However, the tape-to-head spacing can reflect other spatial differences such as, for example, displacement of tape10across the width of head12, rotation of tape10relative to head12, and the like.

A signal decoder26receives the plurality of pulse-shaped signals24and produces decoded output signals38. Signal decoder26includes a plurality of soft output viterbi processors28, a plurality of low density parity check decoders32, and decoder logic36. Each soft output viterbi processor28accepts pulse-shaped signals24from one pulse shaping filter22and produces a series of probabilities as viterbi output signals30. Each probability represents the chance that data is a one or a zero. A general discussion of viterbi processors is presented below. Each low density parity check decoder32receives viterbi output signals30from one soft output viterbi processor28and decodes the signals. The decoder logic36receives the low density parity check decoder output signals34and selects one of the low density parity check decoder output signals34as the signal decoder output signals38. The decoder logic36can be implemented to use a variety of selection criteria such as selecting the signals with the tightest distribution of probabilities based on minimum and maximum values, selecting the signals with the minimum standard deviation, selecting the signals with the least number of indeterminate values, using viterbi metrics, and the like.

FIG. 2depicts an embodiment of the system using a single low density parity check decoder32. In this embodiment, the signal decoder26comprises a plurality of soft output viterbi processors28, decoder logic36, and a single low density parity check decoder32. As above, each soft output viterbi processor28accepts pulse-shaped signals24from one pulse shaping filter22and produces a series of probabilities as viterbi output signals30. The decoder logic36receives the viterbi output signals30and selects one of the viterbi output signals30as the input signals40for the single low density parity check decoder32. The decoder logic36can be implemented to use selection criteria such as selecting the signals with the tightest distribution of probabilities based on minimum and maximum values; selecting the signals with the minimum standard deviation; selecting the signals with the least number of indeterminate values; using viterbi metrics; and the like. The signal decoder output signals38are the output signals from the single low density parity check decoder32.

FIG. 3is a flow diagram of an embodiment of the present invention using multiple low density parity check decoders. A tape head12reads a tape10generating read signals16, as in block101. Tape10is separated from head12by a tape spacing distance14. A pre-processor18pre-processes the read signals16using pre-amplifiers, analog processors, and analog-to-digital converters, as in block102, to produce a digital output stream. A plurality of pulse shaping filters22filter the pre-processed read signals20producing a plurality of pulse-shaped signals24, as in block103. Each pulse shaping filter has at least one filter parameter based on a particular tape spacing relationship. A plurality of soft output viterbi processors28in signal decoder26process the pulse-shaped signals24, as in block104. Each soft output viterbi processor28produces a series of probabilities as viterbi output signals30. A plurality of low density parity check decoders32decodes the viterbi output signals30, as in block105. The decoder logic36selects one of the low density parity check decoder output signals34as the signal decoder output signals38, as in block106.

FIG. 4is a block flow diagram of an embodiment of the present invention using a single low density parity check decoder. A tape head12reads a tape10generating read signals16, as in block201. Tape10is separated from head12by a tape spacing distance14. A pre-processor18pre-processes the read signals16, as in block202. A plurality of pulse shaping filters22filter the pre-processed read signals20producing a plurality of pulse-shaped signals24, as in block203. Each pulse shaping filter has at least one filter parameter based on a particular tape spacing relationship. A plurality of soft output viterbi processors28in the signal decoder26process the pulse-shaped signals24, as in block204. Each soft output viterbi processor28produces a series of probabilities as viterbi output signals30. The decoder logic36selects one of the viterbi processed signals30as the input signals40for the single low density parity check decoder32, as in block205. The low density parity check decoder decodes the selected signals, as in block206. The signal decoder output signals38are the output signals from the single low density parity check decoder32.

FIG. 5is a graphic representation of an output stream of a soft output viterbi processor. A soft output viterbi processor samples the incoming signals and assigns each sample a probability value as to whether it is likely to be a “zero” or a “one.” Processor output, shown generally by300, comprises a stream of probabilities, one of which is indicated by302. Probabilities falling within range304are classified as a “one.” Probabilities falling within range306are classified as a “zero.” Probabilities falling outside of ranges304,306are classified as indeterminate, as indicated by range308. Final conversion of the output stream into a bit stream requires the use of a downstream decoder. As discussed above, the downstream decoder in the present invention is preferably a low density parity check decoder.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. More specifically, those skilled in the art will recognize that the filters, processors, and logic described above could be implemented using hardware, software, or a combination of hardware and software.