Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-11-09
2003-06-17
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C702S180000, C360S053000
Reexamination Certificate
active
06580768
ABSTRACT:
TECHNICAL FIELD
This invention relates to maximum likelihood detection of data recorded as analog signals representing a finite number of states, and, more particularly, to adapting maximum likelihood detection to handle variable data channels.
BACKGROUND OF THE INVENTION
Maximum likelihood detection of data recorded as analog signals and detected from partial response samples is highly advantageous in magnetic disk drives, where the disks and heads are fixed and non-removable. The characteristics of the channel are fixed, including the particular disk media, the particular recording and read heads, the linear velocity and flying height between the disk media and the recording and read heads, and the recording and read electronics. The channel characteristics can be measured and, once known, tend to remain constant.
With constant channel characteristics, the detection capability of maximum likelihood detection can be designed so as to be highly error free. The received partial response samples are equalized so that the signals provided to the maximum likelihood detector tend to precisely match the expected signals.
Additionally, a specific code may be employed which maximizes the distances between the sensed states. Only limited changes are taken into account, such as differences in data rates between inner and outer tracks, minor servo offtrack operation, minor disk defects, and some head wear over time. Thus, a specific maximum likelihood detection circuit can be designed which is specific to the type of disk drive and which will have a low error rate at high recording densities. Further, such minor changes have been accommodated by employing digital FIR (finite impulse response) filters whose coefficients are programmable, thus changing the frequency response of the filters to better equalize the signal being read to match the maximum likelihood detector. Examples include, U.S. Pat. No. 5,321,559, Nguyen et al., U.S. Pat. No. 5,365,342, Abbott et al., and U.S. Pat. No. 5,442,760, Abbott et al.
It becomes more difficult to use such maximum likelihood detection with recording devices which have removable media.
Removable media devices tend to be mass storage devices which allow data to be recorded on portable media that is removed from the device and stored elsewhere, such as in the storage shelves of an automated data storage library, or in true archive storage outside of a drive or library on storage shelves or in boxes and other containers. The amount of data so stored quickly becomes very large and, if a new and upgraded portable media is introduced, there is a desire on the part of the user to resist re-recording all of the archived data onto the upgraded portable media. Hence, a backwards compatibility is typically required for removable media devices.
The characteristics of portable media thus vary between the types of media, but also tend to vary between ones of the same type of media, and within the same media.
Examples of removable media devices include optical disk and optical tape storage, which may be read-only, write-once, and rewritable media, and be different types of media, such as molded, magneto-optic and phase-change media.
Optical media is subject to variation from media to media in recorded data output characteristics based on the type of media, above, variation in media materials between manufacturers and over time, and between recording densities. Optical media is also subject to change as the recording moves between tracks, where the effective head to track speed may change, or as the recording moves from one side of the disk to another, where the effective head to disk surface angle may be tilted.
Another example of removable media devices includes magnetic tape recording, which have media to media variation based on different data densities on the same type of media, different types of media such as chromium-based, nickel-based, ferrous-based media, or between materials used by different manufacturers. Additionally, tape media may have differing thicknesses and therefore differing media to head (flying and contact) characteristics over the recording and read head, resulting in differing head to media spacings. The tape media may also fly at differing head to media spacings as the tape moves from one reel to the next and the tension or angle of the tape to the head varies. Further, the flying characteristics of the tape media may vary as the tape head is moved from one side of the tape to tracks at the other side of the tape.
Maximum likelihood detection in such differing circumstances is exceedingly difficult, and may require a different maximum likelihood detector for each circumstance.
The incorporated Hutchins et al. '019 and '020 applications provide maximum likelihood detectors which allow the programming, or setting, of numerical metric coefficients, adjusting the response of the maximum likelihood detector. The coefficients are preferably derived from the difference between metrics directly associating “0” and “1” states of the recorded signal. The coefficients are respectively applied to each digital sample to generate alternative metrics, and each respective alternative metric is compared to a previous metric. Based on the comparison, one of a plurality of provided metrics is selected which remains within defined positive and negative bounds. Then, the one of the finite number of states represented by the selected metric is identified, and in response to the identified one of the finite states, a maximum likelihood state detector is incremented to a maximum likelihood state dictated by the identified one of the finite states, the incremented maximum likelihood state detects the recorded analog signals.
The numerical metric coefficients of the maximum likelihood detector are based on sample “cases” which are derived from sample outputs for expected waveforms of a particular media, and the metric coefficient numerical values are calculated for the expected sample outputs. Specifically, the sample outputs of the expected waveforms are determined, for example, by measuring a number of actual outputs for sample points of the waveforms, and calculating the mean values of each of the sample points, and the metric coefficient numerical values are calculated for the mean values of the sample outputs, thereby providing the numerical metric coefficients. The equations for deriving the metric coefficients minimize the mean squared error between the received signal and the ideal signal, which is the noise-free signal.
Once determined, the numerical metric coefficients are stored in a lookup table and selected when the portable media having the characteristics is identified as loaded into the device. A portable removable media may be identified externally by a label, or internally by reading a memory chip, reading a universal modulated code from the media, or the first few bytes of the media to be read may have more universal characters which are easily read which identify the media.
A media detector may comprise an optical reader or scanner for reading a label, a wireless interface for reading a chip, or may comprise logic associated with the read channel, and, the numerical metric coefficients are selected from the lookup table for the detected media.
Thus, the numerical coefficients for the specific media must be known in advance, and the predetermined numerical coefficients must be stored and available for use. Further, the characteristics of the specific media must not be variable, so that the same numerical coefficients may be used for the media without change.
However, as discussed above, optical media is subject to change as the recording moves between tracks, where the effective head to track speed may change, or as the recording moves from one side of the disk to another, where the effective head to disk surface angle may be tilted. Tape media may fly at differing head to media spacings as the tape moves from one reel to the next and the tension or angle of the tape to the. head varies. Further, the flying characteristics of the tape
Chin Stephen
Holcombe John H.
International Business Machines - Corporation
Yeh Edith
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