Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1998-01-30
2001-02-06
De Cady, Albert (Department: 2784)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C360S065000
Reexamination Certificate
active
06185716
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the detection of data in noise and is particularly apt for use in digital data storage systems such as magnetic disk drive systems.
BACKGROUND OF THE INVENTION
It is generally true that data needs to be detected in a noisy and/or distorted input signal. For example, in a communications system, data is transmitted from a source location to a destination location via a transmission medium. While propagating through the medium, the data signal picks up noise from the medium and other sources. When the signal is received at the destination location, it is necessary to extract the data from the noisy signal. It is desired that the extracted data be identical to the data transmitted from the source location, but often errors will exist in the extracted data. As can be appreciated, if the detected data is not a sufficiently accurate representation of the transmitted information, the data will be of little or no use to the party receiving the data.
An equally demanding application that requires data to be detected from a noisy input signal is data storage. For example, in a magnetic disk drive system, data is stored as a coded series (i.e., a binary series) of magnetic transitions, frequently referred to as pulses, recorded on the surface of a magnetic disk. When data is transferred to and from the disk, the transitions nominally exhibit a Lorentzian shape, which is generally to a degree distorted. Some distortion is deterministic due to geometry of the head/media system. But, some is variable due to proximity of other transitions, localized deformities in the media, and irregular grain structure of the media. In addition, noise is added to the data signal from head (transducer) resistance, cable parasitics, and electronics. Collectively, noise and variable distortion tends to obscure the data.
As with communication systems, it is very important that the detected data in a data storage system be an accurate representation of the data that was originally stored on the media. To compensate for the occurrence of errors in the read data, most data storage systems apply error correction coding to the data before it is stored on the media. Then, if errors are present in the read data, the system is virtually always capable of correcting the errors before the data is delivered to the user. However, ECC coding adds overhead to the system that decreases the overall amount of user data that can be stored on a particular media, and it adds to system cost, both in proportion to the amount of ECC that must be applied to decrease detected error rates to an acceptable level.
Therefore, it is advantageous to develop a system for detecting data in a noisy and/or distorted signal that reduces the detected error rate. Such a system would be capable of operation with lower levels of error correction coding and would thus allow an increased amount of user data to be stored on a given storage medium.
SUMMARY OF THE INVENTION
The present invention relates to a detection system that is capable of detecting data in a noisy and/or distorted signal with higher than typical accuracy. That is, the system is capable of achieving data detection with a relatively low bit error rate for a given signal to noise ratio. Because noisier, more distorted signals can be tolerated for a given error rate, data can be more tightly packed on the disk surface.
The system of the present invention utilizes a dual detector approach to detect data in an input signal. That is, a first detector system performs an initial data detection on the input data signal and then a second detector system uses the output of the first detector system to reprocess the original input signal. The first detector system detects data in the input signal and creates both a primary bit string and a secondary bit string as a result. The primary bit string is the normal output signal of the first detector. The secondary bit string is similar to the primary bit string but has certain bits in the bit string (i.e., bits of low detection probability) changed by inversion. Subsequent bits in either bit string may also differ because of possible error propagation. The first detector system also outputs information related to the probability that the bits in the primary bit string have been properly detected.
The primary and secondary bit strings are delivered to first and second canceller units, respectively, within the second detector system. The two canceller units each also receive the original input data signal that was read from the media. The canceller units each modify the input data signal based upon a corresponding series of bits from the first detector system, such as by subtracting (alternatively, by adding) cancellation values from the data signal. In a preferred embodiment, the present invention uses a dual decision feedback equalizer (DFE) detector unit as the first detector and a dual canceller/multiplexer unit to perform the secondary detection.
The first detector system includes means for generating a flag signal whenever the magnitude of a modified data sample is less than a predetermined threshold level (i.e., between a positive and negative signal threshold which together create a quality threshold). As such, the flag signal identifies a data bit that has low detection probability. That is, it has higher probability of being in error than non-flagged bits. Note that, if a bit is truly in error, subsequent bits also exhibit higher error probability due to error propagation risk inherent in decision feedback detectors. The flag signal is used by the first detector system to determine whether to invert a corresponding bit in the secondary bit string. The flag signal is also delivered to the second detector system for enabling a possible alternative decision by the pair of canceller units. In addition to the flag signal, the first detector system creates a quality signal that is also related to bit detection probability of the questionable bit. The quality signal is delivered to the second detector system for determining a second threshold value which is used in combination with a margin signal (described below) in selecting the output of the second detector system.
As stated above, the two canceller units are operative for, among other things, modifying the original input data signal based upon a bit string received from the first detector system. In the preferred embodiment, this is done by subtracting a corresponding series of cancellation values from the data signal (or its functional equivalent). Each cancellation value subtracted from the data signal at a particular bit time is related to the output of the first detector system. That is, the cancellation value subtracted within the first canceller is chosen based upon the primary bit string and the cancellation value subtracted in the second canceller is chosen based upon the secondary bit string. In one embodiment, a pair of random access memories (RAMS) are used to store all of the possible cancellation values needed by the system. Each RAM is addressed using a series of bits extracted from the bit strings from the first detector system (i.e., the primary bit string in the first canceller and the secondary bit string in the second canceller).
When the low probability bit flag is raised, the canceller units each accumulate a squared error sum. Each error in the squared error sum is the difference between the actual modified sample input data and the expected modified sample values over a prescribed series of samples. When a squared error sum has been accumulated for the prescribed series of samples, a margin value for the dual canceller is calculated using the two error sums. In a preferred embodiment, the margin value is determined by finding the difference between the two error sums. The margin value is then compared to a second threshold value that is linearly related to the probability signal from the first detector for the original questionable bit. The comparison results in a select signal that is used to determine the output
Cady Albert De
Lamarre Guy
Maxtor Corporation
Ross P.C. Sheridan
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