Detection of hard-disc defect regions using soft decisions
In a hard-disc drive, a defect region on the hard disc is detected by generating two statistical measures (e.g., β 1 (k) and β 2 (k)) based on signal values (e.g., x[n] or y[n]) and soft-decision values (e.g., L[n]) corresponding to the signal values. The measures are compared to detect the location of the defect region of the hard drive. Using the soft-decision values reduces fluctuations in a ratio of the statistical measures compared to a ratio formed from statistical measures that are not based on soft-decision values, resulting in a more-reliable test for detecting defect regions.
1. A machine-implemented method for detecting a defect region on a hard drive, the method comprising:
(a) the machine generating a first measure based on signal values, x[n] y[n] expected signal values corresponding to the signal values, and soft-decision values L[n] corresponding to the signal values;
(b) the machine generating a second measure based on expected signal values and defect-free soft-decision values corresponding to only defect-free regions of the hard drive;
(c) the machine comparing the first and second measures; and
(d) the machine detecting the defect region based on the comparison of step (c).
2. The invention of claim 1 , wherein:
the first measure is based on a moving average of products of (i) the signal values, (ii) the expected signal values, and (iii) magnitudes of the soft-decision values; and
the second measure is based on a moving average of products of (i) squares of the expected signal values and (ii) expectation values of the magnitudes of the defect-free soft-decision values.
3. The invention of claim 2 , wherein:
the first measure β 1 (k) is given by:
β 1 ( k )=Σ i=0 M−1 y[k−i]ŷ[k−i ]abs( L[k−i ])
the second measure β 2 (k) is giving by:
β 2 ( k )=Σ i=0 M−1 ŷ[k−i]ŷ[k−i]E (abs( L ))
wherein:
y[k−i] is the (k−i)th signal value;
ŷ[k−i] is the expected value of the (k−i)th signal value;
L[k−i] is the soft-decision value for the (k−i)th signal value;
E(abs(L)) represents an expectation function applied to an absolute value of the defect-free soft-decision values; and
M is a specified positive integer.
4. The invention of claim 1 , wherein the defect region is associated with either thermal asperity (TA) or media defect (MD).
5. The invention of claim 1 , wherein the signal values are analog-to-digital converter (ADC) output values [n].
6. The invention of claim 1 , wherein the signal values are equalizer input values x[n].
7. The invention of claim 1 , wherein the signal values are equalizer output values y[n].
8. The invention of claim 1 , wherein the comparison determines whether the first measure is less than a specified fraction of the second measure, where the specified fraction is less than one.
9. The invention of claim 1 , wherein the comparison compares a ratio of the first measure and the second measure to a specified threshold.
10. The invention of claim 1 , wherein the expected signal values are mean values of the signal values.
11. The invention of claim 10 , wherein the expected signal values are functions of bit patterns associated with the signal values and one or more adjacent signal values.
12. The invention of claim 1 , wherein the expected signal values are reconstructed from hard-decision values corresponding to the signal values.
13. The invention of claim 12 , wherein the expected signal values are reconstructed by convolving the hard-decision values with a target polynomial.
14. A machine for detecting a defect region on a hard drive, the machine comprising:
(a) means for generating a first measure based on signal values, x[n] y[n] expected signal values corresponding to the signal values, and soft-decision values L[n] corresponding to the signal values;
(b) means for generating a second measure based on expected signal values and defect-free soft-decision values corresponding to only defect-free regions of the hard drive;
(c) means for comparing the first and second measures; and
(d) means for detecting the defect region based on the comparison of means (c).
15. The invention of claim 14 , wherein the machine is a digital signal processor.
16. The invention of claim 14 , wherein the machine is a hard-disc drive.
17. A non-transitory machine-readable storage medium, having encoded thereon program code, wherein, when the program code is executed by a machine, the machine implements a method for detecting a defect region on a hard drive, the method comprising:
(a) generating a first measure based on signal values, x[n] y[n] expected signal values corresponding to the signal values, and soft-decision values L [n] corresponding to the signal values;
(b) generating a second measure based on expected signal values and defect-free soft-decision values corresponding to only defect-free regions of the disk drive;
(c) the machine comparing the first and second measures; and
(d) the machine detecting the defect region based on the comparison of step (c).
18. The invention of claim 14 , wherein:
the first measure is based on a moving average of products of (i) the signal values, (ii) the expected signal values, and (iii) magnitudes of the soft-decision values; and
the second measure is based on a moving average of products of (i) squares of the expected signal values and (ii) expectation values of the magnitudes of the defect-free soft-decision values.
19. The invention of claim 18 , wherein:
the first measure β 1 (k) is given by:
β 1 ( k )=Σ i=0 M−1 y[k−i]ŷ[k−i ]abs( L[k−i ])
the second measure β 2 (k) is giving by:
β 2 ( k )=Σ i=0 M−1 ŷ[k−i]ŷ[k−i]E (abs( L ))
wherein:
y[k−i] is the (k−i)th signal value;
ŷ[k−i] is the expected value of the (k−i)th signal value;
L[k−i] is the soft-decision value for the (k−i)th signal value;
E(abs(L)) represents an expectation function applied to an absolute value of the defect-free soft-decision values; and
M is a specified positive integer.