Disk drive margining read channel by biasing log-likelihood ratios of an iterative decoder

1. A disk drive comprising:

a disk comprising a plurality of data tracks;

a head actuated over the disk; and

control circuitry operable to:

sample a read signal emanating from the head to generate read samples;

generate first log-likelihood ratios (LLRs) in response to the read samples;

bias the first LLRs to generate biased LLRs; and

decode the biased LLRs into a data sequence, wherein the biased LLRs increase an error rate of the data sequence.

2. The disk drive as recited in claim 1 , wherein a sequence detector generates the first LLRs.

3. The disk drive as recited in claim 2 , wherein the sequence detector comprises a Viterbi sequence detector.

4. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to bias the first LLRs to modify a probability density function (PDF) of the first LLRs.

5. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to:

write a test pattern to the disk;

read the test pattern from the disk to generate the read signal; and

bias the first LLRs in response to the test pattern.

6. The disk drive as recited in claim 5 , wherein the control circuitry is further operable to bias the first LLRs to modify a probability density function (PDF) of the first LLRs.

7. The disk drive as recited in claim 6 , wherein the control circuitry is further operable to bias the first LLRs according to:

c ·(first LLR k )− a k ·m

where:

c is a scalar that scales a variance of the first LLRs;

a k represents a symbol of the test pattern; and

m is a scalar that shifts a mean of the first LLRs.

8. The disk drive as recited in claim 7 , wherein biasing the first LLRs modifies the PDF by increasing the variance and shifting the mean toward a center of the PDF.

9. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to:

accumulate the first LLRs into a histogram representing the PDF; and

generate the biased LLRs in response to the histogram.

10. The disk drive as recited in claim 1 , wherein an iterative decoder generates the first LLRs.

11. The disk drive as recited in claim 10 , wherein the control circuitry is further operable to bias the first LLRs only during a subset of iterations executed by the iterative decoder.

12. A method of operating a disk drive comprising a head actuated over a disk comprising a plurality of data tracks, the method comprising:

sampling a read signal emanating from the head to generate read samples;

generating first log-likelihood ratios (LLRs) in response to the read samples;

biasing the first LLRs to generate biased LLRs; and

decoding the biased LLRs into a data sequence, wherein the biased LLRs increase an error rate of the data sequence.

13. The method as recited in claim 12 , wherein a sequence detector generates the first LLRs.

14. The method as recited in claim 13 , wherein the sequence detector comprises a Viterbi sequence detector.

15. The method as recited in claim 12 , further comprising biasing the first LLRs to modify a probability density function (PDF) of the first LLRs.

16. The method as recited in claim 12 , further comprising:

writing a test pattern to the disk;

reading the test pattern from the disk to generate the read signal; and

biasing the first LLRs in response to the test pattern.

17. The method as recited in claim 16 , further comprising biasing the first LLRs to modify a probability density function (PDF) of the first LLRs.

18. The method as recited in claim 17 , wherein the first LLRs are biased according to:

c ·(first LLR k )− a k ·m

where:

c is a scalar that scales a variance of the first LLRs;

a k represents a symbol of the test pattern; and

m is a scalar that shifts a mean of the first LLRs.

19. The method as recited in claim 18 , wherein biasing the first LLRs modifies the PDF by increasing the variance and shifting the mean toward a center of the PDF.

20. The method as recited in claim 15 , further comprising:

accumulating the first LLRs into a histogram representing the PDF; and

generating the biased LLRs in response to the histogram.

21. The method as recited in claim 12 , wherein an iterative decoder generates the first LLRs.

22. The method as recited in claim 21 , further comprising biasing the first LLRs only during a subset of iterations executed by the iterative decoder.