Disk drive excluding servo sub-bursts when computing position error signal

1. A disk drive comprising:

a disk comprising a plurality of servo sectors wherein each servo sector comprises at least one servo burst;

a head actuated over the disk; and

control circuitry operable to:

sample a read signal emanating from the head while reading a servo burst in a servo sector to generate a plurality of sample values;

generate a quality metric in response to a plurality of sample values of a servo sub-burst representing part of the servo burst;

when the quality metric does not exceed a threshold, exclude the servo sub-burst sample values when generating a position error signal (PES) representing a position of the head over the disk;

when the quality metric exceeds the threshold, include the servo sub-burst sample values when generating the PES; and

position the head over the disk in response to the PES.

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

generate a plurality of quality metrics each corresponding to one of a plurality of servo sub-bursts; and

when one of the quality metric exceeds the threshold, exclude the corresponding servo sub-burst from the PES.

3. The disk drive as recited in claim 2 , wherein the servo burst comprises an amplitude based servo burst.

4. The disk drive as recited in claim 2 , wherein:

the disk comprises at least one spiral track comprising a high frequency signal interrupted by a sync mark at a sync mark interval; and

the sync marks define a plurality of servo bursts each comprising a plurality of servo sub-bursts.

5. The disk drive as recited in claim 2 , wherein when the quality metric of a first servo sub-burst in a first servo burst exceeds the threshold, the control circuitry is further operable to substitute the first servo sub-burst with a second servo sub-burst to generate the PES.

6. The disk drive as recited in claim 1 , wherein:

the read signal comprises a periodic signal; and

the servo sub-burst comprises at least one cycle of the periodic signal.

7. The disk drive as recited in claim 1 , wherein the quality metric comprises an amplitude of the read signal.

8. The disk drive as recited in claim 1 , wherein the servo burst comprises a phased based servo burst.

9. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to generate the threshold as a running average of quality metrics generated for servo bursts of previous servo sectors.

10. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to generate the threshold in response to a PES generated for a previous servo sector.

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

generate the quality metric relative to an amplitude of each servo sub-burst;

compare the amplitudes of the servo sub-bursts; and

when the amplitude of a first one of the servo sub-bursts differs from the amplitude of a second and third servo sub-burst:

combine the amplitudes of the second and third servo sub-bursts to generate a combined amplitude signal;

scale the combined amplitude signal to generate a scaled servo burst signal; and

generate the PES in response to the scaled servo burst signal.

12. The disk drive as recited in claim 1 , wherein when the quality metric exceeds the threshold the control circuitry is further operable to save an entry in a defect log identifying a defect in the servo burst.

13. A method of operating a disk drive comprising a disk including a plurality of servo sectors wherein each servo sector comprises at least one servo burst, and a head actuated over the disk, the method comprising:

sampling a read signal emanating from the head while reading a servo burst in a servo sector to generate a plurality of sample values;

generating a quality metric in response to a plurality of sample values of a servo sub-burst representing part of the servo burst;

when the quality metric does not exceed a threshold, excluding the servo sub-burst sample values when generating a position error signal (PES) representing a position of the head over the disk;

when the quality metric exceeds the threshold, including the servo sub-burst sample values when generating the PES; and

positioning the head over the disk in response to the PES.

14. The method as recited in claim 13 , further comprising:

generating a plurality of quality metrics each corresponding to one of a plurality of servo sub-bursts; and

when one of the quality metric exceeds the threshold, excluding the corresponding servo sub-burst from the PES.

15. The method as recited in claim 14 , wherein the servo burst comprises an amplitude based servo burst.

16. The method as recited in claim 14 , wherein:

the disk comprises at least one spiral track comprising a high frequency signal interrupted by a sync mark at a sync mark interval; and

the sync marks define a plurality of servo bursts each comprising a plurality of servo sub-bursts.

17. The method as recited in claim 14 , wherein when the quality metric of a first servo sub-burst in a first servo burst exceeds the threshold, the control circuitry is further operable to substitute the first servo sub-burst with a second servo sub-burst to generate the PES.

18. The method as recited in claim 13 , wherein:

the read signal comprises a periodic signal; and

the servo sub-burst comprises at least one cycle of the periodic signal.

19. The method as recited in claim 13 , wherein the quality metric comprises an amplitude of the read signal.

20. The method as recited in claim 13 , wherein the servo burst comprises a phased based servo burst.

21. The method as recited in claim 13 , further comprising generating the threshold as a running average of quality metrics generated for servo bursts of previous servo sectors.

22. The method as recited in claim 13 , further comprising generating the threshold in response to a PES generated for a previous servo sector.

23. The method as recited in claim 13 , further comprising:

generating the quality metric relative to an amplitude of each servo sub-burst;

comparing the amplitudes of the servo sub-bursts; and

when the amplitude of a first one of the servo sub-bursts differs from the amplitude of a second and third servo sub-burst:

combining the amplitudes of the second and third servo sub-bursts to generate a combined amplitude signal;

scaling the combined amplitude signal to generate a scaled servo burst signal; and

generating the PES in response to the scaled servo burst signal.

24. The method as recited in claim 13 , wherein when the quality metric exceeds the threshold further comprising saving an entry in a defect log identifying a defect in the servo burst.