Programmable gray code bit change generator

1. A method of processing a received signal, comprising:

receiving a code word that is different from an expected code word;

using a logic circuit to determine whether the difference between the received code word and the expected code word is acceptable, including by:

determining, based at least in part on the received code word, one or more disallowed bit changes, wherein each disallowed bit change corresponds to a bit position which is not expected to change when comparing a desired track's code word against an adjacent track's code word;

determining that the difference is acceptable if (1) the received code word matches the desired track's code word or (2) none of the disallowed bit changes change when comparing the received code word against the adjacent track's code word; and

determining that the difference is unacceptable if one or more disallowed bit changes change when comparing the received code word against the adjacent track's code word; and

in the event it is determined that the difference is unacceptable, providing an indication of an error.

2. The method of claim 1 , wherein the received code word is a Gray code word.

3. The method of claim 1 , further comprising in the event it is determined that the difference is acceptable, ignoring the difference or continuing as if there was no difference.

4. The method of claim 1 , wherein receiving a code word comprises reading a portion of a hard disk drive corresponding to a track ID.

5. The method of claim 1 , wherein the logic circuit comprises Delta Gray logic.

6. The method of claim 5 , wherein a shift register or adder is used to modify the input to the Delta Gray logic.

7. The method of claim 1 , wherein the error comprises a disk drive defect.

8. The method of claim 1 , further including, in the event it is determined that the difference is unacceptable, performing error correction.

9. The method of claim 1 , further including, in the event it is determined that the difference is unacceptable, aborting a write operation.

10. The method of claim 1 , wherein the disallowed bit changes further include a bit position which is not expected to change when comparing the desired track's code word against a code word of a track which is X tracks away from the desired track, where X is an integer.

11. The method of claim 10 , wherein X is variable.

12. The method of claim 11 , wherein X is smaller when a servo system is trying to stay on track and larger when the servo system is moving from one track to another track.

13. A system for processing a received signal, comprising:

an input interface configured to receive a code word that is different from an expected code word;

a logic circuit configured to determine whether the difference between the received code word and the expected code word is acceptable, including by:

determining, based at least in part on the received code word, one or more disallowed bit changes, wherein each disallowed bit change corresponds to a bit position which is not expected to change when comparing a desired track's code word against an adjacent track's code word;

determining that the difference is acceptable if (1) the received code word matches the desired track's code word or (2) none of the disallowed bit changes change when comparing the received code word against the adjacent track's code word; and

determining that the difference is unacceptable if one or more disallowed bit changes change when comparing the received code word against the adjacent track's code word; and

an output interface configured to provide an indication of an error in the event it is determined that the difference is unacceptable.

14. The system of claim 13 , wherein the disallowed bit changes further include a bit position which is not expected to change when comparing the desired track's code word against a code word of a track which is X tracks away from the desired track, where X is an integer.

15. The system of claim 14 , wherein X is variable.

16. The system of claim 15 , wherein X is smaller when a servo system is trying to stay on track and larger when the servo system is moving from one track to another track.

17. A computer program product for processing a received signal, the computer program product being embodied in a non-transitory computer readable storage medium and comprising computer instructions for:

receiving a code word that is different from an expected code word;

determining whether the difference between the received code word and the expected code word is acceptable, including by:

determining, based at least in part on the received code word, one or more disallowed bit changes, wherein each disallowed bit change corresponds to a bit position which is not expected to change when comparing a desired track's code word against an adjacent track's code word;

determining that the difference is acceptable if (1) the received code word matches the desired track's code word or (2) none of the disallowed bit changes change when comparing the received code word against the adjacent track's code word; and

determining that the difference is unacceptable if one or more disallowed bit changes change when comparing the received code word against the adjacent track's code word; and

in the event it is determined that the difference is unacceptable, providing an indication of an error.

18. The computer program product of claim 17 , wherein the disallowed bit changes further include a bit position which is not expected to change when comparing the desired track's code word against a code word of a track which is X tracks away from the desired track, where X is an integer.

19. The computer program product of claim 18 , wherein X is variable.

20. The computer program product of claim 19 , wherein X is smaller when a servo system is trying to stay on track and larger when the servo system is moving from one track to another track.