Disk drive centering demodulation windows on spiral tracks during a seek operation

1. A disk drive comprising:

a disk comprising a plurality of spiral tracks, each spiral track comprising a high frequency signal interrupted at a predetermined interval by a sync mark;

a head actuated over the disk; and

control circuitry operable to:

use the head to read the spiral tracks to generate a read signal representing spiral track crossings; and

process the spiral track crossings to first seek the head over a plurality of concentric tracks defined by the spiral tracks, and during the first seek:

first open a first demodulation window in response to a first window offset during a first revolution of the disk, wherein the first demodulation window corresponds to the head approaching a first spiral track;

use the head to read the first spiral track to generate a first spiral track crossing;

generate a first window position error signal (PES) representing a difference between a target framing of the first demodulation window around the first spiral track crossing and a detected framing of the first demodulation window around the first spiral track crossing;

adjust the first window offset in response to the first window PES; and

second open the first demodulation window in response to the adjusted first window offset during a second revolution of the disk.

2. The disk drive as recited in claim 1 , wherein the target framing corresponds to a center of the first spiral track crossing occurring at a center of the first demodulation window.

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

position the head at an initial radial location over the disk; and

initialize the first window offset prior to the first seek.

4. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to evaluate the first window PES to adjust a seek velocity.

5. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to adjust the seek velocity during the first seek.

6. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to adjust the seek velocity after the first seek and prior to a second seek.

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

process the spiral track crossings to maintain the head in a substantial circular path while writing product servo sectors to the disk, including to write the product servo sectors over the first spiral track at a first radial location; and

disable the adjustment to the first window offset during the first seek when the head is at the first radial location.

8. The disk drive as recited in claim 1 , wherein during the first seek the control circuitry is further operable to:

first open a second demodulation window in response a second window offset during the first revolution of the disk, wherein the second demodulation window corresponds to the head approaching a second spiral track;

use the head to read the second spiral track to generate a second spiral track crossing;

generate a second window PES representing a difference between a target framing of the second demodulation window around the second spiral track crossing and a detected framing of the second demodulation window around the second spiral track crossing;

adjust the second window offset in response to the second window PES; and

second open the second demodulation window in response to the adjusted second window offset during the second revolution of the disk.

9. A method of operating a disk drive, the disk drive comprising a head actuated over a disk comprising a plurality of spiral tracks, each spiral track comprising a high frequency signal interrupted at a predetermined interval by a sync mark, the method comprising:

using the head to read the spiral tracks to generate a read signal representing spiral track crossings; and

processing the spiral track crossings to first seek the head over a plurality of concentric tracks defined by the spiral tracks, and during the first seek:

first opening a first demodulation window in response to a first window offset during a first revolution of the disk, wherein the first demodulation window corresponds to the head approaching a first spiral track;

using the head to read the first spiral track to generate a first spiral track crossing;

generating a first window position error signal (PES) representing a difference between a target framing of the first demodulation window around the first spiral track crossing and a detected framing of the first demodulation window around the first spiral track crossing;

adjusting the first window offset in response to the first window PES; and

second opening the first demodulation window in response to the adjusted first window offset during a second revolution of the disk.

10. The method as recited in claim 9 , wherein the target framing corresponds to a center of the first spiral track crossing occurring at a center of the first demodulation window.

11. The method as recited in claim 9 , further comprising:

positioning the head at an initial radial location over the disk; and

initializing the first window offset prior to the first seek.

12. The method as recited in claim 9 , further comprising evaluating the first window PES to adjust a seek velocity.

13. The method as recited in claim 12 , further comprising adjusting the seek velocity during the first seek.

14. The method as recited in claim 12 , further comprising adjusting the seek velocity after the first seek and prior to a second seek.

15. The method as recited in claim 9 , further comprising:

processing the spiral track crossings to maintain the head in a substantial circular path while writing product servo sectors to the disk, including to write the product servo sectors over the first spiral track at a first radial location; and

disabling the adjustment to the first window offset during the first seek when the head is at the first radial location.

16. The method as recited in claim 9 , wherein during the first seek further comprising:

first opening a second demodulation window in response a second window offset during the first revolution of the disk, wherein the second demodulation window corresponds to the head approaching a second spiral track;

using the head to read the second spiral track to generate a second spiral track crossing;

generating a second window PES representing a difference between a target framing of the second demodulation window around the second spiral track crossing and a detected framing of the second demodulation window around the second spiral track crossing;

adjusting the second window offset in response to the second window PES; and

second opening the second demodulation window in response to the adjusted second window offset during the second revolution of the disk.