Two dimensional magnetic recording servo system phase alignment

1. A servo system comprising:

a first equalizer circuit operable to filter digital servo data samples derived from a first read head to yield first equalized data;

a second equalizer circuit operable to filter digital servo data samples derived from a second read head to yield second equalized data;

a first interpolator operable to interpolate the first equalized data to yield a plurality of first interpolated outputs at different phases;

a second interpolator operable to interpolate the second equalized data to yield a plurality of second interpolated outputs at different phases; and

a phase tracking and signal combining circuit operable to select and combine most closely aligned signals from the plurality of first interpolated outputs and the plurality of second interpolated outputs to yield a combined servo data signal.

2. The servo system of claim 1 , wherein the first read head and the second read head are positioned to read a same data track.

3. The servo system of claim 1 , further comprising a servo address mark detector circuit operable to detect a servo address mark in the combined servo data signal.

4. The servo system of claim 3 , further comprising a multiplexer operable to provide one of the combined servo data signal, an output of the first interpolator or an output of the second interpolator to the servo address mark detector circuit.

5. The servo system of claim 4 , further comprising:

a first peak servo address mark detector operable to detect the servo address mark in an output of the first interpolator; and

a second peak servo address mark detector operable to detect the servo address mark in an output of the second interpolator.

6. The servo system of claim 5 , further comprising a head offset estimation circuit operable to estimate an offset of the first read head and the second read head with respect to a data track based at least in part on outputs of the first peak servo address mark detector and of the second peak servo address mark detector.

7. The servo system of claim 1 , further comprising:

a first delay circuit operable to delay the digital servo data samples derived from the first read head by an integer multiple of a sample period when the digital servo data samples derived from the first read head are advanced by at least a full sample period over the digital servo data samples derived from the second read head; and

a second delay circuit operable to delay the digital servo data samples derived from the second read head by an integer multiple of a sample period when the digital servo data samples derived from the second read head are advanced by at least a full sample period over the digital servo data samples derived from the first read head.

8. The servo system of claim 1 , further comprising:

a first digital lowpass filter operable to filter the digital servo data samples derived from the first read head prior to equalization; and

a second digital lowpass filter operable to filter the digital servo data samples derived from the second read head prior to equalization.

9. The servo system of claim 8 , further comprising:

a first zero error phase start detector circuit operable to detect an initial phase offset selected in the first interpolator based on an output of the first digital lowpass filter; and

a second zero error phase start detector circuit operable to detect an initial phase offset selected in the second interpolator based on an output of the second digital lowpass filter.

10. The servo system of claim 1 , wherein the first read head and the second read head comprise an array reader in a two dimensional magnetic recording system.

11. The servo system of claim 1 , wherein the phase tracking and signal combining circuit is operable to apply weighting factors to the most closely aligned signals from the plurality of first interpolated outputs and the plurality of second interpolated outputs before combining them to yield the combined servo data signal.

12. The servo system of claim 1 , further comprising a combined path peak tracking circuit operable to compare three input phases of the combined servo data signal to determine whether the first interpolator and the second interpolator should advance, delay or maintain an output phase.

13. The servo system of claim 1 , wherein the first and second equalizers, first and second interpolators and the phase tracking and signal combining circuit are embodied in an integrated circuit.

14. The servo system of claim 1 , wherein the servo system is embodied in a storage system.

15. A method for phase alignment in a two dimensional magnetic recording servo channel, comprising:

equalizing a first servo signal derived from a first read head to yield a first equalized signal;

equalizing a second servo signal derived from a second read head to yield a second equalized signal;

interpolating the first equalized signal to yield first interpolated signals at a plurality of different phase offsets;

interpolating the second equalized signal to yield second interpolated signals at a plurality of different phase offsets; and

selecting and combining most nearly phase aligned signals of the first interpolated signals and the second interpolated signals to yield a combined servo signal.

16. The method of claim 15 , further comprising searching for a servo address mark in the combined servo signal.

17. The method of claim 15 , further comprising selecting either the first interpolated signals or the second interpolated signals to be searched for the servo address mark instead of the combined servo signal.

18. The method of claim 15 , further comprising detecting a first zero error phase start signal based on the first servo signal, detecting a second zero error phase start signal based on the second servo signal, and selecting initial phase offsets for use in the interpolating of the first equalized signal and in the interpolating of the second equalized signal based at least in part on the first zero error phase start signal and on the second zero error phase start signal.

19. The method of claim 15 , further removing integer multiples of sample period delays in the first servo signal or the second servo signal.

20. A storage device, comprising:

a storage medium operable to store user data and servo data;

a head assembly disposed in relation to the storage medium and operable to read and write data on the storage medium; and

a servo system operable to combine signals from a plurality of read heads, the servo system comprising:

a first equalizer circuit operable to filter digital servo data samples derived from a first of the plurality of read heads to yield first equalized data;

a second equalizer circuit operable to filter digital servo data samples derived from a second of the plurality of read heads to yield second equalized data;

a first interpolator operable to interpolate the first equalized data to yield a plurality of first interpolated outputs at different phases;

a second interpolator operable to interpolate the second equalized data to yield a plurality of second interpolated outputs at different phases; and

a phase tracking and signal combining circuit operable to select and combine most closely aligned signals from the plurality of first interpolated outputs and the plurality of second interpolated outputs to yield a combined servo data signal.