Continuously zoned servo preamble detection and frequency acquisition

1. An apparatus for storing data, comprising:

a storage medium comprising servo data regions and user data regions, the servo data regions comprising a preamble pattern, wherein servo data in the servo data regions is written with a varying clock frequency across the storage medium;

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

a preamble detection circuit configured to search for the preamble pattern in a plurality of frequency bins of an input stream derived from the head assembly.

2. The apparatus of claim 1 , wherein the servo data regions have a constant density across the storage medium.

3. An apparatus for storing data, comprising:

a storage medium comprising servo data regions and user data regions, the servo data regions comprising a preamble pattern, wherein servo data in the servo data regions is written with a varying clock frequency across the storage medium;

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

a preamble detection circuit configured to search for the preamble pattern in a plurality of frequency bins of an input stream derived from the head assembly, wherein the preamble detection circuit comprises a plurality of preamble energy integrators configured to calculate an in-band energy in each of the plurality of frequency bins.

4. The apparatus of claim 3 , wherein the preamble detection circuit comprises an in-band energy selector configured to select an output of one of the plurality of preamble energy integrators having a largest amplitude as a largest in-band energy.

5. The apparatus of claim 4 , wherein the preamble detection circuit is configured to detect the preamble pattern based at least in part on said largest in-band energy.

6. The apparatus of claim 5 , wherein the preamble detection circuit is configured to detect the preamble pattern based on a ratio between the largest in-band energy and the out-of-band energy.

7. The apparatus of claim 3 , further comprising a frequency offset calculator configured to calculate a frequency offset between an expected servo frequency and an actual servo frequency.

8. The apparatus of claim 7 , wherein the frequency offset calculator is configured to calculate the frequency offset based on a phase difference at a beginning of a preamble detection and at an end of the preamble detection.

9. The apparatus of claim 7 , wherein the frequency offset calculator is configured to calculate the frequency offset based on a magnitude of an output of each of the plurality of preamble energy integrators.

10. The apparatus of claim 9 , wherein the frequency offset calculator comprises a lookup table storing frequency offset values, indexed with the magnitude of the output of each of the plurality of preamble energy integrators as a combined index.

11. The apparatus of claim 7 , wherein the frequency offset calculator is configured to apply an update gain to the frequency offset.

12. The apparatus of claim 7 , further comprising a frequency update decision circuit configured to output the frequency offset to a clock synthesizer while the preamble detection circuit detects the preamble in the input stream.

13. The apparatus of claim 7 , further comprising a successive preamble spacing detector configured to detect when the preamble pattern has been detected at least twice in succession at an expected interval before indicating that the preamble has been detected.

14. A method for preamble detection and frequency acquisition in a continuously zoned servo magnetic storage system, comprising:

generating a sample stream from a read head in the magnetic storage system;

calculating a first in-band energy of the sample stream at an expected servo frequency;

calculating a second in-band energy of the sample stream at a negative offset from the expected servo frequency;

calculating a third in-band energy of the sample stream at a positive offset from the expected servo frequency;

selecting the first in-band energy, the second-in-band energy or the third in-band energy having a largest amplitude as the largest in-band energy; and

detecting a preamble pattern in the sample stream based at least in part on the largest in-band energy.

15. The method of claim 14 , further comprising calculating a frequency offset based on the first in-band energy, the second-in-band energy and the third in-band energy.

16. The method of claim 15 , wherein calculating the frequency offset comprises calculating a phase difference between a beginning of the preamble pattern and an end of the preamble pattern.

17. The method of claim 15 , wherein calculating the frequency offset comprises deriving the frequency offset based on a relative magnitude of the first in-band energy, the second-in-band energy and the third in-band energy.

18. The method of claim 14 , further comprising adjusting a clock frequency based on the frequency offset when the preamble pattern is detected.

19. The method of claim 14 , further comprising determining that the preamble pattern has been detected at least twice in succession at an expected interval between instances of the preamble pattern.

20. A storage device, comprising:

a storage medium;

a head assembly disposed in relation to the storage medium;

means for detecting a preamble pattern in a sample stream derived from the head assembly using a plurality of frequency bins; and

means for calculating a frequency offset between an expected servo frequency and an actual servo frequency based on in-band energy values calculated in each of the plurality of frequency bins.