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Systems and methods are disclosed for detection of a selected signal pattern, such as a servo sector preamble, and for frequency offset determination. A circuit may be configured to divide a signal into detection windows of a selected size, and sample the signal a selected number of times within eac

Systems and methods are disclosed for detection of a selected signal pattern, such as a servo sector preamble, and for frequency offset determination. A circuit may be configured to divide a signal into detection windows of a selected size, and sample the signal a selected number of times within each detection window. The circuit may then determine an error value for each detection window based on values of the samples for each detection window, and determine the preamble is detected when a threshold number of most-recently sampled detection windows have error values below a threshold value. The circuit may then organize the sample values corresponding to the preamble into groups, and calculate phase estimates representing a phase at which the groups were sampled. The circuit may determine a frequency offset based on the phase estimates, and modulate the sampling frequency according to the frequency offset.

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1. An apparatus comprising: a data storage device including: a disc data storage medium;a data channel configured to: receive a signal representative of data stored on the disc data storage medium;perform a preamble search operation to detect a preamble signal pattern within the signal, including: s

1. An apparatus comprising: a data storage device including: a disc data storage medium;a data channel configured to: receive a signal representative of data stored on the disc data storage medium;perform a preamble search operation to detect a preamble signal pattern within the signal, including: sample the signal to obtain a selected number of samples corresponding to a detection window;determine an error value for the detection window based on values of the selected number of samples;detect the preamble signal pattern when a selected proportion of most-recently sampled detection windows have error values below a threshold value; andidentify a head location relative to the disc data storage medium based on detecting the preamble signal pattern; andperform data access operations based on the head location. 2. The apparatus of claim 1 comprising the data channel further configured to: modulate a sampling frequency at which the signal is sampled, including: determine selected samples from detection windows having error values below the threshold value;calculate phase estimates representing a phase at which groups of the selected samples were sampled;determine a frequency offset based on the phase estimates; andmodulate the sampling frequency according to the frequency offset. 3. The apparatus of claim 2 comprising the data channel further configured to: determine a phase estimate for each group of selected samples;compare the phase estimates for consecutive groups; anddetermine the frequency offset based on the results of the comparisons of the phase estimates of the consecutive groups. 4. The apparatus of claim 3 comprising the data channel further configured to: compare the phase estimates for the consecutive groups by determining a difference between the phase estimates for the consecutive groups; anddetermine the frequency offset based on a summation of the differences between the phase estimates for the consecutive groups. 5. The apparatus of claim 4 comprising the data channel further configured to: perform a phase unwrapping operation on the differences between the phase estimates to obtain an unwrapped phase difference; anddetermine the frequency offset based on a summation of the unwrapped phase differences. 6. The apparatus of claim 5 comprising the data channel further configured to: once the frequency offset is below a frequency offset threshold, search for a known pattern preceding a sector stored to the disc data storage medium, including: continue to detect the preamble signal pattern;perform the preamble search operation again in the event the preamble signal pattern is lost;identify the known pattern when the preamble signal pattern is not lost; andread data from the sector when the known pattern is identified. 7. The apparatus of claim 6 further comprising: the known pattern is a servo timing mark; andthe sector is a servo sector. 8. The apparatus of claim 6 further comprising: the known pattern is a sync mark; andthe sector is a data sector. 9. The apparatus of claim 6 comprising the data channel includes: an analog to digital converter (ADC) configured to sample the signal at the sampling frequency based on a clock signal;a preamble detection module configured to: determine the error value for each detection window;determine whether the preamble is detected based on the error values of each preamble detection window;calculate the phase estimates;determine the frequency offset;modulate the sampling frequency; anda clock signal generator configured to provide the clock signal to the ADC. 10. The apparatus of claim 9 comprising the preamble detection module further configured to modulate the sampling frequency using an interpolation filter applied to a clock signal. 11. An apparatus comprising: a data storage device having: a disc data storage medium;a data channel configured to: receive a signal representative of data stored on the disc data storage medium;modulate a sampling frequency at which the signal is sampled, including: sample the signal to obtain sample values corresponding to a preamble signal pattern of a sector of the disc data storage medium;organize the sample values into groups based on detection windows in which the sample values were obtained, a detection window representing a selected number of samples obtainable in a set number of periods of the preamble signal pattern;calculate phase estimates representing a phase at which the groups were sampled;determine a frequency offset based on the phase estimates, the frequency offset representing a deviation of a rate at which the signal is being sampled per period relative to an expected sampling rate;modulate the sampling frequency according to the frequency offset to generate a modulated sampling frequency aligned with the expected sampling rate; andread the sector based on the modulated sampling frequency. 12. The apparatus of claim 11 comprising the data channel further configured to: perform a preamble search operation to detect the preamble signal pattern within the signal, including: determine an error value for each detection window based on values of the selected number of samples; anddetect the preamble signal pattern when a threshold proportion of most-recently sampled detection windows have error values below a threshold value. 13. The apparatus of claim 12 comprising the data channel further configured to: determine the error value for each detection window based on a deviation of the sample values from an expected waveform pattern of the preamble signal pattern. 14. The apparatus of claim 12 further comprising: the data channel includes: an analog to digital converter (ADC) configured to sample the signal at the sampling frequency based on a clock signal;a clock signal generator configured to provide the clock signal to the ADCa preamble detection module configured to: determine the error value for each detection window;detect the preamble signal pattern based on the error values of each detection window;calculate the phase estimates;determine the frequency offset; andmodulate the sampling frequency via modifying a rate of the clock signal produced by the clock signal generator. 15. The apparatus of claim 12 comprising the data channel further configured to: determine the preamble signal pattern is detected;continue to determine error values for detection windows after the preamble signal pattern is detected to determine when the preamble signal pattern is lost;determine the frequency offset in response to detecting the preamble signal pattern;search for a servo timing mark for a duration of a search window corresponding to a selected number of periods of the preamble signal pattern;extend the search window while the preamble signal pattern continues to be detected; anddo not extend the search pattern when the preamble signal pattern is no longer detected based on a number of the most-recently sampled detection windows having error values below the threshold value falling below the threshold proportion. 16. The apparatus of claim 11 comprising the data channel further configured to: calculate a phase estimate for each group of sample values;determine a difference between the phase estimates for each pair of consecutive groups; anddetermine the frequency offset based on a summation of the differences between the phase estimates for each pair of consecutive groups. 17. An apparatus comprising: a data channel configured to detect a selected pattern within a signal, including: sample the signal to obtain sample values;divide the sample values into a first set of groups;determine error values for the first set of groups;determine selected groups from the first set of groups that correspond to the selected pattern based on the selected groups having error values below a threshold; anddetect the selected pattern based on the selected groups. 18. The apparatus of claim 17 comprising the data channel further configured to: organize consecutive samples corresponding to the selected groups in a second set of groups;determine a frequency offset between a sampling rate at which the selected groups were sampled and an expected sampling rate based on the selected groups; andmodulate the sampling frequency based on the frequency offset. 19. The apparatus of claim 18 further comprising: the selected signal pattern is a preamble pattern of a servo sector recorded to a disc data storage medium;the data channel further configured to: detect a servo timing mark of the servo sector using the sampling frequency once it has been modulated;read the servo sector once the servo timing mark is detected; andidentify a location of a read head relative to the disc data storage medium based on the servo sector. 20. The apparatus of claim 19 further comprising: the disc data storage medium;the data channel further configured to: detect the selected signal pattern when a selected proportion of most-recently sampled groups from the first set of groups have error values below a threshold value;determine a phase estimate for each group of the selected groups representing a phase at which samples of the selected groups were sampled;compare the phase estimates for consecutive groups of the selected groups; anddetermine the frequency offset based on the phase estimates.