Abstract:
Aspects of the disclosure provide a method for recording. The method includes receiving a tracking signal corresponding to a recording track on a storage medium, phase-locking an internal signal to the tracking signal by reducing a phase error generated based on the internal signal and the tracking signal, generating a phase bias, combining the phase bias into the phase error to phase-shift the internal signal from the tracking signal, and recording on the recording track based on the internal signal.

Description:
INCORPORATION BY REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 11/925,258 filed Oct. 26, 2007, which claims the benefit of U.S. Provisional Application No. 60/863,487, “METHOD AND APPARATUS TO CORRECT WOBBLE PHASE SLIP IN OPTICAL RECORDERS” filed Oct. 30, 2006. The disclosures of the prior applications are incorporated herein by reference in their entirety. 
     BACKGROUND 
     Generally, data can be stored on a recording layer of an optical disc by forming either data pits or data marks on tracks arranged in an interleaved spiral or concentric circles. A path of the track can be continuously modulated to deviate from its centerline in the radial direction. Such modulation about a centerline is generally referred to as wobble. Wobble can be utilized to store information, such as address and location, via various techniques. Techniques include pre-pit, phase modulation and frequency modulation. 
     The wobble can be converted into wobble signal by an optical apparatus. The wobble signal can be a sinusoidal modulated voltage signal that can be used in an optical disc recording system to provide the address and location information for recording. 
     SUMMARY 
     The present disclosure can provide a method and an apparatus to correct wobble phase slip in an optical disc recording apparatus. Such phase slip can occur during, for example, recording. 
     Aspects of the disclosure can be directed to a method and an apparatus of correcting a phase slip detected in a recording apparatus. The method of correcting a phase slip can include receiving a wobble signal of a first frequency, sampling the wobble signal to generate a sampled wobble signal, where the wobble signal is sampled at a second frequency that corresponds to a frequency of a sampling signal, comparing a phase of the sampled wobble signal with a phase of a controlled signal to generate a phase error, modifying the phase error by adding a phase bias, and adjusting the second frequency based on the modified phase error to reduce a magnitude of the modified phase error. 
     The present disclosure can also provide a method of correcting the phase slip, where the wobble signal is a sinusoidal signal and coded with information. The information can be coded using at least one of phase modulation, frequency modulation and pre-pitting. The method can also include decoding information in the wobble signal, and detecting a phase slip based on the information decoded. Additionally, the method can also include determining a number of wobble signal cycles required to correct the detected phase slip based on the decoded information. According to the present disclosure, adding the phase bias can also include changing a magnitude of the phase bias to a target magnitude at a slow average rate. 
     Aspects of the present disclosure can provide a method of correcting the phase slip, where comparing the phase of the sampled wobble signal with the phase of the controlled signal to generate the phase error can include generating a sine signal having the first frequency, multiplying the sampled wobble signal with the sine signal to generate a first multiplied signal, integrating the first multiplied signal over one wobble cycle to generate a quadrature component of the phase error, generating a cosine signal having the first frequency, multiplying the sampled wobble signal with the cosine signal to generate the second multiplied signal, and integrating the second multiplied signal over one wobble cycle to generate an in-phase component of the phase error, and detecting the phase error as a function of the quadrature and the in-phase components of the phase error. Furthermore, the method can include calculating the phase error by an arctangent function of the quadrature and the in-phase components. 
     The present disclosure can also provide a method of correcting phase slip where modifying the phase error by adding the phase bias can further include generating the phase bias, and producing a generated phase error by combining the phase bias and the phase error. The method can also include changing the magnitude of the phase bias to a target magnitude at a slow average rate. The method can further include detecting an unstable situation, and adjusting the modified phase error if the unstable situation is detected. The method of detecting the unstable condition can further include checking a first condition if the magnitude of the phase bias is larger than a predetermined threshold, and checking a second condition if a sign of the phase error and a sign of the phase bias are the same. Additionally, the method can also include adjusting the generated phase error by a magnitude of 2π or adjusting the generated phase error by subtracting the phase bias. 
     The present disclosure can also describe a method of correcting phase slip where modifying the phase error by adding the phase bias can further include shifting a phase of the controlled signal with the phase bias and changing the phase bias to a target magnitude at a slow average rate. 
     Aspects of the disclosure can also include shifting a phase of the sine signal with the phase bias, shifting a phase of the cosine signal with the phase bias and changing the phase bias to a target magnitude at a slow average rate. 
     Additionally, the present disclosure can direct to a method of correcting phase slip that can also include using a sine lookup table to store a cycle of the sine signal, reading from the sine lookup table cyclically according to a first sequence, using a cosine lookup table to store a cycle of the cosine signal, reading from the cosine lookup table cyclically according to a second sequence, shifting the first sequence by an integer number, and shifting the second sequence by the integer number. Further more, the method can also include calculating the integer number according to the phase bias, and changing the phase bias to a target magnitude at a slow average rate or adjusting the magnitude of the integer number from 0 to a target number at a slow average rate. 
     The present disclose can provide an apparatus for correcting a phase slip. The apparatus can include an analog to digital converter (ADC) that can sample a received wobble signal having a first frequency to generate a sampled wobble signal, where the wobble signal can be sampled at a second frequency that can correspond to a frequency of a sampling signal, a wobble demodulator that can generate a phase error based on a difference between a phase of the sampled wobble signal and a phase of a controlled signal, a phase bias generator that can generate a phase bias, the phase bias being used to modify the phase error, and a voltage controlled oscillator (VCO) that can modify the second frequency based on the modified phase error. The wobble signal can be a sinusoidal signal and coded with information, where the information can be coded using at least one of phase modulation, frequency modulation and pre-pitting. A controller can decode the information in the wobble signal and detect a phase slip based on the information decoded. The controller can also determine a number of wobble signal cycles required to correct the detected phase slip based on the decoded information. The phase bias generator can change the phase bias to a target magnitude by a slow average rate. 
     The wobble demodulator in the apparatus can further include a sine signal generator that can generate a sine signal having the first frequency, a multiplier that can multiply the sampled wobble signal with the sine signal to generate a first multiplied signal, an integrator that can integrate the first multiplied signal over one wobble cycle to generate a quadrature component of the phase error, a cosine signal generator that can generate a cosine signal having the first frequency, a multiplier that can multiply the sampled wobble signal with the cosine signal to generate a second multiplied signal, an integrator that can integrate the second multiplied signal over one wobble cycle to generate an in-phase component of the phase error, and a phase detector that can generate the phase error as a function of the quadrature and the in-phase components. Additionally, the phase detector can use an arctangent function to calculate the phase error. 
     The apparatus of correcting the phase slip can also include an adder that combines the phase bias to the phase error to produce a modified phase error. The adder can adjust the modified phase error when an unstable situation is detected. The apparatus can also include a timing loop filter that can transmit the modified phase error into a voltage; the voltage can control the VCO to adjust the frequency of the sampling signal. 
     The apparatus of correcting the phase slip can also include a sine lookup table that can store a cycle of the sine signal that the sine signal generator can provide a value from the sine lookup table according to a first sequence, and a cosine lookup table that can store a cycle of the cosine signal that the cosine signal generator can provide a value from the cosine lookup table according to a second sequence. The sine signal generator can adjust the first sequence according to the phase bias, and the cosine signal generator can adjust the second sequence according to the phase bias. 
     The present disclosure can also describe an apparatus of correcting phase slip where the phase detector can adjust the quadrature and the in-phase components according to the phase bias before generating the phase error. 
     Aspects of the disclosure can provide a data recording system. The data recording system can include a tracking apparatus that can follow a track of a storage medium and receive a wobble signal, a recording apparatus that can present data on the track of the storage medium, an analog to digital converter (ADC) that can sample the received wobble signal having a first frequency to generate a sampled wobble signal, where the wobble signal is sampled at a second frequency that corresponds to a frequency of a sampling signal, a wobble demodulator that can generate a phase error based on a difference between a phase of the sampled wobble signal and a phase of a controlled signal, a phase bias generator that can generate a phase bias, the phase bias being used to modify the phase error, and a voltage controlled oscillator (VCO) that can modify the second frequency based on the modified phase error. 
     Furthermore, the data recording system can also include a sine signal generator that can generate a sine signal having the first frequency, a multiplier that can multiply the sampled wobble signal with the sine signal to generate a first multiplied signal, an integrator that can integrate the first multiplied signal over one wobble cycle to generate a quadrature component of the phase error, a cosine signal generator that can generate a cosine signal having the first frequency, a multiplier that can multiply the sampled wobble signal with the cosine signal to generate a second multiplied signal, an integrator that can integrate the second multiplied signal over one wobble cycle to generate an in-phase component of the phase error, and a phase detector that can generate the phase error as a function of the quadrature and the in-phase components. 
     Additionally, the data recording system can include a sine lookup table that can store a cycle of the sine signal that the sine signal generator can provide a value from the sine lookup table according to a first sequence, a cosine lookup table that can store a cycle of the cosine signal, and the cosine signal generator can provide a value from the cosine lookup table according to a second sequence. The apparatus can also include an adder that can combine the phase bias to the phase error to produce a modified phase error, and can adjust the modified phase error when an unstable situation is detected. Within the apparatus, the sine signal generator can adjust the first sequence according to the phase bias, and the cosine signal generator can adjust the second sequence according to the phase bias. 
     According to another aspect of the disclosure, the phase detector in the apparatus can calculate the phase error by an arctangent function of the quadrature and the in-phase components and can adjust the quadrature and the in-phase components according to the phase bias before the calculation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of this disclosure will be described in detail with reference to the following figures, wherein like numerals reference like elements and wherein: 
         FIG. 1  is a diagram showing an exemplary surface of an optical disc with a spiral track; 
         FIG. 2  is a diagram showing an exemplary wobble timing loop; 
         FIG. 3  is a diagram showing an exemplary wobble demodulator; 
         FIG. 4  is a diagram showing an exemplary characteristic of a wobble demodulator; 
         FIG. 5  is a diagram showing an exemplary timing loop of wobble phase correction by adding a phase bias to a phase error; 
         FIG. 6  is a diagram showing an exemplary timing loop of wobble phase correction by adding the phase bias in the wobble demodulator; 
         FIG. 7  is a diagram showing an exemplary timing loop of wobble phase correction by adding the phase bias in a phase detector; and 
         FIG. 8  is a flow chart outlining an exemplary process of wobble phase correction. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Storage media that can continually store data in a physical media, such as an optical disc, can be very useful because of its relative high storage capability. 
     Even though the recording is continuous, address information may be needed to improve the efficiency of recording. For example,  FIG. 1  is a diagram showing an exemplary surface of an optical disc  100  with a spiral track  105 . On the spiral track  105 , data can be stored on a recording layer by forming either data pits or data marks. During recording, the disc is usually spinning around its center with an angular velocity instructed by a controller. Recording data with a constant linear length of pits and marks is preferred to improve the data storage capability of the optical disc. To assist in maintaining constant length of marks and pits, timing and address information can be encoded in the spiral tracks. The address at a location can be used to guide where to write data to the disc. 
     One technique to code timing and address information is to wobble the track. Wobble can be referred to a continuously sinusoidal deviation of the track from an average centerline. The variation of the track is in the radial direction. Wobble can be utilized to code the address information via various techniques, such as pre-pits, frequency modulation, and phase modulation. The length of a wobble period can be used to determine the recording clock. Normally, one address is given to a group of predetermined number of wobbles. Inside the group, sequential numbers can be used to represent the locations of each wobble in the group. Inside each wobble, there can be numerous channel bit intervals depending on the specification of the optical media. The location of each channel bit interval can be correlated to the phase of the physical wobble. 
     Wobble, which can be the spatial sinusoidal deviation of a physical track, can be converted to an electrical voltage signal, which is sinusoidally modulated. The conversion can be handled by an optical apparatus in a recorder that tracks the wobble using a constant linear velocity. Therefore, the phase of the physical wobble, which is related to the location, can be converted into phase of the wobble signal, which is related to timing. In the optical disc recording system, the wobble signal can serve as an input to a synchronization mechanism to synchronize with another input, which is a signal from an internal oscillator that can be controlled by the recorder. Therefore, the timing of the internal oscillator can be associated with the location of the physical wobble. The internal oscillator can be used to guide the timing of recording. Based on the timing information of the internal oscillator, data can be written on the appropriate location referring to the physical wobble. 
     Occasionally, due to defects, physical disturbance of the recorder or other distortions, the lock to the physical wobble may be lost temporarily. When the lock to the physical wobble is recovered, the phase of the internal oscillator may not be correctly correlated to the location of the physical wobble. There may exist one or more cycles of difference. The discrepancy in the phase of the internal oscillator and the location of the physical wobble is referred as wobble phase slip. 
     Wobble phase slip can cause various types of problems. One particular problem can happen during data recording. Occasionally, the recording process may stop, and then continue. If there exists a phase slip, for example the internal oscillator signal is one wobble cycle behind the physical wobble when the stop happens. When the recording process continues, the optical apparatus tracks the phase of the physical wobble for another time to find out the previous stopping location. Somehow, there may be no wobble phase slip at this time. Because the phase of the internal oscillator is used to record and track the stopping location, the recording can continue one cycle behind the real stopping location. Consequently, the previous written data in the last physical wobble cycle can be overwritten or destroyed. To avoid data damaging, it is desirable that the correction of wobble phase slip can be achieved during the recording session if phase slip happens. In this way, when the recording stops, the internal oscillator signal can record the same phase as the physical wobble. 
     For a general understanding of the features of the present disclosure, reference is made to the optical disc as a specific type of continuous recording storage media for the sake of clarity, familiarity, and ease of description. However, it should be appreciated that the method and apparatus disclosed herein, as discussed below, can be equally applied to any known or later-developed continuous recording storage media. 
     The present disclosure proposes that wobble phase slip can be recovered by artificially adding a phase bias in a synchronization mechanism. 
     As described above, a synchronization mechanism can be used to synchronize an internal oscillator signal and a wobble signal. When the two signals are synchronized, a phase error of the internal oscillator signal and the wobble signal is around zero. When the phase error is not zero, the synchronization mechanism can adjust an offset in order to correct the phase error. After some time, the phase error can return to around zero. According to the present disclosure, the synchronization mechanism can be manipulated by intentionally adding a phase bias. The phase bias can trick the synchronization mechanism that there exists a phase error, so the synchronization mechanism can work by itself to synchronize the internal oscillator and the wobble signal with the desired phase bias. 
     Among the various synchronization mechanisms, a phase lock loop can be used.  FIG. 2  is a diagram showing an exemplary wobble timing loop  200 , which is a phase lock loop using digital signal processing technology. The wobble timing loop  200  can lock the phase of an internal oscillator signal to the phase of an input signal, such as a wobble signal  210 . 
     The wobble timing loop  200  can include an analog to digital converter (ADC)  220 , a wobble demodulator  240 , a timing loop filter  260  and a voltage controlled oscillator (VCO)  280 , coupled together as shown. The ADC  220 , wobble demodulator  240 , timing loop filter  260  and VCO  280  can cooperatively work together to lock the phase of the internal oscillator to the phase of the wobble signal  210 . 
     As mentioned above, the wobble signal  210  can be a continuous analog voltage signal transmitted by an optical apparatus that is capable of tracking the physical wobble. Generally, the optical apparatus can track the physical wobble by a constant linear velocity, and transmit the phase of the physical wobble into the phase of the wobble signal  210 . So, the phase of the wobble signal can move forward with a constant velocity. When the physical wobble is sinusoidally modulated, the wobble signal  210  can be a sinusoidally modulated voltage signal. 
     The ADC  220  can convert the wobble signal  210  into a discrete wobble signal  230  with aid from a sampling signal  290 . When the wobble signal  210  is a sinusoidally modulated continuous analog signal, the discrete wobble signal  230  can be a sinusoidally modulated discrete digital signal. Generally, the sampling signal  290  has a nominal frequency f 0 , which can be L times of the frequency of the wobble signal  210 . The frequency of the sampling signal  290  may change corresponding to the other components of the timing loop  200 . The frequency change of the sampling signal can result in the phase change of the discrete wobble signal  230 . When the sampling frequency is larger, the phase of discrete digital signal can move forward slower, and when the sampling frequency is smaller, the phase of the discrete digital signal can move forward faster. So adjusting the frequency of the sampling signal  290  can shift the phase of the discrete wobble signal  230 . 
     The wobble demodulator  240  can be designed to compare the phase of the discrete wobble signal  230  to the phase of an internal oscillator (not shown) that can be controlled by the recorder, and output a phase error signal  250  representing a phase difference. An exemplary demodulator will be described in detail below. 
     The timing loop filter  260  can output a voltage signal  270  based on the phase error signal  250 . For example, when the average of the phase error signal  250  is around zero, the timing loop filter  260  can output a voltage signal of value V 0 . When the average of the phase error signal is larger than zero, the timing loop filter  260  can output the voltage signal  270  of value larger than V 0 . When the average of the phase error signal is smaller than zero, the timing loop filter  260  can output the voltage signal  270  of value smaller than V 0 . 
     The VCO  280  can generate the sampling signal  290  with a frequency controlled by the voltage signal  270 . For example, when the voltage signal  270  is of value V 0 , the frequency of the sampling signal  290  can be the nominal frequency f 0 . When the voltage signal  270  is of value larger than V 0 , the frequency of the sampling signal  290  can be larger than f 0 . When the voltage signal  270  is of value smaller than V 0 , the frequency of the sampling signal  290  can be smaller than f 0 . In addition, the nominal frequency f 0  can be L times of the frequency of the wobble signal  210 , so that there are L sampling points in one wobble signal cycle. When the frequency of the sampling signal  290  keeps at the nominal frequency, the L sampling points in each wobble cycle can be evenly spaced. Consequently, the phase of the discrete wobble signal  230  can move forward with a consistent velocity. 
     The components in the wobble timing loop  200  can cooperatively work together to drive the phase error signal  250  to be around zero. When the phase error signal  250  is zero, which can mean that there is no phase difference between the discrete wobble signal  230  and the internal oscillator signal. This further means that the wobble signal  210  is sampled at the right sampling points with the aid of the sampling signal  290 . The timing loop filter  260  can output a voltage signal of value V 0 . The VCO  280  can continuously generate the sampling signal  290  of frequency f 0  to keep sampling the wobble signal  210  at the same intervals in each wobble cycle. Therefore, the phase of the discrete wobble signal  230  can keep moving forward at the same rate as the internal oscillator signal, such that the phase error signal  250  can be kept around zero. 
     When phase error signal  250  is not zero, this can mean that the phase of the discrete wobble signal  230  and the phase of the internal oscillator signal are not the same. For example, the discrete wobble signal  230  can be ahead of the internal oscillator signal. Consequently, the timing loop filter  260  can output the voltage signal  270  of value V that can be larger than V 0 . Based on larger voltage signal  270 , the VCO  280  can generate the sampling signal  290  of frequency f that can be larger than f 0 . Since larger sampling frequency can slow down the phase of the discrete wobble signal  230 , so the phase difference of the discrete wobble signal  230  and the internal oscillator signal can become smaller. If the phase difference still exists, the timing loop  200  can work the similar way as described above to continuously reduce the difference. After some time, the phase difference can be close enough to zero. Finally, the phase error signal  250  can return to around zero. 
     The wobble demodulator  240  can be designed via various techniques to compare the phases.  FIG. 3  is a diagram showing an exemplary wobble demodulator. The exemplary wobble demodulator is a digital quadrature demodulator  240 A. The digital quadrature demodulator  240 A can generate a phase error  250  based on the phase difference of the discrete wobble signal  230  and an internal oscillator. The digital quadrature demodulator  240 A can utilize two parallel signal processing paths to calculate a quadrature component and an in-phase component of the phase difference. Then a phase detector can generate the phase error  250  based on the quadrature and the in-phase components of the phase difference. 
     The path to calculate the quadrature component can include three components: a sine signal generator  315 , a multiplier  305  and an integrator  325 . 
     The sine signal generator  315  can cyclically provide a sine signal  320  of one cycle to the multiplier  305 . Generally, the sine signal  320  can be controlled to have the same frequency as the wobble signal  210 . Therefore, the sine signal generator  315  can be treated as an internal oscillator that can oscillate at a controlled frequency, which can be the same as the wobble signal  210 . 
     The multiplier  305  can multiply the discrete wobble signal  230  with the sine signal  320 , and provide an output signal  310  to the integrator  325 . Both the sine signal  320  and the discrete wobble signal  230  can have about the same frequency as the wobble signal  210 . Because of that, the output signal  310  can have two parts, a high frequency part and a low frequency part. The high frequency part can generally have a frequency that can be about twice the frequency of the wobble signal  210 . The low frequency part can be proportional to a cosine value of a phase difference of the discrete wobble signal  230  and the sine signal  320 . 
     The integrator  325  can integrate the signal  310  over one cycle of the wobble signal. The integration of the high frequency part over one wobble cycle can be zero. Therefore, the integrator  325  can operate like a low pass filter that can average out the high frequency part and output a low frequency signal  330 . The low frequency signal  330 , which can be proportional to the cosine value of the phase difference, is a quadrature component of the phase difference. 
     The path to calculate the in-phase component can also include three components: a sine signal generator  345 , a multiplier  335  and an integrator  355 . 
     The sine signal generator  345  can cyclically provide a cosine signal  350  of one cycle to the multiplier  335 . Generally, the cosine signal  350  can be controlled to have the same frequency as the wobble signal  210 . Therefore, the cosine signal generator  345  can be treated as an internal oscillator that can oscillate at a controlled frequency, which can be the same as the wobble signal  210 . 
     Generally the cosine signal  350  and the sine signal  320  can have the same phase and frequency. The phase can be referred as a phase of the internal oscillator. The frequency can be referred as a frequency of the internal oscillator. 
     The multiplier  335  can multiply the discrete wobble signal  230  with the cosine signal  350 , and provide an output signal  340  to the integrator  355 . Both the cosine signal  350  and the discrete wobble signal  230  can have about the same frequency as the wobble signal  210 . Because of that, the output signal  340  can have two parts, a high frequency part and a low frequency part. The high frequency part can generally have a frequency that can be about twice the frequency of the wobble signal  210 . In addition, the low frequency part can be proportional to a sine value of the phase difference of the discrete wobble signal  230  and the cosine signal  350 . 
     The integrator  355  can integrate the signal  340  over one cycle of the wobble signal. The integration of the high frequency part over one wobble cycle can be zero. Therefore, the integrator  355  can operate like a low pass filter that can average out the high frequency part and output a low frequency signal  360 . The low frequency signal  360 , which can be proportional to the sine value of the phase difference, is the in-phase component of the phase difference. 
     Both the quadrature component  330  and the in-phase component  360  can be received by a phase detector  365 . The phase detector  365  can generate a phase error signal  250  based on the analysis of the quadrature component  330  and in-phase component  360 . The phase error signal  250  can be related to the phase difference of the discrete wobble signal  230  and the phase of the internal oscillator  315  or  345 . Various techniques can be used in the phase detector to calculate the phase error signal  250 . For example, the phase error signal  250  can be calculated by an arctangent function atan(Q, I) with the quadrature component  330  (Q) and the in-phase component  360  (I) as the inputs to the function. In such an example, the phase error signal  250  is an angle in the range of [−π, π]. This angle can be the co-terminal angle of the real phase difference of the discrete wobble signal  230  and the internal oscillator  315  or  345 . 
       FIG. 4  is a diagram showing a characteristic of the exemplary wobble demodulator  240 A. A series of parallel lines having a slope of about one can represent a response of the wobble demodulator  240 A. The phase error  250 , which is the output of the wobble demodulation, can be in the range of [−π, π], no matter the range of the real phase difference. 
     When the real phase difference of the discrete wobble signal  230  and the internal oscillator signal is in the range of [−π, π] the phase error  250  can be the same as the real phase difference, the relationship of the phase error  250  and the real phase difference can be represented by demodulator response line  410 . 
     When the real phase difference of the discrete wobble  230  and the internal oscillator signal is out of the range of [−π, π], the phase error  250  is the co-terminal angle of the real phase difference in the range of [−π, π]. For example, if the real phase difference of the discrete wobble signal  230  and the internal oscillator is in the range of [−3π, −π], the response of the wobble demodulator  240 A can be represented by demodulator response line  430 . As can be seen, the phase error  250  is in the range of [−π, π], and can be larger than the real phase difference by 2π. If the real phase difference is in the range of [π, 3π], the corresponding response of the wobble demodulator  240 A can be represented by line  420 . As can be seen, the phase error  250  is still in the range of [−π, π], and can be less than the real phase difference by 2π. 
     If a demodulator having the characteristic as shown in  FIG. 4 , such as the demodulator  240 A, is used in the timing loop  200 , the timing loop  200  can synchronize the discrete wobble signal  230  and the internal oscillator to have zero phase error. In another word, the timing loop  200  by itself can pull the response of the demodulator moving along a response line from an initial response to a destination response that is on the X axis, such as response  440 , response  490  and response  425 . The response line can be the line that includes the initial response. The destination response can be a response having zero phase error. For example, a response line  410  can include a response  405 , which is the initial response. The timing loop  200  can pull the response of the demodulator  240 A moving along the response line  410  from the response  405  to the response  490 , which is zero phase error and zero real phase difference. 
     However, when the two signals are synchronized to have zero phase error, the real phase difference can be non-zero, such as 2π or −2π. The real phase difference can be a value of an integer times 2π. 
     For another example, when the real phase difference is outside range of [−π, π], such as an initial response  415  which is belong to a response line  430 . The timing loop  200  can pull the demodulator response along the response line  430  moving to the destination response  440 , which is zero phase error but −2π phase difference, equivalent to one wobble cycle. 
     As can be seen, although the timing loop  200  can lock the phase of the discrete wobble signal  230  and the internal oscillator signal to have zero phase error, the real phase difference of the two signals may be one or more wobble cycles. This can be referred to as wobble phase slip. The present embodiments can correct the wobble phase slips knowing the number of cycles of wobble phase slip. 
     Generally, a controller in the optical recording system can expect information in the physical wobble. When the expected information arrives at a different wobble cycle, the controller can detect that wobble phase slip occurred, and can determine a number of wobble cycles that need to be corrected. The controller can then trigger a phase slip correction process to correct the phase slip. 
       FIG. 5  is a diagram showing a timing loop  500  using an exemplary method to correct the wobble phase slips. The timing loop  500  in  FIG. 5  is similar to the timing loop  200 , except instead of using the phase error signal  250  as the input to the timing loop filter  260 , the timing loop  500  can use a modified phase error signal  530 . The modified phase error  530  can be provided by an adder  510 , which can add the phase error signal  250  with a phase bias signal  520 . The phase bias signal  520  can be generated by a phase bias generator  540  under the control of a controller  550 . 
     Instead of driving the phase error signal  250  to zero, the timing loop  500  can drive the modified phase error signal  530  to zero. Under normal working condition, which can be, wobble phase slip free, the phase bias signal  520  can be zero. Then the timing loop  500  is similar to the timing loop  200 , and can be used as the timing loop  200 . When the wobble phase slip presents, the phase bias generator  540  can be triggered to generate the phase bias signal  520  according to an instruction signal from the controller  550 . Because the modified phase error signal  530  is the sum of the phase error signal  250  and the phase bias signal  520 . To make the modified phase error  530  to be zero, the phase error signal  250  can be the opposite of the phase bias signal  520 . Accordingly, the real phase difference can change too. 
     An exemplary method for wobble phase slip correction can be explained with reference to  FIG. 4  and  FIG. 5 . 
     When a wobble phase slip presents, the real phase difference can be non-zero, but the phase error can be zero. For example, the physical wobble can be one wobble cycle ahead of the internal oscillator, then the discrete wobble signal  230  can be one cycle ahead of the internal oscillator. Thus, the real phase difference can be 2π, and the phase error signal  250  can be zero, as indicated by a response  425  on the  FIG. 4 . 
     Initially, when the phase bias signal  520  is zero, the modified phase error signal  530  is a sum of the phase error signal  250  and the phase bias signal  520 , so can be zero. 
     After the wobble phase slip has been detected, the phase bias generator  540  can be triggered to generate a positive phase bias  520  that is added to the phase error  250 . Thus the modified phase error  530 , which is the sum of the phase error signal  250  and the phase bias signal  520 , can also be positive. As describe above, the timing loop filter  260  can generate a voltage signal  270  of value V that is larger than the nominal value V 0 , and the VCO  280  can generate a sampling signal  290  of a frequency f that is larger than the nominal frequency f 0 . Consequently, larger sampling frequency can slow down the discrete wobble signal  230 , and make the real phase difference to be smaller. 
     More specifically, the phase bias signal  520  can be π/4. Initially, the demodulator response can be around response  425 , which has 2π real phase differences, and zero phase error. So the modified phase error  530  can be π/4. 
     As described above, the timing loop  500  can pull the demodulator response along a response line to make the modified phase error signal  530  to be zero. In this example, in order to make the modified phase error signal  530  to be zero, the phase error signal has to be negative to compensate for the positive phase bias signal  520 . So the timing loop  500  can pull the demodulator response along the response line  420 , which is the response line including the initial response  425 , in a direction of negative phase error to a destination response  455 , which has −π/4 phase error, and 7π/ 4 real phase difference. Accordingly, the modified phase error  530  can be zero when the demodulator response is at the destination response  455 . 
     Theoretically, changing the phase bias  520  slowly from 0 to 2π, the real phase difference can move from 2π to zero. Accordingly, the wobble phase slip can be corrected. 
     Practically, when the phase bias  520  approaches π, the timing loop  500  can work unstably. As can be seen, the demodulator response line  420  breaks at a response  460 , where the phase error is −π. When the phase bias  520  approaches π, the phase error signal  250  can approaches −π, and the modified phase error  530  is zero. However, because of noise or disturbance, the phase error signal  250  can be π, which is represented by response  470 . This can make the modified phase error  530  to be 2π instead of zero. When the phase error signal jumps between π and −π, the timing loop performance may be unstable. 
     Various techniques can be used to remedy this situation. One technique is based on the observation that the sign of the phase error signal  230  changes when the situation occurs. When the sign of the phase error  230  changes, the modified phase bias  530  can be adjusted, such as setting the phase bias signal  520  to be zero, or using equation 1: 
                     φ   modified     =     {             φ   error     +     φ   bias     -     2   ⁢   π               if   ⁢           ⁢     φ   error       &gt;     0   ⁢           ⁢   and   ⁢           ⁢     φ   bias       &gt;     π   /   2                   φ   error     +     φ   bias     +     2   ⁢   π               if   ⁢           ⁢     φ   error       &lt;     0   ⁢           ⁢   and   ⁢           ⁢     φ   bias       &lt;       -   π     /   2                   φ   error     +     φ   bias           otherwise         }             (   1   )               
where φ error  is the phase error signal  250 , φ bias  is the phase bias signal  520 , and the φ modified  is the modified phase error signal  530 .
 
     By this way, the modified phase error  530  can be adjusted to avoid the unstable situation. Accordingly, the system can be made stable, and the real phase difference can be continuously pulled from 2π to 0 if the phase bias  520  changes from 0 to 2π. 
     A simplified conceptual example can be used to describe the phase slip correction process according to the present embodiments. Assumptions can be made that each wobble cycle can include four channel bit intervals. When wobble phase slip occurs, such as the physical wobble is one cycle ahead of the internal oscillator signal, a phase bias can be added in the timing loop to make the timing loop  500  to adjust the sampling frequency to be higher, so to slow down the discrete wobble signal. Then one wobble cycle can include more channel bit intervals, such as six channel bit intervals. Thus, two wobble cycles can include 12 channel bit intervals, which can be equivalent to three cycles of the internal oscillator. Consequently, after two wobble cycles of the adjustment, the internal oscillator signal can catch up the wobble signal. The timing loop can then return to the normal condition. 
       FIG. 6  is a diagram showing another exemplary timing loop  600  of wobble phase correction. Instead of adding phase bias in the phase error signal  250  in the above example, the phase bias can be added in the wobble demodulator. As illustrated in  FIG. 6 , phase bias  520  can be added in the sine and cosine signal generators  315  and  345 . As described above, the sine and cosine signal generator can perform the function of an internal oscillator. The phase error signal  250  outputted from the wobble demodulator  240 A can be related to the real phase difference of the discrete wobble signal  230  and the internal oscillator. The timing loop  600  can keep the phase error signal to around zero. 
     The exemplary method can be explained using a similar example as previous method. In the example, initially, the phase of the discrete wobble signal  230  is more than the phase of the internal oscillator by 2π. Therefore, the real phase difference can be 2π, and the phase error can be zero, which can be represented by point  425  in  FIG. 4 . 
     The method can begin with adding a negative phase bias in the internal oscillator, which includes both the sine signal generator  315  and the cosine signal generator  345 . This can result in a non-zero phase error signal  250 . The timing loop  600  can keep the phase error signal  250  to be around zero. In order for the phase error signal  250  to be around zero, the timing loop  600  can shift the phase of the discrete wobble signal  230  by a negative value about the same as the negative phase bias. Therefore, the real phase difference of the discrete wobble signal  230  and the internal oscillator can also change from 2π by the negative value. Slowly changing the phase bias from 0 to −2π, the timing loop  600  can shift the phase of the discrete wobble signal  230  by −2π, thus the real phase difference of the discrete wobble signal  230  and the internal oscillator can change from 2π to 0. Accordingly, the wobble phase slip can be corrected. 
     It is noted that the phase error signal  250  can be kept around zero in this exemplary method, so the method can effectively eliminate the unstable situation in the above exemplary method without any additional adjustments. 
     Often the sine and cosine signal generators in the demodulator can be implemented as look-up tables. For example, the sine signal generator  315  can include L rows of cells. The row number of a cell can correspond to an angle in the range of a wobble cycle, and the value in the cell can be the sine value of the angle. The L angles that related to the L row numbers can be evenly spaced in a wobble cycle. When the phase bias is zero, which is regarded as the nominal working condition, the sine and cosine signal generators can sequentially and cyclically provide a value from the L rows to the multipliers. For example, the sine and cosine signal generators can provide values of the following rows sequentially: 1, 2, 3, . . . , L−2, L−1, L, 1, 2, 3, . . . , L−2, L−1, L, 1, 2, . . . . 
     When the phase bias is non-zero, the sine and cosine signal generators can shift the row number by m referring to the nominal working condition, and still sequentially and cyclically provide a value in the L rows to the multipliers. m can be calculated by equation 2: 
                   m   =     round   ⁡     (       L   ⁢           ⁢     φ   bias         2   ⁢   π       )               (   2   )               
For example, if m is two, instead of providing the values of the above sequence, the sine and cosine signal generator can provide value in a row according to the following row sequence: 3, 4, 5, . . . , L, 1, 2, 3, 4, 5, . . . , L, 1, 2, 3, 4, . . . . In such a configuration, the operation of increasing the phase bias from 0 to 2π, can be accomplished by increasing the m from zero to L.
 
       FIG. 7  is a diagram showing another exemplary timing loop of wobble phase correction. Similar to the timing loop  200 , the wobble timing loop  700  can keep the phase error signal  250  around zero. 
     In the exemplary wobble timing loop  700 , the phase bias  520  can be added in the phase detector. Various techniques can be used in the phase detector to add phase bias, such as rotating the incoming data by the phase bias  520 . For example, a new set of quadrature and in-phase components (Q′, I′), which are corresponding to the quadrature and in-phase components of a modified phase difference, can be calculated and served as the input to an arctangent function. Thus, the modified phase difference is the sum of the phase difference and the phase bias. The calculation of the new set can be based on the quadrature and the in-phase components inputted in the phase detector and sine and cosine values of the phase bias signal  520 , such as using equations 3 and 4:
 
 Q′=Q ×cos(φ bias )+ I ×sin(φ bias )  (3)
 
 I′=I ×cos(φ bias )− Q ×sin(φ bias )  (4)
 
     The exemplary method can be explained similar to the previous methods. In the example, initially, the phase of the discrete wobble signal  230  is more than the phase of the internal oscillator by 2π. Therefore, the real phase difference can be 2π, and the phase error can be zero, which can be represented by point  425  in  FIG. 4 . 
     The method can begin with adding a positive phase bias. This can be equivalent to changing the phase of the internal oscillator by a negative phase bias in the above exemplary method showing in  FIG. 6 . Similar to the above example, phase error signal  250  is then non-zero. As already known, the timing loop  700  can keep the phase error signal  250  to be around zero. In order for the phase error signal  250  to be around zero, the timing loop  700  can shift the phase of the discrete wobble signal  230  by a negative value. Therefore, the real phase difference of the discrete wobble signal  230  and the internal oscillator can also change from 2π by the negative value. Slowly changing the phase bias from 0 to 2π, the timing loop  700  can shift the phase of the discrete wobble signal  230  by −2π, thus the real phase difference of the discrete wobble signal  230  and the internal oscillator can change from 2π to 0. The wobble phase slip can be corrected. 
     It is noted that the phase error signal  250  can be kept around zero in the this exemplary method, so this exemplary method can effectively eliminate the unstable situation in the exemplary method showing in  FIG. 5  without any additional adjustments. 
       FIG. 8  is a flow chart outlining an exemplary process for wobble phase slip correction. The process begins in step S 800 , and proceeds to step S 810 , wherein a wobble signal is received and decoded. Then the process proceeds to step S 820 , where a judgment is made as to whether a phase slip is detected. As described above, the wobble signal can carry information. The information can be used to detect a number of phase slip. If there is no phase slip, the process proceeds to step S 860  and terminates. If a phase slip presents, the process proceeds to step S 830  to begin the process of phase slip correction. 
     In step S 830 , a target phase bias can be determined. For example, to correct a phase slip of one cycle, the process can determined 2π as the target phase bias. Then the process proceeds to step S 840 . 
     In step S 840 , a phase bias can increase by a small amount. Initially, the phase bias is zero. Each time the process executes the step S 840 , the phase bias increases by a small amount. In addition, the phase bias may be modified to avoid the unstable situation. For example, equation 1 can be used when the phase bias is applied in the manner of  FIG. 5 . The process then proceeds to step S 850 . 
     In step S 850 , a judgment is made as to if the phase bias is equal to the target phase bias. If the answer is no, the process continues to step  840 , where the phase bias continues to increase by a small amount. If in step S 850 , the answer is yes, the process of phase slip correction has been completed, the process proceeds to step S 860  and terminates. 
     For the ease and clarity of description, the embodiments are presented with the examples of wobble phase slip of one wobble cycle. However, for a phase slip of multiple wobble cycles, the presented embodiments can be simply repeated or extended to correct the phase slip of multiple cycles. 
     For the ease and clarity of description, the embodiments are presented with a phase bias changing from 0 to either 2π or −2π in a timing loop. However, after the change of phase bias is more than π, as long as the timing loop is stable, the phase bias can be reset to zero, and the timing loop can work by itself to correct the rest of the phase error that is within [−π, π]. 
     While the invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, embodiments of the invention as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the spirit and scope of the invention.