Patent Publication Number: US-8526279-B1

Title: Method and apparatus for detecting wobble defects in optical recording system

Description:
INCORPORATION BY REFERENCE 
     This present disclosure is a continuation of U.S. application Ser. No. 13/311,353, filed on Dec. 5, 2011, which is a continuation of U.S. application Ser. No. 12/713,865, filed on Feb. 26, 2010, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/157,394, filed on Mar. 4, 2009, the contents of each of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Generally, a storage medium, such as an optical storage disc, wobbles a recording track to embed timing and address information. The timing and address information assists an optical recording device to record data at appropriate locations of the wobbled recording track. For example, the optical recording device can include an optical pick-up unit coupled with a wobble channel to extract the timing and address information. The optical pick-up unit generates a wobble signal corresponding to the wobbled recording track, and the wobble channel extracts the timing and address information from the wobble signal. Defects in the storage medium can cause disturbances to the timing and address information, and can cause loss of track to the timing and address information. 
     SUMMARY 
     Aspects of the disclosure can provide an apparatus. The apparatus includes a pick-up unit, such as an optical pick-up unit, a wobble channel and a defect detector. The pick-up unit generates a push-pull signal corresponding to a wobbled track of a storage medium, such as found on an optical disc. The wobble channel receives the push-pull signal, obtains a wobble signal from the push-pull signal, and calculates a wobble amplitude metric based on the wobble signal. The defect detector compares the wobble amplitude metric to a threshold to detect wobble defects. 
     In an embodiment, the wobble channel includes an envelope detector. The envelope detector detects a peak-to-peak envelope amplitude of the wobble signal. Then, the defect detector compares the peak-to-peak envelope amplitude to the threshold to detect the wobble defects. 
     In another embodiment, the wobble channel further includes a wobble demodulator to demodulate the wobble signal into an in-phase component and a quadrature component. Then, the wobble amplitude metric is calculated based on at least one of the in-phase component and the quadrature component. In an example, the wobble amplitude metric is calculated based on only one of the in-phase component or the quadrature component, such as an absolute value of the in-phase component, an absolute value of the quadrature component, and the like. In another example, the wobble amplitude metric is calculated based on a maximum of the in-phase component and the quadrature component, such as a maximum of absolute values of the in-phase component and the quadrature component. In another example, the wobble amplitude metric is calculated based on a quadratic mean of the in-phase component and the quadrature component, such as the quadratic mean, a square of the quadratic mean, and the like. 
     According to an aspect of the disclosure, the defect detector generates a defect signal indicative of a wobble defect when the wobble amplitude metric is smaller than the threshold. Further, at least one of the pick-up unit and the wobble channel is controlled based on the defect signal. 
     Aspects of the disclosure can provide a method of detecting wobble defects. The method includes generating a wobble signal in response to a wobbled track of a storage medium, calculating a wobble amplitude metric based on the wobble signal, comparing the wobble amplitude metric to a threshold, and detecting wobble defects based on the comparison. 
     Additionally, aspects of the disclosure can provide an integrated circuit (IC). The IC includes a wobble channel and a defect detector. The wobble channel receives a push-pull signal, obtains a wobble signal from the push-pull signal, and calculates a wobble amplitude metric based on the wobble signal. The defect detector compares the wobble amplitude metric to a threshold to detect wobble defects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG. 1  shows a block diagram of a medium apparatus example and an optical disc example according to an embodiment of the disclosure; 
         FIG. 2  shows a block diagram of a wobble channel example coupled with an optical pick-up unit example according to an embodiment of the disclosure; 
         FIG. 3A  shows a block diagram of a wobble demodulator example coupled with a defect detector example according to an embodiment of the disclosure; 
         FIG. 3B  shows another block diagram of a wobble demodulator example coupled with a defect detector example according to an embodiment of the disclosure; 
         FIG. 4  shows a block diagram of a timing loop filter example coupled with a digital voltage control oscillator (DVCO) example according to an embodiment of the disclosure; 
         FIG. 5  shows a block diagram of a front-end portion in a wobble channel example according to an embodiment of the disclosure; and 
         FIG. 6  shows a flow chart outlining a process example for detecting wobble defects according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a block diagram of a medium apparatus example  100  and a storage medium, such as an optical disc example  190  according to an embodiment of the disclosure. The medium apparatus  100  includes a processor  110 , an optical drive  115 , a random access memory (RAM) unit  130 , and a non-volatile memory  140 . These elements can be coupled together as shown in  FIG. 1 . 
     The optical drive  115  includes various components, such as an optical pick-up unit (OPU)  120 , a wobble channel  125  having a defect detector, and the like. The OPU  120  generates various electrical signals, such as push-pull signal, data signal, and the like, based on optical properties on the optical disc  190 . The wobble channel  125  includes suitable circuits to obtain a wobble signal from the push-pull signal, and further obtain information embedded in the wobble signal. The embedded information can assist the OPU  120  to record data on the optical disc  190 . Further, the defect detector of the wobble channel  125  detects wobble defects from the wobble signal, and generates a defect signal that is indicative of the detected wobble defects. The defect signal can be suitably used by the optical drive  115  to reduce disturbances due to the wobble defects. 
     The optical disc  190  can be any suitable optical disc, such as CD, DVD-R, DVD-RW, DVD+R, DVD+RW, HD, Blu-Ray, and the like. Generally, the optical disc  190  includes a spiral recording track, for example, in the form of a spiral groove adjacent to a spiral land. On the spiral recording track, user data can be stored on a recording layer by forming either data pits or data marks of different lengths and different spacings. The length of a pit, or the space between two pits denotes different information content or data information. To assist maintaining a uniform pit/mark length and pit/mark spacing, timing information and address information are embedded in the spiral groove and spiral land during disc manufacturing. In an example, the timing information is embedded by wobbling the spiral groove and/or the spiral land. Further, the address information is embedded by various techniques, such as land pre-pits, wobble phase modulation, and the like. In addition, disc information, such as manufacture, optical properties, and the like, is also embedded in the spiral groove and/or the spiral land during disc manufacturing. 
     The OPU  120  can be suitably configured to generate electrical signals in response to the embedded information on the optical disc  190 . In an embodiment, the OPU  120  includes servomechanisms (not shown) to direct a laser beam to a location of the optical disc  190 . The laser beam is reflected from the location of the optical disc  190 . The reflected laser beam has light properties that correspond to information embedded at the location of the optical disc  190 . The light properties are detected by a light detector (not shown) of the OPU  120 . Further, the light detector of the OPU  120  converts the light properties to various electrical signals, such as a push-pull signal, and the like, for other components of the optical drive  115  to extract the embedded information. 
     In addition, the OPU  120  can be suitably configured to record user data on the optical disc  190  based on the extracted embedded information, such as the timing information, the address information, the disc information, and the like. In an embodiment, the servomechanisms of the OPU  120  are suitably controlled to direct a recording laser beam to a recording location of the optical disc  190 . The recording location is determined based on the obtained address information. In addition, the recording laser beam is configured according to the obtained disc information, and the turn-on time of the recording laser beam is determined based on the obtained timing information. 
     The wobble channel  125  obtains the wobble signal from the push-pull signal, and detects wobbles in the wobble signal. Based on the detected wobbles, the wobble channel  125  obtains various information to assist controls of the optical drive  115 . More specifically, the wobble channel  125  locks an internal clock to the wobble signal to obtain the embedded timing information in the wobbles. Further, the wobble channel  125  extracts the embedded address information, the embedded disc information, and the like, based on the locked wobble signal. Then, the extracted information is used by the optical drive  115  to control, for example, the servomechanisms, the recording laser, and the like. 
     However, the wobble signal can be disturbed due to wobble defects on the optical disc  190 . The disturbances in the wobble signal can be detected by the defect detector in the wobble channel  125 . Thus, appropriate actions can be taken to mitigate the effects of the wobble defects. For example, when wobble defects are detected, the controls of the servomechanisms and the recording laser can be maintained according to their previous statuses instead of being changed based on the wobble signal. 
     In an example when wobble defects on the optical disc  190  are not detected, the wobble defects disturb the wobble signal. The disturbances in the wobble signal can cause the wobble channel  125  to inaccurately detect the wobbles, and lose locking of the internal clock to the wobble signal. Thus, the timing information and the address information may be erroneously extracted, and the controls of the servomechanisms and the recording laser may be erroneously performed based on the disturbed wobble signal. 
     The processor  110  of the medium apparatus  100  executes system and application codes. The non-volatile memory  140  holds information even when power is off. Thus, the non-volatile memory  140  can be used to store system and application codes, such as firmware. The RAM unit  130  is readable and writable. Generally, the RAM unit  130  has a fast access speed. In an example, data and the codes are stored in the RAM unit  130  during operation, such that the processor  110  accesses the RAM unit  130  for the codes and the data instead of the non-volatile memory  140 . 
     It is noted that the medium apparatus  100  can include more than one processor  110 . In an example, the optical drive  115  includes a processor to execute software instructions for controlling the various components of the optical drive  115 . It is also noted that the non-volatile memory  140  can include various non-volatile memory devices, such as battery backup RAM, read-only memory (ROM), programmable ROM (PROM), flash PROM, electrical erasable PROM (EEPROM), magnetic storage, optical storage, and the like. The RAM unit  130  can also include various RAM devices, such as DRAM, SRAM, and the like. 
     The medium apparatus  100  can include various other components. In an embodiment, the medium apparatus  100  includes a user input module  160 . The user input module  160  enables a user to control operations of the medium apparatus  100 . The user input module  160  includes various user input devices, such as keyboard, mouse, touch screen, and the like. In addition, the user input module  160  includes interfaces for coupling external user input devices with the medium apparatus  100 . 
     In another embodiment, the medium apparatus  100  includes an audio/video (A/V) module  150 . The AN module  150  includes various video and audio devices, such as microphone, display screen, and the like. In addition, the A/V module  150  includes interfaces that couple external video and audio devices with the medium apparatus  100 . In an example, the optical disc  190  stores video data and audio data. The video devices and audio devices play the video data and the audio data stored on the optical disc  190 . 
     In another embodiment, the medium apparatus  100  includes communication modules, such as a network module  170 , a wireless communication module  180 , and the like. The network module  170  and the wireless communication module  180  enable the medium apparatus  100  to transmit the data stored on the optical disc  190  to other devices, or to store data received from the other devices onto the optical disc  190 . 
     For ease and clarity of description, the embodiments are presented with a bus type architecture; however, it should be understood that any other architectures can also be used to couple components within the medium apparatus  100 . 
       FIG. 2  shows a block diagram of a wobble channel example  225  coupled with an optical pick-up unit (OPU) example  220  in an optical drive according to an embodiment of the disclosure. The wobble channel example  225  is a more detailed example of the wobble channel  125  in  FIG. 1 , and the OPU  220  is a more detailed example of the OPU  120  in  FIG. 1 . The wobble channel  225  includes a front-end analog portion  230 , a wobble demodulator  240  and a defect detector  260 . These elements are coupled together as shown in  FIG. 2 . 
     The OPU  220  includes a detector, such as a quadrant photo detector array  210  shown in  FIG. 2 . The quadrant photo detector array  210  includes four detectors to detect a light beam  215 , and generates various signals, such as a push-pull signal (PPS), based on the light beam  215 . In the  FIG. 2  example, the push-pull signal is generated according to Eq. 1:
 
 PPS =( I   a   +I   b )−( I   c   +I   d )  Eq. 1
 
where I a , I b , I c  and I d  are current signals generated by the four detectors in response to the light beam  215  reflected from a wobbled recording track on a storage medium.
 
     The front-end analog portion  230  receives the push-pull signal, regulates the push-pull signal, and outputs a wobble signal. The front-end analog portion  230  regulates the push-pull signal with analog techniques for various purposes, such as amplification, compensation for offsets, adjusting appropriate dynamic range, and the like. In an example, the front-end analog portion  230  includes an offset loop that adjusts the offsets of the push-pull signal. In another example, the front-end analog portion  230  includes a gain loop that adjusts an amplifier gain to regulate the push-pull signal to an appropriate dynamic range. Thus, the outputted wobble signal is suitable for subsequent circuit components to handle. 
     The wobble demodulator  240  receives the wobble signal, and extracts the timing information from the wobble signal. More specifically, the wobble demodulator  240  includes a phase-locked loop that locks an internal clock signal to the wobble signal to keep tracking timings embedded in the wobbled recording track. Then, the internal clock signal is used by components of an optical drive, such as the optical drive  115 , to extract, for example, address information, disc information and the like, embedded in the wobbled recording track. In addition, the internal clock signal is used to record user data with regard to the wobbled recording track. Thus, the extraction and recording operations depend on a locking quality of the internal clock signal to the wobble signal. 
     Generally, the phase-locked loop locks the internal clock signal to the wobble signal based on a phase error. The phase error indicates a phase difference of the internal clock signal and the wobble signal, for example. The phase-locked loop pulls the phase error towards a stable point, such as zero. However, the phase error may shift from the stable point, due to various reasons, such as noises, disturbances, interferences, defects, and the like. The shifted phase error can result in errors in the decoded information. 
     Wobble defects, such as scratches, block dots, and the like, can cause large shifts in the phase error. In extreme cases, the wobble defects can cause loss of timing lock of the internal clock signal to the wobble signal. In an example, when the light beam  215  is reflected from a defective location on the wobbled recording track, the difference between the sum of I a  and I b , and the sum of I c  and I d  is reduced. Thus, the amplitude of the push-pull signal is reduced. In an extreme case, the push-pull signal does not correspond to the wobbled recording track. In such case, the phase error can be substantially random. The random phase error can cause the phase-locked loop to lose locking of the internal clock signal to the wobble signal. 
     According to an aspect of the disclosure, the defect detector  260  detects the wobble defects based on a wobble amplitude metric. The wobble amplitude metric is calculated based on the wobble signal. In an embodiment, the wobble amplitude metric is provided to the defect detector  260  by the front-end analog portion  230 . Then, the defect detector  260  detects wobble defects based on the wobble amplitude metric provided by the front-end analog portion  230 . 
     In another embodiment, the wobble amplitude metric is provided to the defect detector  260  by the wobble demodulator  240 . Then, the defect detector  260  detects wobble defects based on the wobble amplitude metric provided by the wobble demodulation  240 . 
     The defect detector  260  outputs a defect signal corresponding to the wobble defect detection based on the wobble amplitude metric. In an example, the defect detector  260  outputs logic one when the wobble amplitude metric is smaller than a threshold and outputs logic zero when the wobble amplitude metric is larger than the threshold. In an embodiment, the defect signal is provided to a controller  270 . The controller  270  can control the operations of components in the optical drive, such as the OPU  220 , the wobble channel  225 , and the like, based on the defect signal. 
     In an example, the controller  270  controls a servomechanism (not shown) within the OPU  220  based on the defect signal. More specifically, when the defect signal indicates no defect in the wobble signal, the servomechanism operates based on the wobble signal, for example, adjusts parameters based on the wobble signal. When the defect signal indicates wobble defects in the wobble signal, the servomechanism ignores the wobble signal for a time duration, or until the defect signal indicates no defect in the wobble signal. The defect signal can be used by other components, such as an offset loop, a gain loop, a timing loop, and the like, to reduce disturbances due to the wobble defects. 
     In an embodiment, the front-end analog portion  230 , the wobble demodulator  240  and the defect detector  260  are implemented as integrated circuit modules in one or more integrated circuit (IC) chips. The IC chips can further include other circuit modules, such as controller module, encoder module, decoder module, memory module, network module, and the like. The IC chips can be coupled with the OPU  220  in an optical drive. 
       FIG. 3A  shows a block diagram of a wobble demodulator  340  coupled with a defect detector example  360 A according to an embodiment of the disclosure. The wobble demodulator  340  is implemented as a timing loop including a quadrature demodulator  320 , an analog-to-digital converter (ADC)  339 , a timing loop filter  370 , and a voltage control oscillator (VCO)  380 . These elements are coupled together as shown in  FIG. 3A . 
     The ADC  339  converts the wobble signal into a discrete wobble signal based on a sampling clock signal from VCO  380 . The quadrature demodulator  320  computes a phase error signal between an internal clock (not shown) and the discrete wobble signal. 
     The timing loop filter  370  receives the phase error signal, and outputs a voltage signal based on the phase error signal. The voltage signal is received by the VCO  380  to generate the sampling clock signal. 
     The quadrature demodulator  320  demodulates the discrete wobble signal with regard to the internal clock. More specifically, the quadrature demodulator  320  includes two parallel signal processing paths to generate a quadrature component and an in-phase component of the discrete wobble signal with regard to the internal clock. The path to generate the in-phase component includes a sine signal generator  343  of the internal clock, a multiplier  341  and an integrate and dump filter (I&amp;D)  342 . The path to generate the quadrature component includes a cosine signal generator  346  of the internal clock, a multiplier  344  and an integrate and dump filter (I&amp;D)  345 . Subsequently, the quadrature and the in-phase components are used by a phase and amplitude detector  350  to generate the phase error as well as a wobble amplitude metric. The wobble amplitude metric is used by the defect detector  360 A to detect wobble defects. 
     During operation, for example, the sampling clock signal from the VCO  380  samples the wobble signal to obtain the discrete wobble signal. On the in-phase path, the discrete wobble signal is multiplied with a sine signal of the internal clock by the multiplier  341 . Further, the multiplied signal is integrated over a period by the integrate and dump filter  342  to obtain the in-phase component. 
     On the quadrature path, the discrete wobble signal is multiplied with a cosine signal of the internal clock by the multiplier  344 . Further, the multiplied signal is integrated over a period by the integrate and dump filter  345  to obtain the quadrature component. Subsequently, the quadrature component and the in-phase component are used by the phase and amplitude detector  350  to detect the phase error, for example using an arctangent function. In addition, the phase and amplitude detector  350  calculates a wobble amplitude metric based on the in-phase component and the quadrature component. In an example, the phase and amplitude detector  350  calculates a quadratic mean of the in-phase component and the quadrature component, as shown by Eq. 2:
 
Quadratic Mean==√{square root over ( Q   2   +I   2 )}  Eq. 2
 
where Q denotes the quadrature component, and I denotes the in-phase component. Then, the wobble amplitude metric is calculated based on the quadratic mean. In an example, the wobble amplitude metric is the quadratic mean. In another example, the wobble amplitude metric is a square of the quadratic mean.
 
     Further, the timing loop filter  370  obtains a feedback portion based on the phase error. The feedback portion is used by the VCO  380  to adjust the sampling clock signal, such as its phase and frequency. Therefore, the sampling clock signal samples the wobble signal with a desired frequency and a desired phase. In an example, the wobble demodulator  340  is configured to lock the internal clock to the wobble signal with zero phase error. 
     The internal clock is used by other components of an optical drive to control operation timings. Therefore, the other components can operate corresponding to the wobble signal as a result of the internal clock being locked to the wobble signal. However, due to wobble defects and other reasons, such as noises, and the like, the phase error of the wobble signal and the internal clock can be errantly shifted from, for example, zero. The non-zero phase error can result in actions of the wobble demodulator  340  to erroneously attempt to lock the internal clock to the wobble signal having defect disturbances. The actions of the wobble demodulator  340  can cause the internal clock to lose locking to the wobble signal. 
     The defect detector  360 A declares a defect based on the wobble amplitude metric provided by the phase and amplitude detector  350 . In an embodiment, the defect detector  360 A includes a comparator  361 . The comparator  361  compares the wobble amplitude metric with a threshold to detect wobble defects. In an example, if the wobble amplitude metric is smaller than the threshold, the defect detector  360 A detects a wobble defect, and outputs logic one as the defect signal; otherwise, the defect detector  360 A outputs logic zero as the defect signal. 
     In an embodiment, the defect signal can be used by the timing loop filter  370  to reduce disturbances due to the wobble defects. In an example, the timing loop filter  370  is configured to disregard the phase error corresponding to the defect signal being logic one. 
       FIG. 3B  shows another block diagram of the wobble demodulator  340  coupled with a defect detector example  360 B according to an embodiment of the disclosure. The defect detector  360 B includes a multiplexer  362  coupled with the comparator  361 . The multiplexer  362  receives the in-phase component from the integrate and dump filter  342 , the quadrature component from the integrate and dump filter  345 , and a combination of the in-phase component and the quadrature component, such as a quadratic mean of the in-phase component and the quadrature component, a maximum of the in-phase component and the quadrature component, and the like, provided by the phase and amplitude detector  350 . The multiplexer  362  can be suitably controlled to select one of the in-phase component, the quadrature component, and the combination of the in-phase component and the quadrature component, and provide the selected to the comparator  361 . Then, the comparator  361  compares the selected to a suitable threshold, and generates the defect signal based on the comparison. 
     It is noted that suitable operations can be conducted on the in-phase component and/or the quadrature component. In an example, positive values are used in the defect detector  360 B to detect defects. Thus, absolute values of the in-phase components and the quadrature component are calculated and provided to the multiplexer  262 . Further, the absolute values are used to determine the maximum of the in-phase component and the quadrature component. 
     It is noted that the multiplexer  362  can be removed, and the comparator  361  is suitably coupled to the integrated and dump filter  342 , the integrated and dump filter  345  and/or the phase and amplitude detector  350 . In an example, the comparator  361  is coupled to the integrate and dump filter  342  to receive the in-phase component, and the comparator  361  generates the defect signal based on comparing the in-phase component to a suitable threshold. In another example, the comparator  361  is coupled to the integrate and dump filter  345  to receive the quadrature component, and the comparator  361  generates the defect signal based on comparing the quadrature component to a suitable threshold. In another example, the comparator  361  is coupled to the phase and amplitude detector  350  to receive a maximum of the in-phase component and the quadrature component, and the comparator  361  generates the defect signal based on comparing the maximum of the in-phase component and the quadrature component to a suitable threshold. In another example, the comparator  361  is coupled to both the integrate and dump filter  342  and the integrate and dump filter  345  to receive both the in-phase component and the quadrature component. The comparator  361  can include any suitable comparison algorithm, or any suitable comparison logic to generate the defect signal based on comparing the in-phase component and/or the quadrature component, or functions of the in-phase and/or quadrature components to suitable thresholds. 
       FIG. 4  shows a block diagram of a timing loop filter example  470  coupled with a digital voltage control oscillator (DVCO) example  480  according to an embodiment of the disclosure. The timing loop filter  470  includes a phase path and a frequency path to generate a control signal for the DVCO  480 . The phase path includes a first multiplier  472  to generate a phase component. The frequency path includes a second multiplier  474  and an integrator  475  to generate the frequency component. The integrator  475  includes a first adder  476  and a register  478 . Further, the timing loop filter  470  includes a second adder  479  to combine the phase component and the frequency component to generate the control signal. These elements are coupled together as shown in  FIG. 4 . 
     The first multiplier  472  multiplies the phase error with a phase update gain to generate the phase component. The phase update gain can be tunable. For example, the phase update gain can be tuned to one of 16 values. In an example, when wobble defects are detected, for example, as indicated by the defect signal being logic one, the phase update gain is tuned to a suitable value, such as zero, to disregard the phase error. 
     The second multiplier  474  multiplies the phase error with a frequency update gain. Then, the integrator  475  integrates the multiplied phase error to generate the frequency component. More specifically, the first adder  476  adds the multiplied phase error to a previous frequency component to generate a current frequency component. The register  478  holds the previous frequency component. The previous frequency components is then used to generate the current frequency component. The frequency update gain can be tunable. For example, the frequency update gain can be tuned to one of 16 values. In an example, when wobble defects are detected, for example, as indicted by a defect signal being logic one, the frequency update gain is tuned to a suitable value, such as zero, to disregard the phase error. 
     Further, the second adder  479  combines the phase component and the frequency component to generate the control signal for the DVCO  480 . In an embodiment, the DVCO  480  includes a digital representation of a voltage signal. The control signal is used to adjust the digital representation of the voltage signal. The DVCO  480  further includes a digital to analog converter (DAC) (not shown). The DAC converts the digital representation to the voltage signal. Further, the voltage signal is used to control a voltage control oscillator (VCO) to generate the sampling clock accordingly. 
       FIG. 5  shows a block diagram of a front-end portion example  530  coupled to a defect detector  560  according to an embodiment of the disclosure. The front-end portion  530  includes an offset adder  531 , a variable gain amplifier (VGA)  532 , a continuous time filter (CTF)  533 , an analog to digital converter (ADC)  539 , an envelope detector  536 , a gain loop controller  534 , and an offset loop controller  535 . These elements are coupled together as shown in  FIG. 5 . 
     The offset adder  531 , the VGA  532 , and the CTF  533  regulate a received analog signal to have desired properties. For example, the offset adder  531  adjusts the analog signal with an offset provided by the offset loop controller  535 . The VGA  532  amplifies the analog signal with a gain controlled by the gain loop controller  534 . The CTF  533  truncates a noise bandwidth of the analog signal, for example. 
     The ADC  539  obtains a discrete wobble signal by sampling the regulated analog signal. The envelope detector  536  detects envelopes of the discrete wobble signal. Then, the offset loop controller  535  generates the offset based on the envelopes, and the gain loop controller  534  generates the gain based on the envelopes. Also, the defect detector  560  detects wobble defects based on the envelopes. In an embodiment, the defect detector  560  detects the wobble defects based on a peak-to-peak envelope amplitude. In an example, the defect detector  560  includes a comparator  561 . The comparator  561  compares the peak-to-peak envelope amplitude to a threshold to generate a defect signal. For example, when the peak-to-peak envelope amplitude is smaller than the threshold, the comparator  561  outputs logic one as a defect signal to indicate a wobble defect; otherwise, the comparator  561  outputs logic zero. 
     It is noted that the envelope detector  536  and the front-end portion  530  can be suitably adjusted to detect wobble signal envelopes before the wobble signal is sampled by the ADC  539 . 
       FIG. 6  shows a flow chart outlining a process example  600  for an optical drive to detect wobble defects according to an embodiment of the disclosure. The process  600  starts at S 610 , and proceeds to S 620 . 
     At S 620 , the optical drive generates a wobble signal in response to a wobbled recording track. In an example, the optical drive includes an OPU, such as the OPU  220 , and a wobble channel, such as the wobble channel  225 . The OPU directs a laser beam onto the wobbled recording track, and detects a reflected laser beam. The OPU generates a push-pull signal in response to the reflected laser beam. The wobble channel  225  regulates the push-pull signal to generate the wobble signal. 
     At S 630 , the optical drive calculates a wobble amplitude metric. In an embodiment, the wobble channel  225  includes an envelope detector to detect a peak-to-peak envelope amplitude of the wobble signal. In another embodiment, the wobble channel  225  includes a wobble demodulator. The wobble demodulator demodulates the wobble signal with regard to an internal clock to generate an in-phase component and a quadrature component. The in-phase component and the quadrature component can be used to calculate a phase error of the wobble signal to the internal clock. In addition, at least of the in-phase component and the quadrature component can be used to calculate the wobble amplitude metric. In an example, the wobble amplitude metric is calculated based on only the in-phase component. For example, the wobble amplitude metric is calculated as an absolute value of the in-phase component, or a scaled in-phase component. In another example, the wobble amplitude metric is calculated based on a maximum of the in-phase component and the quadrature component. For example, the wobble amplitude metric is calculated as a maximum of absolute values of the in-phase component and the quadrature component. In another example, the wobble amplitude metric is calculated based on a quadratic mean of the in-phase component and the quadrature component. For example, the wobble amplitude metric is calculated as the quadratic mean or a square of the quadratic mean. 
     At S 640 , the optical drive compares the wobble amplitude metric to a threshold. In an example, the optical drive compares the peak-to-peak envelope amplitude to a threshold. In another example, the optical drive compares an absolute value of the in-phase component to a threshold. In another example, the optical drive compares the maximum of the absolute value of the in-phase component and the absolute value of the quadrature component to a threshold. In another example, the optical drive compares the quadratic mean of the in-phase component and the quadrature component to a threshold. 
     At S 650 , the optical drive detects wobble defects based on the comparison. In an example, when the wobble amplitude metric is smaller than the threshold, the optical drive outputs a defect signal indicative of a wobble detect, such as the defect signal being logic one. Otherwise, the optical drive outputs logic zero as the defect signal, for example. The defect signal can be used by the optical drive to adjust various components to reduce disturbances due to the wobble defects. Then, the process proceeds to S 660  and terminates. 
     It is noted that the process  600  can be continuously performed by the optical drive during a recording process. It is also noted that the threshold can be static or dynamic, and can be suitably calibrated for different wobble amplitude metrics. 
     While the invention has been described in conjunction with the specific embodiments thereof that are proposed as examples, 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 scope of the invention.