# Patent

## Title
Method and device for protecting a servo in reading signals on a defect disc

## Abstract

A device for protecting a servo in reading signals on a defect disc from disturbance and instability is provided. The device includes a defect detection unit and a logic combination unit. The defect detection unit receives a plurality of defect detection signals to detect various defects for setting a plurality of defect flag signals, wherein the plurality of defect detection signals at least include an envelope signal of a RF signal and bit modulation signals. The logic combination unit performs logic operation on the defect flag signals in order to detect a specified defect. The logic combination unit indicates a servo control unit generating low-pass output signals according to said plurality of defect flag signals for controlling related electromechanical devices to prevent from disturbance and instability when the specified defect is detected. A method for protecting a servo in reading signals on a defect disc is also provided.

## Background
RELATED APPLICATIONS
The present application is based on, and claims priority from, U.S. Application No. 60/662,351, filed Mar. 17, 2005, the disclosure of which is hereby incorporated by reference herein in its entirety.


BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to the field of device protection of Optical disc drive (ODD). More particularly, the present invention relates to a method and device that protects a servo of ODD in reading signals on a defect disc.
2. Description of the Prior Art
Nowadays, disc-type storage media are broadly used in keeping data due to their storage capacity. Such disc-type storage media like optical discs, i.e. CD-R discs, CD-RW discs, DVD-R discs, DVD-RW discs, DVD+R discs, DVD+RW discs, or DVD-RAM discs etc., also provide better protection to the data stored on them against damage. However, these characteristics mentioned above do not mean the optical discs are faultless storage media for storing data because some defects might either take place on their surfaces. For example, a deep scratch, a shallow scratch, and even a fingerprint. These defects could result in not only reading or writing errors but also a system disturbance while the system reads or writes data. Hence, it is an important thing to detect existing defects for protecting the system from a disturbed or instable situation.
It is well known to use the difference of signal amplitude, such as an RF level (RFLVL) or a sub-beam added (SBAD) signal, to detect an existing defect. FIG. 1A illustrates signals of a deep defect detected by applying well-known RFLVL detection. As shown in FIG. 1A, a defect detection applying the RFLVL is illustrated. An RF signal 110 has a hollow region 112 in a time period 120. That means the corresponding data of the hollow region 112 is damaged by a defect, so that the RF signal 110 in the time period 120 cannot be read out. Further, the depth of the hollow region 112 represents the depth of the defect. An RFLVL signal 114, which is formed from the RF signal 110 passing a low pass filter, shows the envelope of the RF signal 110. A detection threshold 130 is a fixed DC referred voltage level. As the RFLVL signal 114 is lower than the detection threshold 130 in the time period 120, a defect flag signal 140 is raised from “0” to “1”. Moreover, a FE/TE signal 150 respectively generates a positive surge 152 and a negative surge 154 at the beginning and the end of the time period 120 to indicate a focusing and a tracking error signal. However, while the defect flag signal 140 is set from “0” to “1”, a servo system, such as a focusing or a tracking servo, and a data path control system, such as a preamplifier, a slicer, or a phase lock loop (PLL), can detect a defect signal and then reduce the potential disturbance and instability through applying some appropriately protective methods and devices.
FIG. 1B illustrates signals of a shallow defect detected by applying well-known RFLVL detection. In FIG. 1B, an RF signal 110-1 has a hollow region 112-1 in a time period 120-1. That also means the corresponding data of the hollow region 112-1 is damaged by a defect, so that the RF signal 110-1 in the time period 120-1 cannot be totally read out. But, the depth of the hollow region 112-1 is not deep as the hollow region 112 shown in FIG. 1A since it might just result from a shallow defect, such as a shallow scratch. An RFLVL signal 114-1 shows the envelope of the RF signal 110-1. A detection threshold 130-1 is a fixed DC referred voltage level like the detection threshold 130 shown in FIG. 1A. Obviously, the RFLVL signal 114-1 is always higher than the detection threshold 130-1 because the shallow defect does not make the hollow region 112-1 deep enough. Hence, not only a defect flag signal 140-1 has no response to the shallow defect, but also a FE/TE signal 150-1 has no apparently change except a little noise. Furthermore, since the shallow defect is not detected, some protective methods and devices are not triggered to protect the system from the potential disturbance and instability. In other words, the servo systems and the data path control systems are easily affected by the disturbance and instability in this defect situation.
Similarly, referring to FIG. 1C, illustrating signals of a fingerprint detected by applying well-known RFLVL detection, an RF signal 110-2 has a hollow region 112-2 in a time period 120-2. That means the corresponding data of the hollow region 112-2 is slightly affected by a defect, so that the RF signal 110-2 in the time period 120-2 has weaker amplitudes. Also, the depth of the hollow region 112-2 is not deep like the hollow region 112-1 shown in FIG. 1B, since it might just result from a shallow defect, such as a fingerprint. An RFLVL signal 114-2 shows the envelope of the RF signal 110-2 and a detection threshold 130-2 is a fixed DC referred voltage level like the detection threshold 130 shown in FIG. 1A. The RFLVL signal 114-2 is always higher than the detection threshold 130-2 in this defect situation, because the shallow defect does not make the hollow region 112-2 deep enough. Thus, not only a defect flag signal 140-2 has no response to the shallow defect, but also a FE/TE signal 150-2 has no apparently change except a little noise. This situation is similar to the situation described in FIG. 1B; the servo systems and the data path control systems cannot be safely protected. On the other hand, however, the defects shown in FIG. 1B and FIG. 1C further include different statuses according to their damaged depth, width and direction; some defects might still have original data, but others have only destroyed data. Therefore, it is difficult to determine the defect flag signal simply by the detection threshold comparison.
In view of the drawbacks mentioned with the prior art of device protection, there is a continued need to develop a new and improved method and device that overcomes the disadvantages associated with the prior art of device protection. The advantages of this invention are that it solves the problems mentioned above.
SUMMARY OF THE INVENTION
A device for protecting a servo in reading signals on a defect optical disc from disturbance and instability is provided. The device includes a defect detection unit and a logic combination unit. The defect detection unit receives a plurality of defect detection signals to detect various defects for setting a plurality of defect flag signals, wherein the plurality of defect detection signals at least include an envelope signal of a RF signal and bit modulation signals. The logic combination unit performs logic operation on the defect flag signals in order to detect a specified defect. The logic combination unit indicates a servo control unit generating low-pass output signals according to said plurality of defect flag signals for controlling related electromechanical devices to prevent from disturbance and instability when the specified defect is detected.
A method for protecting a servo in reading signals on a defect optical disc from disturbance and instability is further provided in present invention. The method receives a plurality of defect detection signals for setting a plurality of defect flag signals, wherein the plurality of defect detection signals at least include an envelope signal of a RF signal and bit modulation signals. Thereafter, performing logic operation on the defect flag signals in order to detect a specified defect. Next, low-pass output signals are generated according to the plurality of defect flag signals for controlling related electromechanical devices to prevent from disturbance and instability.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
FIG. 1A illustrates signals of a deep defect detected by applying well-known RFLVL detection;
FIG. 1B illustrates signals of a shallow defect detected by applying well-known RFLVL detection;
FIG. 1C illustrates signals of a fingerprint detected by applying well-known RFLVL detection;
FIG. 2 illustrates a schematic defect detection device block diagram in accordance with the present invention;
FIGS. 3A ̃3F illustrate flow charts of some defect detection in according with the present invention;
FIG. 4 illustrates different defect signals detected by applying some defect detection in accordance with the present invention; and
FIG. 5A and FIG. 5B illustrate a preferred embodiment of the servo in accordance with the present invention and related signals thereof.



DESCRIPTION OF THE PREFERRED EMBODIMENT
Some embodiments of the invention will now be described in greater detail. Nevertheless, it should be noted that the present invention can be practiced in a wide range of other embodiments besides those explicitly described, and the scope of the present invention is expressly not limited except as specified in the accompanying claims.
Moreover, some irrelevant details are not drawn in order to make the illustrations concise and to provide a clear description for easily understanding the present invention.
FIG. 2 illustrates a schematic defect detection device block diagram in accordance with the present invention. In FIG. 2, a servo control unit 210 handles the related electromechanical devices, such as the spin rate of a spindle motor 204, the move of a sled motor 206, and the slightly tracking and focusing move of a lens 208, through a power driver 212. That is, the servo control unit 210 can make the lens 208 not only aim at the right track of a disc 202 but also have a well focus for data reading and transferring. Through roughly moving a pick-up head 209 and slightly tracking move of the lens 208 at the horizontal direction, and slightly focusing move of the lens 208 at the vertical direction, the servo control unit 210 can make the lens 208 focus well on the right track of the disc 202. A data path control unit 280 includes a preamplifier 220, a slicer 230, a phase lock loop (PLL) 240, and a decoder 250. The preamplifier 220 receives data signals from the pick-up head 209 and generates various signals, such as RF signals for data process, servo control signals, i.e. a FE/TE signal, for the servo control unit 210, and other signals, such as bit modulation signals, i.e. eight to fourteen bit modulation (EFM) signals and RF level (RFLVL) signals, etc., for defect detection. The slicer 230 digitalizes the RF signals transferred from the preamplifier 220. The PLL 240 synchronizes the digitalized RF signals to a system clock and counts the length of the digitalized RF signals according to the system clock. The decoder 250 decodes the length of the digitalized RF signal to a host (not shown).
A defect detection unit 260 receives the various signals from the preamplifier 220, and EFM signals from the slicer 230 and the PLL 240 to detect different kinds of defects through different defect detections to set corresponding defect flag signals. Wherein, the different defect detections include ADefect detection, ADefect1 detection, EFMDefect detection, RPDefect detection, Interruption detection, and DSPDefect detection, so as to set ADefect flag signal, ADefect1 flag signal, EFMDefect flag signal, RPDefect flag signal, Interruption flag signal and DSPDefect flag signal. A microprocessor or a Digital Signal Processor (DSP) could be used as the defect detection unit 260. The firmware of the foregoing defect detections could be stored in the detection unit 260 to perform corresponding defect detection. A logic combination unit 270 performs an appropriate logic operation, simply, such as an OR operation or an AND operation, on the defect flag signals to precisely improve the defect detection. As the operation result indicates in a defect situation, the logic combination unit 270 triggers defect protection methods and devices to protect the corresponding units, such as the servo control unit 210, the preamplifier 220, the slicer 230, the PLL 240, and the decoder 250.
FIGS. 3A ̃3F show the flow charts of defect detection method in accordance with present invention. Referring to FIG. 3A, ADefect detection is illustrated. In step 311, comparing an RFLVL signal with an ADefect level. Wherein the RFLVL signal is the envelope of an RF signal and the ADefect level is a fixed DC referred voltage level. An ADefect flag is set to “1” in step 315 while the RFLVL signal is lower than the ADefect level. When the RFLVL signal is higher than the ADefect level, and then judging whether the RFLVL signal is in defect delay time or not (step 312). In step 314, the ADefect flag is set to “1” while the RFLVL signal is in defect delay time. However, while the RFLVL signal is higher than the ADefect level and is not in the defect delay time, the ADefect flag is set to “0” in step 313. The ADefect detection is appropriately used for detecting a deep defect, such as a scratch. When the ADefect flag signal transit from “0” to “1”, it means a defect is detected.
Referring to FIG. 3B, the flow charts of ADefect1 detection is illustrated. All steps in FIG. 3B are similar to those in FIG. 3A. In step 321, comparing an RFLVL signal with an ADefect1 level. Wherein the RFLVL signal is the envelope of an RF signal and the ADefect1 level is a fixed DC referred voltage level. The difference between the ADefect level and ADefect1 level is that the ADefect1 level is higher than the ADefect level. Hence, the ADefect1 detection is more sensitive than ADefect detection for shallow defect and fingerprint. An ADefect1 flag is set to “1” in step 325 while the RFLVL signal is lower than the ADefect1 level.
When the RFLVL signal is higher than the ADefect1 level, and then judging whether the RFLVL signal is in defect delay time or not (step 322). In step 324, the ADefect1 flag is set to “1” while the RFLVL signal is in defect delay time. However, while the RFLVL signal is higher than the ADefect1 level and is not in the defect delay time, the ADefect1 flag is set to “0” in step 323. When the ADefect1 flag signal transit from “0” to “1”, it means a defect is detected.
Referring to FIG. 3C, the flow charts of EFMDefect detection is illustrated. In step 331, while a data sector or a data frame has at least n1 RF patterns are shorter than a first predetermined data length, the EFMDefect flag is set to “1”. For example, the first predetermined data length is 3 T for both CD and DVD data. In step 332, while the data sector or the data frame has at least n2 RF patterns are longer than a second predetermined data length, the EFMDefect flag is set to “1”. For example, the second predetermined data length is respectively 11 T and 14 T for CD and DVD data. In step 333, while the data sector or the data frame has at least n3 RF patterns are longer than a serious data length, such as 18 T, the EFMDefect flag is set to “1”. On the other hand, while a data sector or a data frame has at least n4 RF patterns are between the first and the second predetermined data length, the EFMDefect flag is set to “0”. The EFMDefect detection is appropriately used for detecting an abnormal data length and it is real-time defect detection. Wherein, the EFMDefect detection is more sensitive while the variables n1, n2, n3, and n4 have small values. When the EFMDefect flag signal transit from “0” to “1”, it means a defect is detected.
Referring to FIG. 3D, the flow charts of Interruption detection is illustrated. All steps in FIG. 3D are similar to those in FIG. 3A. In step 341, comparing an RFLVL signal with an Interruption level. Wherein the RFLVL signal is the envelope of an RF signal and the Interruption level is a fixed DC referred voltage level. The Interruption level setting is higher than the RFLVL signal in order to detect a defect resulted from strong reflection. An Interruption flag is set to “1” in step 345 while the RFLVL signal is higher than the Interruption level. When the RFLVL signal is lower than the Interruption level, and then judging whether the RFLVL signal is in defect delay time or not (step 342). In step 344, the Interruption flag is set to “1” while the RFLVL signal is in defect delay time. However, while the RFLVL signal is lower than the Interruption level and is not in the defect delay time, the Interruption flag is set to “0” in step 343. When the Interruption flag signal transit from “0” to “1”, it means a defect is detected.
Referring to FIG. 3E, the flow charts of RPDefect detection is illustrated. All steps in FIG. 3E are similar to those in FIG. 3A. In step 351, comparing an RFRP signal with an RPDefect level. Wherein the RFRP signal could be the peak or the bottom envelope of an RF signal and also could be the peak to the bottom of the RF signal and the RPDefect level is a fixed DC referred voltage level. An RPDefect flag is set to “1” in step 355 while the RFRP signal is lower than the RPDefect level. When the RFRP signal is higher than the RPDefect level, and then judging whether the RFRP signal is in defect delay time or not (step 352). In step 354, the RPDefect flag is set to “1” while the RFRP signal is in defect delay time. However, while the RFRP signal is higher than the RPDefect level and is not in the defect delay time, the RPDefect flag is set to “0”, in step 353. When the Interruption flag signal transit from “0” to “1”, it means a defect is detected. The RPDefect detection detects a defect via further processing the RF signal thus it is more sensitive for detecting defects. Due to its sensitive ability to detect defects, the RPDefect detection is suitably used to detect a small scratch and an interruption defect.
Referring to FIG. 3F, the flow charts of DSPDefect detection is illustrated. All steps in FIG. 3F are similar to those in FIG. 3A. In step 361, comparing an absolute difference value between an RFLVL and an RFLVL_LPF with a predetermined threshold. Wherein, the RFLVL_LPF signal is a slowly falling signal of the RFLVL signal passed a low pass filter. A DSPDefect flag is set to “1” in step 365 while an absolute difference value between an RFLVL and an RFLVL_LPF is bigger than the predetermined threshold. When the absolute difference value between the RFLVL and the RFLVL_LPF is smaller than the predetermined threshold, and then judging whether the moment is in defect delay time or not (step 362). In step 364, the DSPDefect flag is set to “1” while the moment is in defect delay time. However, When the absolute difference value between the RFLVL and the RFLVL_LPF is smaller than the predetermined threshold and the moment is not in the defect delay time, the DSPDefect flag is set to “0” in step 363. When the DSPDefect flag signal transit from “0” to “1”, it means a defect is detected. The DSPDefect detection detects a defect through a variable threshold thus a fixed DC referred voltage level is unnecessary.
As shown in FIG. 4, some defect signals detected by applying the defect detection in accordance with the present invention are illustrated. An RF signal 41 has a deep hollow thus its envelope signal 411 also has the deep hollow. According to the ADefect1 and the ADefect detection mentioned before, an ADefect1 flag signal 416 and an ADefect flag signal 415 are respectively set from “0” to “1” while the envelope signal 411 is lower than an ADefect1 level 402 and an ADefect level 401. The EFMDefect flag signal 417 is set from “0” to “1” as well because the hollow is wide enough and generates abnormal data length. The Interruption flag signal 419 has no response to the hollow since the envelope signal 411 is always smaller than an Interruption level 404. An RFRP signal (peak hold) 412 and an RFRP (bottom-inverse) signal 413 respectively show the peak envelope and the inversed bottom envelope of the RF signal 41. Further, an RFRP (peak-bottom) signal 414 is formed through the RFRP (peak hold) signal 412 subtracting the RFRP (bottom-inverse) signal 413. An RPDefect flag signal 418 is set from “0” to “1” as the RFRP (peak-bottom) signal 414 is lower than an RPDefect level 405. The deep hollow caused by a deep defect, such as a scratch, can be detected out through the ADefect, the ADefect1, the EFMDefect, and the RPDefect detections, since its depth and width are deep and wide enough for those defect detections.
An RF signal 42 has a shallow and narrow hollow thus its envelope signal 421 also has the same form. According to the ADefect1 detection, an ADefect1 flag signal 426 is set from “0” to “1” while the envelope signal 421 is lower than the ADefect1 level 402. An RFRP (peak hold) signal 422 and an RFRP (bottom-inverse) signal 423 respectively show the peak envelope and the inversed bottom envelope of the RF signal 42. Further, an RFRP (peak-bottom) signal 424 is formed through an RFRP (peak hold) signal 422 subtracting an RFRP (bottom-inverse) signal 423. An RPDefect flag signal 428 is set from “0” to “1” as the RFRP (peak-bottom) signal 424 is lower than the RPDefect level 405. However, an ADefect flag signal 425, an EFMDefect flag signal 427, and an Interruption flag signal 429 have no response to the shallow and narrow hollow, since the envelope signal 421 is always higher than the ADefect level 401, unsatisfying the conditions of the EFMDefect detection mentioned before, and is always lower than the Interruption level 404, respectively. The shallow and narrow hollow probably caused by a shallow scratch can be only detected out through the ADefect1 and the RPDefect detection, since its depth and width are insufficient for other defect detection.
An RF signal 43 has a shallow and wide hollow thus its envelope signal 431 also has the same form. An ADefect1 flag signal 436 is set from “0” to “1” while the envelope signal 431 is lower than the ADefect1 level 402. An EFMDefect flag signal 437 is set from “0” to “1” as well, because the hollow is wide enough and generates abnormal data length. An RFRP (peak hold) signal 432 and an RFRP (bottom-inverse) signal 433 respectively show the peak envelope and the inversed bottom envelope of the RF signal 43. Further, an RFRP (peak-bottom) signal 434 is formed through the RFRP (peak hold) signal 432 subtracting the RFRP (bottom-inverse) signal 433. An RPDefect flag signal 438 is set from “0” to “1” as the RFRP (peak-bottom) signal 434 is lower than the RPDefect level 405. However, an ADefect flag signal 435 and an Interruption flag signal 439 have no response to the shallow and width hollow, since the envelope signal 431 is always higher than the ADefect level 401 and is always lower than the Interruption level 404. The shallow and wide hollow possibly caused by a shallow defect can be only detected out through the ADefect1, the EFMDefect and the RPDefect detection, since its depth and width are insufficient for other defect detections.
An RF signal 44 has a shallow and wide hollow thus its envelope signal 441 also has the same form. An ADefect1 flag signal 446 is set from “0” to “1” while the envelope signal 441 is lower than the ADefect1 level 402. An RFRP (peak hold) signal 442 and an RFRP (bottom-inverse) signal 443 respectively show the peak envelope and the inversed bottom envelope of the RF signal 44. Further, an RFRP (peak-bottom) signal 444 is formed through the RFRP (peak hold) signal 442 subtracting the RFRP (bottom-inverse) signal 443. An RPDefect flag signal 448 has no response to the shallow and width hollow, since the RFRP (peak-bottom) signal 444 is always higher than the RPDefect level 405. Moreover, an ADefect flag signal 445, an EFMDefect flag signal 447, and an Interruption flag signal 449 neither have no response to the shallow and wide hollow, since the envelope signal 441 is always higher than the ADefect level 401, unsatisfying the conditions of the EFMDefect detection mentioned before, and is always lower than the Interruption level 404, respectively. The shallow and wide hollow probably resulted from a fingerprint can be just detected out via the ADefect1 detection in this situation, since its depth and width are very deficient for other defect detections.
As for an RF signal 45 and an RF signal 46, both of them are caused from strong signal strengths, such as strong optical reflection, also called an interruption defect. The RF signal 45 has strong amplitudes at its peak and its bottom envelope thus its peak envelope signal 451 has the corresponding form. An EFMDefect flag signal 457 is set from “0” to “1” since the interruption defect is wide enough and generates abnormal data length. An Interruption flag signal 459 is also set from “0” to “1” as the envelope signal 451 is higher than the Interruption level 404. As for other flag signals, an ADefect1 flag signal 456 and an ADefect flag signal 455 have no response to the envelope signal 451 because the envelope signal 451 is always higher than the ADefect1 level 402 and the ADefect level 401. An RFRP (peak hold) signal 452 and an RFRP (bottom-inverse) signal 453 respectively show the peak envelope and the inversed bottom envelope of the RF signal 45. Further, an RFRP (peak-bottom) signal 454 is formed through the RFRP (peak hold) signal 452 subtracting the RFRP (bottom-inverse) signal 453. An RPDefect flag signal 458 has no response to this kind of interruption defect, since the RFRP (peak-bottom) signal 454 is higher than the RPDefect level 405 at all times. This kind of interruption defect can be just detected out via the EFMDefect and the Interruption detection mentioned before.
The RF signal 46 forms an inversed hollow from its bottom envelope thus its peak envelope signal 461 has the corresponding form. An EFMDefect flag signal 467 is set from “0” to “1”, since the interruption defect is wide enough and generates abnormal data length. An RFRP (peak hold) signal 462 and an RFRP (bottom-inverse) signal 463 respectively show the peak envelope and the inversed bottom envelope of the RF signal 46. Further, an RFRP (peak-bottom) signal 464 has a deep hollow formed by the RFRP (peak hold) signal 462 subtracting the RFRP (bottom-inverse) signal 463. An RPDefect flag signal 468 is set from “0” to “1” while the RFRP (peak-bottom) signal 464 is lower than the RPDefect level 405. An Interruption flag signal 469 is set from “0” to “1” while the envelope signal 461 is higher than the Interruption level 404. However, an ADefect1 flag signal 466 and an ADefect flag signal 465 have no response to the signal 461 because the envelope signal 461 is higher than the ADefect1 level 402 and the ADefect level 401. This kind of interruption defect can be only detected out via the EFMDefect, the RPDefect, and the Interruption detection mentioned before.
Generally speaking, the ADefect1 detection is more suitable than the ADefect detection for small and shallow scratch detection. The RPDefect detection is more sensitive for small scratch detection. Hence, it should be understood that the defect detection mentioned in the present invention could be combined in variety for particular defect detection. For example, combining the ADefect1 and the EFMDefect detection via a logic “OR” operation for small scratch detection, or combining the ADefect1 and the EFMDefect detection via a logic “AND” operation for small scratch detection except unwanted fingerprint, etc.
Method and device for protecting a servo are disclosed in present invention. Through applying aforementioned defect detection and a suitable combination thereof to protect a servo from instability. Wherein, the suitable combination of the ADefect detection and DSPDefect detection is preferred. However, it should be understood that the aforementioned defect detection could trigger the protection method and device by individual and various combinations.
As shown in FIG. 5A and FIG. 5B, a preferable servo control unit 210 embodiment and related signals thereof are illustrated. In FIG. 5A, the servo control unit 210 includes a focusing controller 510 and a tracking controller 520. The focusing controller 510 and the tracking controller 520 respectively receive a focusing error (FE) signal 502 and a tracking error (TE) signal 506 from preamplifier 220, so as to generate a focusing output (F00) signal 504 and a tracking output (TR0) signal 507 to power driver 212. The servo control unit 210 further includes a low pass filter 512 and a low pass filter 522. The low pass filter 512 and the low pass filter 522 respectively filter FE signal 502 and TE signal 506, and then respectively generate a low-pass focusing error (FE_LPF) signal and a low-pass tracking error (TE_LPF) signal. The focusing controller 510 and tracking controller 520 respectively receive the FE_LPF signal and the TE_LPF signal. The servo control unit 210 detects a defect flag signal. When a defect is detected and a defect flag signal from the logic combination unit 270 is received, the FE signal 502 and TE signal 506 are respectively replaced by the FE_LPF signal and the TE_LPF signal. Then, the FE_LPF signal and the TE_LPF signal are respectively output to the focusing controller 510 and the tracking controller 520, so as to generate the F00 signal 504 and the TR0 signal 507.
In FIG. 5B, the RFLVL signal 501 symbolizes two defects in the time period 560. Such defects cause a fluctuant FE signal 502 sent from a preamplifier 220. Furthermore, since the two defects occur so close, the FE signal 502 has no enough time to totally respond. A slowly responding FE signal 5021 drawn in a solid line thus replaces an idea FE signal 5021 drawn in a dash line. In order to rapidly pull up the slowly responding FE signal 5021, an F00 signal 504 generates a spike signal 5041 corresponding to the FE signal 502. While the spike signal 5041 compensates the F00 signal 504, it also causes the servo control unit 210 instability and disturbance at the same time. In a word, whether the spike signal 5041 or those positive and negative surges caused by the defects, they make the servo control unit 210 instable and disturbed. Likewise, the above-mentioned situations are similar to those situations of the FE signal 502 and F00 504 being respectively replaced by the TE signal 506 and the TR0 signal 507.
However, as two defects are detected at time period 560 by the servo control unit 210, and then a defect flag signal is received from a logic combination unit 270. Next, the servo control unit 210 chooses an FE_LPF signal and a TE_LPF signal to instead of the FE signal 502 and the TE signal 506 respectively to input a focusing controller 510 and a tracking controller 520, so as to generate the F00 signal 504 and the TR0 signal 507. Wherein, the FE_LPF signal and the TE_LPF signal are respectively the FE signal 502 and the TE signal 506 passed from low passed filters 512 and 522. By doing so, the spike signal, those positive and negative surges, and the signal F00 504 and the TR0 signal 507 affected by the defects can be improved. Furthermore, the servo control unit 210 could be protected from a disturbed or instable situation Besides, the servo control unit 210 can also zero the FE signal and the TE signal (as the FE/TE signal 513 shown in FIG. 5B), and/or mute or partially mute the output signals of the focusing controller 510 and the tracking controller 520 (as the F00/TR0 signal 515 shown in FIG. 5B), or even digitally zero the values of the FE signal and the TE signal after an analog-to-digital conversion so as to reduce the effect by the defects. Also, the servo control unit 210 can hold the FE signal, TE signal, F00 signal, and TR0 signal just before the defect happens. If the defect happens, those signals could replace the FE/TE signals affected by the defects.
Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from what is intended to be limited solely by the appended claims.

## Claims

1. A device for protecting a servo system in reading signals on a defective optical disc from disturbance and instability, said device comprising:
a defect detection unit configured to receive a plurality of defective RF signals, at least comprising an ADefect signal, an ADefect1 signal, an interruption signal, a RPDefect signal, and a DSPDefect signal, from the optical disc and to set a plurality of defect flag signals, at least comprising an ADefect flag, an ADefect1 flag, an interruption flag, a RPDefect flag, and a DSPDefect flag, by the defect detection unit, wherein the ADefect flag is set to 1 while a RFLVL signal is lower than the level of the ADefect signal, the ADefect1 flag is set to 1 while the RFLVL, signal is lower than the level of the ADefect1 signal, the interruption flag is set to 1 while the RFLVL signal is higher than the level of the interruption signal, the RPDefect flag is set to 1 while a RFRP signal is lower than the level of the RPDefect signal, and the DSPDefect flag is set to 1 while an absolute difference value between the RFLVL, signal and an RFLVL_LPF signal is bigger than a predetermined threshold,
wherein the RFLVL signal is the envelope of an RF signal and each of the levels of the ADefect, ADefect1, interruption, RPDefect signals is a fixed DC referred voltage level, and the level of the ADefect1 signal is higher than the level of the ADefect signal, and the RFRP signal is the peak or the bottom envelope of the RF signal, and the RFLVL_LPF signal is a slowly falling signal of the RFLVL signal passed a low pass filter;
a logic combination unit, configured to perform logic operation on said defect flag signals in order to trigger defect protection for said servo system; and
a servo control unit configured to generate low-pass output signals according to said plurality of defect flag signals for controlling a plurality of electromechanical devices in said servo system when a specified defect is detected.

2. The device according to claim 1, wherein said servo control unit mutes or partially mutes a focusing output signal and a tracking output signal when said specified defect is detected.