Abstract:
A method of controlling write power for recording data into sectors of an optical storage medium. The optical storage medium comprises a plurality of sectors, each having a header, a data recording area, and a redundant area. The method comprises sampling a write power output from an optical emitter when the redundant area of the sectors is pointed thereby; generating a write power control force according to a deviation of the sampled write power from an ideal write power corresponding to a write power command; and adjusting the write power according to the write power error control force. Additionally, the redundant area is not a gap section or a mirror region. A circuit and method for determining whether to carry out automatic power control or not during recording data on an optical storage medium are also disclosed.

Description:
BACKGROUND  
       [0001]     The invention relates to data recording on an optical storage medium and, in particular, to automatic power control (APC) for recording data on an optical storage medium.  
         [0002]     In general, data are recorded and reproduced on an optical storage medium such as an optical disc in a unit of area called a sector defined along a recording track. A sector typically comprises four sections, a header section providing physical identification data (PID), a gap section for controlling the laser power, a data section for recording information, and a buffer section for redundancy. The four sections are arranged successively in a sector. Data are recorded only in the data section. The laser power needs to be optimally controlled for recording and reproduction, respectively.  
         [0003]     It is desirable to maintain the intensity of the laser beam on a storage medium at prescribed values while recording. If a laser diode is used as the laser source, the characteristic of light-emitting power against drive current changes dramatically with ambient temperature as well as the duration of the laser diode operation.  FIGS. 1A and 1B  show examples of the light emitting power vs. drive current characteristics of a laser diode, wherein the threshold value I th  denotes a drive current at which the laser diode starts to emit light. As shown in  FIG. 1A , the threshold value I th  increases and the slope coefficient η or a slope of a curve of the light-emitting power vs. drive current characteristic decreases as the temperature increases. On the other hand, as shown in  FIG. 1B , the threshold value I th  increases and the slope coefficient η decreases with long term operation of the laser. Thus, maintaining the drive current of a laser diode at prescribed values under various conditions is necessary. Therefore, in an apparatus for recording and reproducing information optically, the laser is controlled so as to maintain the optical intensity of the laser beam on an optical storage medium at prescribed values.  
         [0004]      FIG. 2  is a block diagram of an optical recording circuit with automatic power control (APC). A photodiode  17  detects a light originally generated by a laser diode  16  and generates a photocurrent proportional to the laser power. A current-to-voltage (I/V) converter  18  converts the photocurrent to a voltage. The voltage is sampled by a sample and hold (S/H) circuit  10  and then compared with an erase power command  11  in an erase power loop error compensator  12 . As a result, an erase power loop error compensator  12  generates an erase power control force for the laser driver  13 . The laser diode driver  13  thus adjusts the laser power of a laser diode  16  accordingly. The closed loop system enables the erase power generation exactly following the erase power command  11 . Accordingly, the laser power is very stable and unaffected by temperature variation. As for write power control, an open loop is typically used with reference to the erase power for write power estimation. In  FIG. 2 , a write power command  14  controls the laser driver  13  through a mapping table for adjusting the write laser power of the laser diode  16 .  
         [0005]      FIG. 3  shows waveforms of signals required for data recording on an optical storage medium. A signal WPC in  FIG. 3  stands for a waveform of a write pulse combination for recording. A write pulse with multi-pulse waveform is used to form pits, while an erase pulse with DC level is used to form lands. Signals WFPDSH and EFPDSH are respectively pulled high when pits and lands are formed. An output signal of the photodiode is sampled as a feedback signal of the closed loop automatic power control (APC) system. When a land is formed, a DC erase power is used for APC. The signal EFPDSH can control the S/H circuit to sample an output signal of the I/V converter, which is relative to the output power of the photodiode. Since a multi-pulse write pulse is used to burn pits, and the laser power switches rapidly, the output bandwidth of the photodiode is limited and the photodiode &amp; I/V converter cannot respond to the power transition in time. The S/H circuit requires a period to settle the sampled signal. Accordingly, APC may not be carried out for calibrating the write power during a write period. The write power of the laser diode is typically adjusted through a mapping table or prediction methods based on an APC calibrated erase power. However, such methods are more suitable for laser diodes with a linear output characteristic. In most cases, a laser diode has a non-linear output characteristic. Thus, the write power is not precisely controlled and is not stable.  
       SUMMARY  
       [0006]     An embodiment of a write power control method for recording data into sectors of an optical storage medium comprises sampling a write power output from an optical emitter when a redundant area of the sectors is pointed thereby; generating a write power control force according to a deviation of the sampled write power from an ideal write power corresponding to a write power command; and adjusting the write power according to the write power error control force. The optical storage medium comprises a plurality of sectors, each having a header, a data recording area, and a redundant area.  
         [0007]     An embodiment of a method for determining when to carry out automatic power control comprises starting a counter when accessing the beginning of a sector; generating a redundant area start signal when a redundant area in the sector is going to be accessed; latching a value of the counter according to the redundant area start signal; and starting automatic power control if the latched counter value is less than a first threshold value.  
         [0008]     An embodiment of a circuit for determining when to carry out automatic power control comprises a counter, a decision circuit, an encoder, a normal write strategy generator, a multiplexer, a laser diode driver, an automatic power control circuit, a sample and hold circuit, and an automatic power control pulse generator. The decision circuit, latching a value of the counter and generating a selection signal accordingly, has an input coupled to the counter. The encoder has an output coupled to the decision circuit. The normal write strategy generator has an input coupled to the encoder. The multiplexer has a first data input coupled to the normal write strategy generator and a selection input, receiving the selection signal, coupled to the decision circuit. The laser diode driver is coupled to an output of the multiplexer. The automatic power control circuit, controlling the laser diode driver to adjust a power of a laser diode, is coupled to the laser diode driver. The sample and hold circuit, feeding back the power of the laser diode, is coupled to the automatic power control circuit. The automatic power control pulse generator is coupled to the sample and hold circuit and a second data input of the multiplexer. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]      FIGS. 1A and 1B  show examples of the light emitting power vs. drive current characteristic of a laser diode.  
         [0010]      FIG. 2  shows a block diagram of an optical recording circuit for automatic power control (APC).  
         [0011]      FIG. 3  shows signals generated for recording data on an optical storage medium.  
         [0012]      FIG. 4A  shows a sector field layout of a rewritable data zone.  
         [0013]      FIG. 4B  shows actions of a counter according to an embodiment of the invention.  
         [0014]      FIG. 4C  shows physical identification data used for synchronization according to an embodiment of the invention.  
         [0015]      FIG. 5  is a flowchart showing a method for determining whether to carry out automatic power control or not during data recording according to an embodiment of the invention.  
         [0016]      FIG. 6  shows a circuit for determining whether to carry out automatic power control or not during data recording according to an embodiment of the invention.  
         [0017]      FIG. 7  shows a block diagram of the automatic power control circuit shown in  FIG. 6  according to an embodiment of the invention.  
         [0018]      FIG. 8A  shows an exemplary EFM pattern output from the encoder for normal writing.  
         [0019]      FIGS. 8B and 8C  respectively show exemplary waveforms of a write pulse and an erase pulse corresponding to the EFM pattern shown in  FIG. 8A .  
         [0020]      FIG. 8D  shows a write pulse combination derived from the write and erase pulses shown in  FIGS. 8B and 8C .  
         [0021]      FIG. 8E  shows a signal EFPDSH corresponding to the EFM pattern shown in  FIG. 8A  for controlling the sample and hold circuit during normal writing.  
         [0022]      FIG. 8F  shows a sector field layout of an optical storage medium.  
         [0023]      FIG. 8G  shows an exemplary EFM pattern output from the encoder for carrying out write power APC.  
         [0024]      FIGS. 8H and 8I  respectively show exemplary waveforms of a write pulse and an erase pulse corresponding to the EFM pattern shown in  FIG. 8G .  
         [0025]      FIG. 8J  shows a write pulse combination derived from the write and erase pulses shown in  FIGS. 8H and 8I .  
         [0026]      FIG. 8K  shows a signal WFPDSH corresponding to the EFM pattern shown in  FIG. 8G  for controlling the sample and hold circuit during APC writing. 
     
    
     DETAILED DESCRIPTION  
       [0027]     According to the specification of a DVD-RAM format, a sector field layout of a rewritable data zone is shown in  FIG. 4A . The regions in a sector  407  other than header  401  are all recordable fields. A buffer section  403  therein has a length of 24 to 25 Bytes, i.e. 384 to 400 channel bits. The content in the buffer section  403  is negligible during normal reading. Due to track run-out or spindle spin speed variations, an actual length of data recording is different from a physical length of a data recording area  405 . The buffer section  403  provides a redundant area for data recording when the actual length of data is longer than the physical length of the data recording area  405 . Thus, the header  401  of a next sector is prevented from damaged by the laser power during optical recording. Additionally, there is no data loss in optical recording when the actual length of data exceeds the length of the data recording area  405 .  
         [0028]     For DVD-RAM, a spindle spins at a zoned constant linear velocity (ZCLV). Under the same spinning speed, 2× for example, the data transfer rate is the same for any track in the same zone. A time period of each sector is fixed. A counter for timing utilizes a fixed clock instead of a wobble clock. Theoretically, a final counter value for each sector field is the same. To make the counter more reliable, physical identification data PID 1 ˜ 4  are used for synchronization, as shown in  FIG. 4C . When one of the physical identification data PID 1 ˜ 4  is decoded, a corresponding counter value is reloaded into the counter.  
         [0029]     Take a 2× transfer rate for example, a counter counts from 0 to 9999 with a fixed clock frequency of 13.53MHz for each sector, as shown in  FIG. 4B . There are 2697 Bytes in a sector field and the header therein has a length of 130 Bytes. Accordingly, the counter counts to about 481 (which is derived by (130/2697)×10000−1) at the end of the header. There are 24 to 25 Bytes in the buffer section. When the buffer section starts, the counter value is between 9906 (((2697−25)/2697)×10000−1) and 9910 (((2697−24)/2697)×10000−1), the value n in  FIG. 4B  is thus between 9906 and 9910. The counter value, however, may count to a value not within a range between 9906 and 9910 due to track run-out or spindle spin speed variations. If the counter value is larger than 9910, the actual length of data recording in the sector is longer than the physical length of the data recording area of the sector, and a part of the buffer section is used for data recording. If the counter value is smaller than 9906, the actual length of data recording in the sector is shorter than the physical length of the data recording area of the sector, which extends the recordable area of the sector.  
         [0030]      FIG. 5  shows an embodiment of a method for determining whether to carry out automatic power control or not during data recording. The method comprises starting a counter when accessing the beginning of a sector of the optical storage medium (step  501 ); generating a redundant area start signal when a redundant area is going to be accessed (step  503 ); latching a value of the counter according to the redundant area start signal (step  505 ), and starting automatic power control if the latched counter value is smaller than a first threshold value (step  507 ). The optical storage medium comprises a plurality of sectors, and each sector comprises a header, a data recording area, and a redundant area. More specifically, the optical storage medium is an optical disc with a DVD-RAM format, and the sector field layout for DVD-RAM is shown in  FIG. 4A , where the redundant area is the buffer section  403 . Preferably, the buffer section  403  follows the data recording area  405 . If the counter value latched at the beginning of the buffer section  403  is smaller than a first threshold value, for example, 9930 in the case of  FIG. 4B , the buffer section  403  can be used for write power control. To the contrary, if the counter value latched at the beginning of the buffer section  403  is larger than the first threshold value, there is not enough space for automatic power control, and the automatic power control will not be activated. To prevent damage to the header of a subsequent sector by the write power, the automatic power control has to end before accessing the next header. Accordingly, a second threshold can be set to stop the automatic power control. If the counter value is larger than the second threshold,  9980  for example, the automatic power control is stopped.  
         [0031]      FIG. 6  shows an embodiment of a circuit for determining whether to carry out automatic laser power control or not for optical recording. The circuit comprises a counter  601 , a decision circuit  602 , an encoder  603 , a write strategy generator  604 , a multiplexer  606 , a laser diode driver (LDD)  607 , an automatic power control (APC) circuit  608 , a sample and hold (S/H) circuit  609 , and an automatic power control pulse generator  605 . The decision circuit  602  has an input coupled to the counter  601 . The decision circuit  602  latches a value of the counter and generates a selection signal APC_ACTION accordingly. The encoder  603  has an output coupled to the decision circuit  602 . The write strategy generator  604  has an input coupled to the encoder  603 . The automatic power control pulse generator  605  is coupled to the encoder  603  for generating a write pulse and an erase pulse for recording a redundant area of the optical storage medium. The multiplexer  606  has a first data input coupled to the write strategy generator  604 , a second data input coupled to the automatic power control pulse generator  605 , and a selection input coupled to the decision circuit  602 . The laser diode driver  607  is coupled to an output of the multiplexer  606 . The automatic power control circuit  608  is coupled to the laser diode driver  607 . The sample and hold circuit  609  is coupled to the automatic power control circuit  608 .  
         [0032]     Take a 2× transfer rate for example, the counter  601  counts from 0 to 9999 with a fixed clock frequency of 13.53MHz for each sector. During normal data recording, the counter  601  counts from 482 to n−1as shown in  FIG. 4B . The encoder  603  provides encoded data to the write strategy generator  604  during normal data recording. When normal data recording ends, the encoder  601  transmits a redundant area start signal to the decision circuit  602 . The decision  602  latches the counter value n when receiving the redundant area start signal from the encoder  601 . If the counter value n latched at the beginning of the buffer section is smaller than a first threshold value, the output signal APC_ACTION of the decision circuit  602  is pulled high. When the output signal APC_ACTION of the decision circuit  602  is at a high state, the multiplexer  606  selects output signals, an APC write pulse and an APC erase pulse, of the APC pulse generator  605 , and outputs them to the laser diode driver  607 . The laser diode  607  also receives output signals, a write power and an erase power, of the automatic power control circuit  608 , and generates a driving current to drive the laser diode  610  according to the received signals. The photo diode  611  generates a current proportional to the laser power, and the current is converted to a voltage by an I/V converter  612 . The voltage is sampled by a sample and hold circuit  609  and transmitted to the automatic power control circuit  608  to control the write power and erase power.  
         [0033]      FIG. 7  shows a block diagram of the automatic power control circuit shown in  FIG. 6  according to an embodiment of the invention. The automatic power control circuit  608  comprises an erase power loop error compensator  722 , a delay  720 , a substractor  729 , and a write power loop error compensator  725 . The erase power loop error compensator  722  receives an erase power command  721 . The delay  720  is coupled to the sample and hold circuit  609 . The subtractor  729  has inputs respectively coupled to the delay  720  and the sample and hold circuit  609 . The write power loop error compensator  725  is coupled to an output of the subtractor  729 . The write power loop error compensator  725  also receives a write power command  724 .  
         [0034]     In  FIG. 7 , the photo diode  611  generates a current proportional to the laser power and the current is converted to a voltage by an I/V converter  612 . In an erase power loop, the voltage is sampled by the sample and hold circuit  609  and compared with an erase power command  721  in the erase power loop error compensator  722 . As a result, the erase power loop error compensator  722  generates an erase power control force, and the laser diode driver  607  adjusts the erase power of the laser diode  610  according thereto. Thus, a closed erase power loop system is formed. The closed loop system enables the erase laser power generation exactly following the erase power command  721 . Accordingly, the laser erase power is very stable and is not affected by temperature variation.  
         [0035]     The erase laser power sampled by the sample and hold circuit  609  is transmitted to the subtractor  729  via a delay unit  720 . In a write power loop, the write laser power is sampled by the sample and hold circuit  609  and subtracted by the feedback erase laser power such that a write power control force corresponding to the write power command  724  is generated by the write power loop error compensator  725 . As a result, the laser diode driver  607  adjusts the write power of the laser diode  610  according to the write power control force. Thus, a closed write power loop system is formed. The closed loop system enables the write laser power generation exactly following the write power command  724 . Accordingly, the laser write power is very stable and is not affected by temperature variation.  
         [0036]     FIGS.  8 A˜ 8 K illustrate waveforms of signals for laser write and erase power control according to an embodiment of the invention. FIG. BA shows an exemplary EFM pattern output from the encoder for optical recording during normal writing.  FIGS. 8B and 8C  respectively show waveforms of a write pulse and an erase pulse corresponding to the EFM pattern of  FIG. 8A .  FIG. 8D  shows a normal write pulse combination derived by combining the pulses of  FIGS. 8B and 8C  during normal writing.  FIG. 8E  shows a signal EFPDSH controlling the sample and hold circuit to sample the output signal of the photo diode. The sampled value reflects the erase power during normal writing, and can be used to control laser power.  FIG. 8F  shows a sector field layout of an optical storage medium, and normal writing includes recording data on the data section.  FIG. 8G  shows an exemplary EFM pattern output from the encoder for carrying out write power APC in a redundant area.  FIGS. 8H and 8I  respectively show waveforms of a write pulse and an erase pulse for carrying out write power APC in a redundant area.  FIG. 8J  shows a write pulse combination derived by combining the pulses of  FIGS. 8H and 8I  for carrying out write power APC in a redundant area.  FIG. 8K  shows a signal WFPDSH controlling the sample and hold circuit for carrying out write power APC in a redundant area. Write power APC is carried out in a buffer section as shown in  FIG. 8F . An encoder generates an EFM pattern of  FIG. 8G  having pits with a length of XT and lands with a length of YT. The waveforms of write and erase pulses shown in  FIGS. 8H and 8I  correspond to the EFM pattern of  FIG. 8G , and the combination of the write and erase pulses is shown in  FIG. 8J . Control signal WFPDSH shown in  FIG. 8K  controls the sample and hold circuit to sample the output signal of the photo diode. The sampled value reflects the write power. As long as the length of lands (YT) is well controlled, an output signal of the photodiode exactly reflects the write power and therefore can be used as a feedback signal for write power APC. On the other hand, the normal write pulse combination and the corresponding signal EFPDSH are used for erase power APC during normal writing.  
         [0037]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and the advantages would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.