Abstract:
A method for using a compact disc decoder to correct a code error in digital data read out from a compact disc includes extracting address data out of at least one sector read from the compact disc, and using an error detection circuit to read a condition of an input error flag corresponding to the address data of the sector, wherein if the condition of the input error flag indicates that the address data contains an error, then the extracting circuit extracts address data of another sector. The method also includes when the input error flag indicates that no error is present in the address data of the sector, the sector is referred to as a first sector, and the selection circuit selects address data of the first sector without referring to address data of any sectors read before the first sector.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a compact disc decoder, and more specifically, to a compact disc decoder that uses an input error flag to correct address errors in header data read from the compact disc.  
           [0003]    2. Description of the Prior Art  
           [0004]    Compact discs (CDs) are commonly produced using two formats: CD Read Only Memory (CD-ROM) and CD Digital Audio (CD-DA). In each of these formats, digital data is read off of the CD, and processed by a CD-ROM drive. In addition, CD-DA discs may also be played in an audio CD player. In U.S. Pat. No. 5,621,743 entitled “CD-ROM Decoder for Correcting Errors in Header Data”, Tomisawa discloses a prior art CD-ROM decoder, which is included herein by reference.  
           [0005]    Please refer to FIG. 1. FIG. 1 is a functional block diagram of a CD drive that is capable of decoding CD-ROM and CD-DA discs according to the prior art. A pickup unit  2  receives reflected light of a laser beam irradiated to a compact disc  1 , converts the intensity of the reflected light into a voltage signal representing the intensity value, and supplies the signal to an analog signal processing unit  3 . The analog signal processing unit  3  reads out digital data written in the compact disc  1  from the input signal, and outputs, in series, the digital data having a format similar to the given format. The output from the analog signal processing unit  3  is connected to an input of a digital signal processing unit  4 , which carries out processing of the digital data input from the analog signal processing unit  3  in accordance with the proper digital data format, CD-ROM format or CD-DA format. The signal processing in the digital signal processing unit  4  maintains compatibility with a digital audio CD system, and includes, for example, demodulation of 14 bit digital data to 8 bit data and code error detection/correction based on Reed-Solomon code. A CD-ROM decoder  5  and a CD-DA decoder  39  respectively provide additional code error correction for the CD-ROM data or CD-DA data fed from the digital signal processing unit  4  and transfer the CD-ROM or CD-DA data, which has substantially no errors, to a host computer. A buffer RAM  6  is connected to the CD-ROM decoder  5  and the CD-DA decoder  39  to temporarily store the CD-ROM or CD-DA data, which has been supplied from the digital signal processing unit  4  to the CD-ROM decoder  5  or the CD-DA decoder  39 , for a given period. A control micro computer  7  controls operation of the analog signal processing unit  3 , digital signal processing unit  4 , CD-ROM decoder  5 , and CD-DA decoder  39  in accordance with the operation programs so that each unit carries out the respective processing at the correct time.  
           [0006]    Please refer to FIG. 2. FIG. 2 shows a typical data format for a sector of conventional CD data. The CD data output from the digital signal processing unit  4  shown in FIG. 1 is divided into a number of sectors, and each sector is 2352 bytes and includes a synchronization signal (12 bytes), header (4 bytes) and user data (2336 bytes) as shown in FIG. 2, and as is well known in the art.  
           [0007]    [0007]FIG. 3 is a functional block diagram of the conventional CD-ROM decoder  5 . A descramble circuit  11  provides descramble processing for the 2340 bytes of the 2352 bytes (1 sector) of CD ROM data input, disregarding the 12 byte synchronization signal, and outputs data which is recovered to be a given format. A write buffer  12  extracts 2336 bytes of data (hereinafter referred to as user data) from the data output from the descramble circuit  11  and writes the user data through a first data bus  16  into the buffer RAM  6 . A header register  13  takes in 4 bytes of the data output from the descramble circuit  11  and transfers the header information via a second data bus  17  to the control micro computer  7 . A synchronization signal detection circuit  14  detects a 12 byte synchronization signal assigned to the leader portion of the respective sectors of the input data and supplies a timing signal representing the beginning of the sector&#39;s CD-ROM data input to an operation control circuit  25 , details of which will be described below. When the synchronization signal is not detected, data showing the detection error is fed to the control micro computer  7  via the second data bus  17 . An error flag register  15  extracts an error flag indicating that errors are still left after the error correction by the digital signal processing unit  4  arranged before the CD-ROM decoder  5  and transfers the information via the second data bus  17  to the control micro computer  7 .  
           [0008]    A write address generator  18  generates a series of addresses at a constant cycling period to designate a write address of the CD-ROM data which is to be written into the buffer RAM  6  from the write buffer  12 . A leading address generator  19  receives an address of the buffer RAM  6 , to which the leader portion of the respective sectors is to be written, from the address generator  18 . After keeping the received addresses until completion of the writing operation for a sector of the CD-ROM data, the leading address generator  19  feeds the addresses to the first data bus  16 . The leading addresses are also fed to the control micro computer  7  via the second data bus  17  so as to produce preset data for a transfer address generator  21 . An error correction circuit  20  takes in the leading address data via the first data bus  16  and sequentially reads out, based on the address data, the CD-ROM data which was written into the buffer RAM  6 . The error correction circuit  20  then detects and corrects a code error on the basis of the error detection code (EDC) and error correction code (ECC), which have been set in the user data. When the data has been subjected to given error correction processing in the above described manner, it is again written into the buffer RAM  6 .  
           [0009]    The transfer address generator  21  is loaded with the preset data corresponding to the leading address of the buffer RAM  6 , at which time the reading out of the CD-ROM data begins. In response to a command from a buffer controller  22 , the transfer address generator  21  generates a series of addresses beginning from an address corresponding to the preset data. The generated addresses are fed via the first data bus  16  to the buffer RAM  6  and used for the designation of the readout address of the CD-ROM data which has been subjected to the error correction processing. A transfer byte counter  23  is loaded with preset data representing the CD-ROM data to be read out from the buffer RAM  6  and then decrements (counts down) the preset data value every time a sector of the CD-ROM data is read out from the buffer RAM  6 . At the point when a given count is completed, the counter  23  supplies a stop command to the buffer controller  22 . A transfer buffer  24  receives, via the first data bus  16 , the CD-ROM data which has been read out in accordance with the address generated by the transfer address generator  21  and transfers the data to the host computer. Each preset data loaded on the transfer address generator  21  and transfer byte counter  23 , respectively, is generated by the control micro computer  7  based on the leading address fed from the leading address generator  19  and a transfer command given by the host computer.  
           [0010]    The operation control circuit  25  counts the time period taken for the completion of error correction made by the error correction circuit  20 , on the basis of a timing signal from the synchronization signal detection circuit  14  and generates another timing signal indicating the completion of the error correction operation. The error correction processing is carried out inside the error correction circuit  20  after taking in a sector of CD-ROM data from the buffer RAM  6 , during which the next one sector of CD-ROM data is being written in the buffer RAM  6 .  
           [0011]    An interrupt command generator  26  receives either the timing signal from the operation control circuit  25  or the stop command from the transfer byte counter  23  and feeds an interrupt command to the control micro computer  7 . In response to the interrupt command, the control micro computer  7 , which carries out the operation control for the analog signal processing unit  3  and digital signal processing unit  4  on a time sharing basis, suspends the operation which is being carried out at that point and allows the CD-ROM decoder  5  to perform the next operation. In other words, by interrupting the current operation in response to the interrupt command, the control micro computer  7  may drive the buffer controller  22  to start the data transfer from the buffer RAM  6  to the host computer.  
           [0012]    Please refer to FIG. 4. FIG. 4 shows address data judging circuitry located in the header register  13  of the CD-ROM decoder  5 . The header register includes an input address register  31 , an address incrementor  32 , an address information register  33 , a selector  34 , and a comparator  35 .  
           [0013]    The input address register  31  receives and stores three bytes of address data representing minutes (MIN), seconds (SEC) and frame number (FRAME) from among the four bytes of header information assigned for every sector of the CD-ROM data. The three bytes of address data are extracted by the descramble circuit  11  and supplied to the input address register  31 . The value of FRAME data causes an increment of the SEC data every 75 frames, and the value of SEC data causes an increment of the MIN data every 60 seconds (4,500 frames). One frame is equal to one sector (2352 bytes).  
           [0014]    The address incrementor  32  reads out address data from either the input address register  31  or the address information register  33  and adds a value of 1 to the address data, which is then supplied to the address information register  33 . A value of 1 is successively added to a frame number, and when the frame number reaches  74 , it rolls to “0” and increments the SECOND value. The SECOND value is incremented from “0” to “ 59 ”, and by the next addition of a value “ 1 ”, the SECOND value returns to “0” and a value “1” is added to the MINUTE value. The MINUTE value is incremented from “0” to “81”, and by the next addition of a value “1”, it returns to “0”.  
           [0015]    The address information register  33  receives and stores address information output from the address incrementor  32 . When the address data from the address incrementor  32  is taken into the address information register  33 , the next address information is supplied to the input address register  31 . The selector  34  receives both the address information read out from the input address register  31  and the address information register  33 , respectively, and selects one of them for output. This selection is made based on a control pulse supplied by the comparator  35 . The comparator  35  also receives both address information output from the input address register  31  and the address information register  33  and compares the two values to determine whether or not these two values coincide. In accordance with the comparison result, the comparator  35  generates a control pulse, which is supplied to the selector  34 . When the two values of address information stored in the respective registers  31  and  33  coincide, address data read out from the input address register  31  is selected to be output. On the other hand, when the two addresses do not coincide, address data from the address information register  33  is selected to be output. At the same time, a selection pulse is supplied to the address incrementor  32 . When the two address information values stored in the respective registers  31  and  33  coincide, the value from the input address register  31  is taken into the address incrementor  32 , and when these two do not coincide, address data from the address information register  33  is taken into the address incrementor  32 .  
           [0016]    Address data stored in the address information register  33  is shifted by one sector from that in the input address register  31  at the same point in time. However, because of the addition of a value “ 1 ”, the two values of address information stored in the respective registers  31  and  33  must coincide as long as there is no code error. When a code error occurs, data from the input address register  31  becomes discontinuous, while address data stored at the same time in the address information register  33  is still continuous, resulting in differences between the address information in the two registers. For instance, please refer to FIG. 5. FIG. 5 is a timing diagram showing operation of the address data judging circuitry located in the header register  13 . As shown in FIG. 5, even if the data value stored in the input address register  31  presents a value of “03:15:A7”, which was supposed to be “03:15:74”, following a value of “03:13:73” due to a code error, the address information input to the address information register  33  still maintains regularity and presents a correct value of “03:15:74”, following a value-of “03:15:73”. When the address information read out from the respective register  31  and  33  do not coincide, the comparator  35  determines that address data stored in the input address register  31  contains a code error, which makes the selector  34  select and output the address information read out from the address information register  33 . Simultaneously, to maintain the regularity of the address data which is to be supplied next from the address incrementor  32  to the address information register  33 , the address information in the address information register  33  is taken into the address incrementor  32  instead of the address information from the input address register  31 . With reference to FIG. 5 again, in the case that data having a value “03:15:A7” is read out from the input address register  31  due to a code error, address information “03:15.74” stored in the address information register  33  is taken into the address incrementor  32  and a value “1” is added to it. As a result, the correct succeeding address information “03:16:00” is stored in the address information register  33  without losing regularity.  
           [0017]    However, there is a shortcoming with the address data judging circuitry of the header register  13  in the CD-ROM decoder  5 . As shown in FIG. 5, the first address information read into the input address register  31  is “03:15:73”. Since this is address information read from a first sector that the CD-ROM decoder receives, and the address information register  33  has no knowledge of what the first sector address is, the address information register  33  receives the same address information located in the input address register  31 . Starting with the second sector, the address information register  33  has knowledge of what the next sector address will be, and the judging circuitry is able to serve its purpose. Unfortunately, with the first sector, there is no way of knowing if the address information corresponding to the first sector is correct. That is, if address information of the first sector is wrong, then the address information register  33  will contain wrong address information for all subsequent sectors.  
           [0018]    The problem just mentioned is a problem not only in the CD-ROM decoder  5 , but in the CD-DA decoder  39  as well. Please refer to FIG. 6. FIG. 6 is a functional block diagram of the conventional CD-DA decoder  39 . Main data is fed into a Cross Interleaved Reed-Solomon (CIRC) decoder  48 , which decodes the main data and detects any errors present in the main data. Subcode data is first fed into a subcode buffer  40 , and a synchronization signal detection circuit  41  detects a 2 byte synchronization signal contained in leader portions of the respective sectors of the input subcode data. Q subcode information is then fed from the subcode buffer  40  into a Q-code buffer  42 . A cyclic redundancy code (CRC) check circuit  46  is used to check errors of data stored in the Q-code buffer  42 . A Q-code address register  44  is used to perform a similar task as the header register  13  in the CD-ROM decoder  5 . That is, the Q-code address register  44  also contains address judging circuitry like that shown in FIG. 4. Unfortunately, the address judging circuitry in the CD-DA decoder  39  contains the same problem as the address judging circuitry in the header register  13  of the CD-ROM decoder  5 . Namely, if address information contained in a first read sector is not correct, the address information register  33  will have incorrect address information data, and no proper address correction can take place on address information of subsequent sectors.  
         SUMMARY OF INVENTION  
         [0019]    It is therefore a primary objective of the claimed invention to provide a method for using a compact disc decoder to provide correct address information for all sectors read from the compact disc in order to solve the above-mentioned problems.  
           [0020]    According to the claimed invention, a method for using a compact disc decoder to correct a code error in digital data read out from a compact disc, which is divided into sectors, is disclosed. The compact disc decoder includes an extracting circuit for extracting address data from the digital data, an error detection circuit for detecting presence of an error in the address data, a correction data generating circuit for receiving the address data and producing correction data, and a selection circuit for selecting address data or correction data. The method comprises using the extracting circuit to extract the address data out of at least one sector read from the compact disc, and using the error detection circuit to read a condition of an input error flag corresponding to the address data of the sector, wherein if the condition of the input error flag indicates that the address data contains an error, then the extracting circuit extracts address data of another sector, and when the input error flag indicates that no error is present in the address data of the sector, the sector is referred to as a first sector, and the selection circuit selects address data of the first sector without referring to address data of any sectors read before the first sector.  
           [0021]    It is an advantage of the claimed invention that the compact disc decoder reads the value of the input error flag corresponding to each sector. The input error flag allows the compact disc decoder to immediately know if there is an error in address information data contained in the sector, and to re-read the sector if an error was detected. In this way, address correction is greatly enhanced since errors in address information data are not propagated to subsequent sectors.  
           [0022]    These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0023]    [0023]FIG. 1 is a functional block diagram of a CD drive that is capable of decoding CD ROM and CD-DA discs according to the prior art.  
         [0024]    [0024]FIG. 2 shows a typical data format for a sector of conventional CD data.  
         [0025]    [0025]FIG. 3 is a functional block diagram of a conventional CD-ROM decoder.  
         [0026]    [0026]FIG. 4 shows address data judging circuitry located in a header register of the CD-ROM decoder.  
         [0027]    [0027]FIG. 5 is a timing diagram showing operation of the address data judging circuitry located in the header register.  
         [0028]    [0028]FIG. 6 is a functional block diagram of a conventional CD-DA decoder.  
         [0029]    [0029]FIG. 7 is a functional block diagram of a CD-ROM decoder according to the present invention.  
         [0030]    [0030]FIG. 8 is a detailed block diagram of an address decision circuit shown in FIG. 7.  
         [0031]    [0031]FIG. 9 is a functional block diagram of a present invention CD-DA decoder.  
         [0032]    [0032]FIG. 10 is a state diagram illustrating control functions of a programmable data-select controller for the CD-ROM decoder and the CD-DA decoder of the present invention.  
         [0033]    [0033]FIGS. 11A and 11B are timing diagrams illustrating control of the CD-ROM decoder and the CD-DA decoder of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0034]    Please refer to FIG. 7. FIG. 7 is a functional block diagram of a CD-ROM decoder  205  according to the present invention. The CD-ROM decoder  205  of the present invention substitutes for the conventional CD-ROM decoder  5  illustrated in FIG. 3, and same reference numbers will be used for identical parts. The difference between the CD-ROM decoder  5  and the present invention CD-ROM decoder  205  is the present invention uses an address decision circuit  50  instead of the header register  13  used in the prior art CD-ROM decoder  5 .  
         [0035]    The address decision circuit  50  contains an address information register  54  for receiving sector address information and for outputting correct sector address information, an update limiter  52  for passing error information found in the error flag from the error flag register  15  to the address information register  54 , and a programmable data-select controller  56  for helping to control operation of the address information register  54  and the update limiter  52 . The error flag register  15  reads the error information from a C2 pointer of the CD-ROM, and generates the corresponding error flag. The error flag sent from the error flag register  15  to the update limiter  52  notifies the update limiter of an error in address information corresponding to a sector being read by the CD-ROM decoder  205 . If a value of the error flag indicates an error, the address decision circuit  50  acts accordingly, as will be explained below.  
         [0036]    Please refer to FIG. 8. FIG. 8 is a detailed block diagram of the address decision circuit  50  shown in FIG. 7. The address decision circuit  50  is similar to the address data judging circuitry located in the header register  13  of the prior art CD-ROM decoder  5 , as shown in FIG. 4. In fact, the address information register  54  is nearly identical to the address data judging circuitry used in the prior art. The address information register  54  contains the input address register  31 , the address incrementor  32 , the address information register  33 , the selector  34 , and the comparator  35 . The functionality of the address information register  54  is nearly identical to that of the address judging circuitry of the prior art. That is, address information is received by the input address register  31  and fed to the address incrementor  32 , which in turn increments the address information and sends it to the address information register  33 . In fact, the only difference between the address information register  54  and the address judging circuitry of the prior art lies in interaction with the update limiter  52  and the programmable data-select controller  56 . As stated earlier, the update limiter  52  receives error information from the error flag register  15  corresponding to each sector that is read from the CD. The update limiter  52  also sends the error information to the programmable data-select controller  56  for control operations.  
         [0037]    The programmable data-select controller  56  is used to control the update limiter  52  and the address information register  54 , and includes a counter  60  for counting numbers of successive sectors with or without a corresponding error, a programmable monitor register  62  for storing programmed information about how many successive sectors with or without errors are needed to change a state of the address decision circuit  50 , a comparator  64  for comparing a value in the counter  60  with a value in the programmable monitor register  62 , and a select controller  66  for controlling the selector  34  located in the address information register  54 . The programmable data-select controller  56  functions as a state machine for controlling the address decision circuit  50 , as will be explained below.  
         [0038]    Please refer to FIG. 9. FIG. 9 is a functional block diagram of a present invention CD-DA decoder  239 . The CD-DA decoder  239  of the present invention substitutes for the conventional CD-DA decoder  39  illustrated in FIG. 6, and same reference numbers will be used for identical parts. The difference between the CD-DA decoder  39  and the present invention CD-DA decoder  239  is the present invention uses an address decision circuit  70  instead of the Q-code address register  44  used in the prior art CD-DA decoder  39 . The function of the address decision circuit  70  used in the CD-DA decoder  239  is nearly identical to the function of the address decision circuit  50  used in the CD-ROM decoder  205 , and same reference numbers will be used for identical parts. In addition, the address decision circuit  70  is functionally identical to the address decision circuit  50  of the CD-ROM decoder  205 . Each has an update limiter  52  and a programmable data-select controller  56 . Moreover, a Q-code information register  74  of the address decision circuit  70  is analogous to the address information register  54  of the address decision circuit  50  that was shown in FIG. 8. With the CD-DA decoder  239 , error information is produced from the CRC check circuit  46 , and fed to the update limiter  52 . As will be explained below, error information is used as input to a state machine that controls the address decision circuit  70  and ensures that proper addresses are used for sectors read from the CD.  
         [0039]    Please refer to FIG. 10. FIG. 10 is a state diagram illustrating control functions of the programmable data-select controller  56  for the CD-ROM decoder  205  and the CD-DA decoder  239  of the present invention. When performing a read operation on a CD, a first sector that is read from the CD will be referred to as a first sector, even if the sector is located in the middle of the CD. When reading the first sector, the programmable data-select controller  56  will be in an idle state  100 . If address information of a sector is successfully read without the presence of an error, the controller  56  then executes an update limiter pass, and advances to an update address state  102 . When in the update address state  102 , if an address of one sector is read in the presence of a corresponding error signal, the controller  56  executes an address check fail, and returns to the idle state  100 . On the other hand, when in the update address state  102 , if the N most recent sector addresses are successfully read without an error, the controller  56  executes N address check passes, and advances to a correct address state  104 . Finally, the controller  56  will stay in the correct address state  104  unless M address check failures occur for M sectors, in which case the controller will return to the idle state  100 . Preferred values of M and N are 2 and 2, although any values can be used according to the present invention. Furthermore, data can be extracted from sectors read from the CD when the controller is in the update address state  102  or the correct address state  104 .  
         [0040]    Please refer to FIG. 11A and FIG. 11B with reference to FIG. 8 and FIG. 10. FIGS. 11A and 11B are timing diagrams illustrating control of the CD-ROM decoder  205  and the CD-DA decoder  239  of the present invention. As shown in FIG. 11A, the programmable data-select controller  56  starts off in the idle state  100 . Then an address “12:44:56” is read from a first sector and stored in the input address register  31 . Since this is the address of the first sector, the address is also stored in the address information register  33 . However, error information provided to the update limiter  52  reveals that the input error flag has a value of “1”, which indicates an error for the address information of this sector. Therefore, the address read for this sector is not valid, and the controller  56  continues to stay in the idle state  100 . Then another address “12:34:57” is read from another sector, which is also considered a first sector because of the idle state  100 , and the address is stored in the input address register  31 . Since this is the address of the first sector, the address is also stored in the address information register  33 . However, for the sector with the address “12:34:57”, the input error flag indicates no corresponding error. Therefore, the controller  56  executes an update limiter pass function, increments the address value in the address information register  33 , stores a value of 1 in the counter  60 , and advances to the update address state  102  for the next sector. Next, the address “12:34:58” is read into the input address register  31  while the controller  56  is in the update address state  102 . That means that the address values in the input address register  31  and the address information register  33  are compared by the comparator  35  and one of the values is selected by the selector  34 , as was the case with the prior art. Since there is no corresponding error indicated by the input error flag, the counter  60  is incremented to have a value of 2, meaning N (which is equal to 2 in this example) successive sector addresses have been read without error. Therefore, the controller advances to the correct address state  104 . The address in the address information register  33  is incremented from “12:34:58” to “12:34:59”. Finally, another sector address “13:34:59” is read with a corresponding error indicated by the input error flag. Since the next sector address read in to the input address register  31  is “13:34:59”, and not equal to the address in the address information register  33 , the selector  34  chooses the address in the address information register  33  to be the correct address for this sector.  
         [0041]    As shown in FIG. 11B, the controller starts off in the correct address state  104 , and reads in an address “12:35:56” without a corresponding error. The address value in the address information register  33  is incremented to “12:35:57”. Still in the correct address state  104 , an address “12:37:57” is read into the input address register  31  with a corresponding error indicated by the input error flag. Since the addresses in the input address register  31  and the address information register  33  are not equal, the selector  34  chooses the address in the address information register  33  to be the correct one. The address value in the address information register  33  is then incremented to “12:35:58”. The counter  60  indicates that this is the first sector address read with an error, and M successive sector addresses with errors have not yet been read (M is equal to 2 in this example), so the controller  56  remains in the correct address state  104 . Another address “02:35:58” is read into the input address register  31  with a corresponding error indicated by the input error flag. The correct address of “12:35:58” is selected from the address information register  33 . Because the counter  60  indicates  2  successive sector addresses have been read with a corresponding error, the controller switches to the idle state  100 . Finally, a new first sector address of “12:35:51” is read into the input address register  31  and also copied into the address information register  33 . Since there was no corresponding error indicated by the input error flag, the controller advances to the update address state  102  for the next state.  
         [0042]    In summary, the present invention is similar to the prior art, but also has the added advantage of using the input error flag to indicate the validity of a sector address. This identifies an error in a sector address immediately when it happens, even if the error occurs in the address of the first sector. Therefore, unlike the prior art, an error of the first sector address will not be propagated to subsequent sectors. Furthermore, the address decision circuits  50  and  70  are used to respectively control the CD-ROM decoder  205  and the CD-DA decoder  239  according to a status of the input error flag and numbers of successive sector addresses with or without an error. Furthermore, just as with the prior art, the comparator  35  and selector  34  are still used in the present invention to choose correct values from the input address register  31  or the address information register  33 . As a result, the present invention retains all advantages of the prior art, while at the same time using values in the input error flag to better identify sector addresses that contain errors.  
         [0043]    Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.