Methods of and apparatus for seeking a target address on a record medium

0001This invention relates to methods of and apparatus for seeking a target address on a record medium.

0002On an optical recording medium in the form of a disc, such a an optical disc or an opto-magnetic disc, spiral or concentric tracks are formed, with each track being divided into a plurality of sectors. On an optical recording medium in the form of a card, such as an optical card, each band-like record area is divided into a plurality of record tracks. At the prescribed position in the recording block, such a at each sector or each recording track, there are formed identification signals, including error detection codes such a synchronisation (sync) patterns, addresses or cyclic redundancy check (CRC) codes, as a result of pre-formatting by the card or disc supplier or formatting preceding the start of use of a new disc. It is by the address information included in the identification signals that a random access can be made to the recorded data on the record medium.

0003The identification part of each block or sector in which are recorded these identification signals is also termed the ID part or the address part in the broad sense of the term. In the case of certain optical discs, for example, there are multiple writings, for example three writings, of a unit of the identification information formed of an address consisting of a track address, a sector address and an error detection code (EDC) for detecting address errors.

0004It has also been proposed in regard to, for example, optical discs that the CRC code be replaced by an error correction code in order to deal with the higher error rates associated with optical discs. When accessing a prescribed sector on such a disc for signal recording or reproduction, the pick-up head movement is controlled while checking the current address by reproducing the identification signals. In reproducing the address a decoding operation such as the aforementioned error detection or correction operation is executed. This results in a complicated decoding process for error checking at the time of address reading or reproduction. Above all, when the information unit is formed by multiple recording or writing, it becomes necessary to determine the address by using, for example, a majority logic procedure after error detection or correction of each information unit, with a resulting complication in the hardware or the decoding algorithm, and prolonged computing time, thus presenting difficulties in achieving high speed accessing.

0005Such methods are further disclosed in Japanese patent specifications JP-A-60/50667 and JP-A-58/177537 (Patent Abstracts of Japan, vol. 9, no. 17 (P-375) (1900) 23 July 1985 and vol. 8, no. 18 (P-250) (1455) 26 January 1984, respectively).

0006According to the present invention there is provided a method of seeking a target address on a record medium on which has been recorded data divided into a plurality of blocks in correlation with an address for each block and an error correction code for each address also recorded on the record medium; said method being characterized by the steps of: <ul id="ul0001" list-style="none"><li>(a) forming from a supplied target address an error correction code to be appended to said target address;</li><li>(b) reading data recorded on said record medium, and extracting the address therefor together with the corresponding error correction code;</li><li>(c) comparing said target address and the error correction code formed therefrom with said extracted address and corresponding error correction code; and</li><li>(d) repeating steps (b) and (c) until the comparison in step (c) indicates that the number of faulty bits in the extracted address and error correction code is within the range of the error correction capability of the error correction code and then designating the currently accessed block to be the block corresponding to the target address.</li></ul>

0007According to the present invention there is also provided a target address seeking apparatus for use with a signal reproducing means for reproducing signals recorded on a record medium on which recorded data are divided into a plurality of blocks in correlation with an address for each block and an error correction code for each address is also recorded on the record medium; said target address seeking apparatus being characterized by: <ul id="ul0002" list-style="none"><li>(a) means for forming from a supplied target address an error correction code to be appended to said target address;</li><li>(b) means for extracting an address having an error correction code from the output signals of said signal reproducing means;</li><li>(c) means for comparing said target address and the error correction code formed from said target address with said extracted address and said extracted error correction code; and</li><li>(d) means for controlling the operation of said signal reproducing means on the basis of the results of the comparison by said comparing means in such a way that the currently accessed block is designated to be the block corresponding to the target address if the number of faulty bits in the extracted address and error correction code is within the range of error correction capability of the error correction code.</li></ul>

0008The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: <ul id="ul0003" list-style="none"><li>Figure 1 is a block diagram showing parts of an optical disc drive for explaining a first embodiment of the present invention;</li><li>Figure 2 shows an example of the recording format for signals recorded on the optical disc;</li><li>Figure 3 illustrates an error check operation;</li><li>Figure 4 is a block diagram showing parts of an optical disc drive for explaining a further embodiment of the present invention; and</li><li>Figure 5 shows a recording format for the optical disc, used in the explanation of Figure 4.</li></ul>

0009Figure 1 illustrates an apparatus for accessing a data block based on address seeking in accordance with an embodiment of the present invention. Signals are recorded on or reproduced from an optical disc 1, as an example of the recording medium, by an optical pick-up head 2. The signals from the pick-up head 2 are supplied to an address reading or reproducer circuit 3 where the address of the block or sector currently reproduced is read, and the thus read address signals are sent to a comparator circuit 5. These signals from the pick-up head 2 are also supplied to a data reproducer 4, so that data reproduced from the optical disc 1 are supplied to an output terminal 8.

0010An example of a proposed signal recording format for each block or sector on the optical disc 1 is shown in Figure 2. In this figure, a track on the optical disc 1 and an enlarged sector identification part or ID part are shown highly schematically. Each track consists of a plurality of sectors, with each sector consisting in turn of a pre-formatted identification part IDR and a data part DTR for recording the sector data. At the head of the identification part IDR is a sync signal PLOS for controlling the operation of a phase locked loop (PLL) circuit for producing clock signals at the time of data reading. Contiguous to this sync signal PLOS is a sector in which identifying address information UT is written three times in succession. Thus, three units UT1, UT2 and UT3 are arranged in this order contiguous to the sync signal PLOS, so as to be used as the recording units of the sector identifying address information. Each unit UT1 to UT3 is formed by a leading sync pattern address mark SPA, an address AD consisting of a track address TA and a sector address SA and an error correction code ECC, consisting, for example of Bose-Chaudhuri-Hoquehem (BCH) codes, as address error check codes. As an example, the track address TA is formed by sixteen bits while the sector address SA is formed by eight bits, so that the address is formed by a sum total of twenty-four bits. The length of the error correction code ECC is equal to the sum of the address data bits, and thus is set to twenty-four bits.

0011When recording or reproducing data signals on or from the optical disc 1 having the aforementioned recording format for each sector or block, under control of, for example, a host computer, it becomes necessary to access the block or sector of the target address desired to be recorded or reproduced. In accessing the sector or block of the target address, it has been customary to read the identification part IDR of each sector to produce the address data. In this case, the addresses AD of the units UT1 to UT3 are subjected to error detection or correction processing by using the error detecting code (EDC) or error correcting code (ECC) and the resulting address data are compared to one another. When these address data are not coincident with one another, the address data shoving the maximum degree of coincidence, when relying for example on a majority logic scheme, are adopted as the valid address. For example, if the address data are formed by triple writing, the address data represented by two coincident data are adopted. This valid address is compared with the target address so that the pick-up head 2 is controlled to be shifted towards the block indicated by the target address. However, in such a method, the current address cannot be determined until the totality of the addresses of the multiple units have been decoded for error checking. In addition, a continuous operation of time-consuming decoding is required, while the amount of hardware is also increased.

0012In the present embodiments, the address error checking is performed by encoding the error check code based on the target address in lieu of decoding the target address data based on the error check code. In the first embodiment, shown in Figure 1, a target address ADt supplied through an input terminal 6 from a host computer (not shown), for example, to the optical disc device is supplied to an error correction code (ECC) encoder 7 where the error correction code ECCt is appended to this address as the error check code. The set comprising the target address ADt and the error correction code ECCt are supplied to the comparator circuit 5 for simple or bit-wise comparison with the set comprising the address and the error correction code included in the reproduced signal obtained at the address reproducer circuit 3. If the two sets are coincident within a prescribed allowable range, that is, the symbol or bit error is within the error correction capability of the error correcting code, then the reproduced address is determined to be the target address, so that a coincidence output is supplied to an output terminal 9. The manner in which this coincidence output is used to control the accessing operation will be explained further herein.

0013Figure 3 is an example of the reproduced signal obtained upon reproduction of the signals recorded on the optical disc 1 in which the 24-bit error correction codes (ECC) as the error check codes, for example, are formed by triple writing or recording, similarly to the recording format of Figure 2. Referring to Figure 3, the sets of addresses and the error check codes making up the units UT1, UT2 and UT3 formed by triple writing or recording are reproduced in the chronological order of AD1, ECC1, AD2, ECC2, AD3 and ECC3.

0014During the simple or bit-wise comparison operation, the first step is to compare the set of AD1 and ECC with the set of the target address ADt and error correction code ECCt. The comparison operation at this point is a simple or bit-by-bit comparison. Thus, when <u style="single">a</u> bits out of the total of forty-eight bits formed of the twenty-four address bits and the twenty-four error correction code bits are faulty or in error, with the number <u style="single">a</u> being four as an example, which is within the error correction capability of the error correction code, the block or sector corresponding to the address currently reproduced is determined to be the block corresponding to the aforementioned target block ADt. Thus, the target address seeking operation is controlled by judging whether the number of the faulty bits is coincident with or lesser than the aforementioned bit number <u style="single">a</u> which is set as the allowable range or threshold value.

0015It will be noted that when a coincidence within the allowable range has been ascertained to exist at the time the first unit UT1, of the three units formed by triple recording as described above, has been reproduced, then this address can be assumed to be the target address and thus it is not necessary to conduct a comparison of the remaining two units UT2 and UT3. Similarly, when the aforementioned coincidence is not ascertained to exist in the first unit UT1 but is ascertained to exist in the second unit UT2, then the bit-wise comparison can be terminated without regard to the remaining third unit UT3.

0016For the aforementioned error correction code ECC, a (48, 24) enlarged BHC code formed by twenty-four bits can be employed. Since this code system has a minimum distance between the codes of twelve and an error correction capability of five bits, an error of <u style="single">a</u> is less than or equal to four bits can be completely corrected. Thus, the reproduced address has been made coincident with the aforementioned target address by the seeking process when the symbol or bit error caused by bit-wise comparison is within <u style="single">a</u> is less than or equal to four bits. In this way the decoding operation which usually needs a complex algorithm and entails additional hardware may be omitted since it is possible to ascertain the reproduced address and access the desired block or sector by performing a decode operation using simpler hardware, without impairing the operational reliability. It will be noted that an error correction code such as a Reed Solomon code may be used in place of the BCH code.

0017It is also possible to use an allowable range or threshold of error for checking the coincidence of the read or reproduced signals which is different from that for checking the coincidence of the written or recorded signals. For example, with a threshold <u style="single">a</u> for the reproduced or read signals, a threshold <u style="single">b</u> for the recorded or written signals can be selected to be less than the threshold <u style="single">a</u>. This means that a narrower allowable range is used with more stringent coincidence conditions at the time of signal recording than at the time of signal reading or reproduction.

0018It is to be noted in this connection that, at the time of signal reading, since the data have been previously written, the demand is to read the data at any rate despite increased reading difficulties. On the other hand, during signal recording, it is advisable not to write data in a less desirable block or sector where the address is possibly difficult to read, so that the data once written can later be read positively. When using a (48,24) enlarged BCH code, the preferred threshold values are <u style="single">a</u>=4 and <u style="single">b</u>=1.

0019For further improving the operational reliability, the address currently reproduced may be determined to be the aforementioned target address only when a comparison difference within the threshold value <u style="single">a</u> has been ascertained for the reproduced signals of at least <u style="single">k</u> units of a total of <u style="single">n</u> units formed by n-times recording or writing. In this case, it is unnecessary to wait until all of the <u style="single">n</u> units have been reproduced, but the address currently reproduced can be instantly determined to be the aforementioned target address at the time the coincidence has been ascertained for the sequentially reproduced <u style="single">k</u> units.

0020It will be noted that the shifting of the optical pick-up head 2 until reaching the target address can be conventionally controlled. For example, the difference between the read address value from the address reproducer circuit 3 and the target address value is determined by the comparator circuit 5 and a head driving actuator 12 is driven through a head driving control circuit 11 as a function of the difference so that the pick-up head 2 is moved through a head supporting plate 10 radially across the optical disc 1 or in the direction A, so that the difference between the reproduced address and the target address will become zero or a predetermined value. The address reproducer circuit 3 can be designed so that the address of the currently reproduced sector next to a preceding sector on the optical disc 1 is estimated by, for example, incrementing by "1" the reference address read from and determined for the preceding sector, the thus estimated address is compared with the actually reproduced address and the latter is supplied as the desired address when the compared addresses coincide with each other. As an alternative, an address reproducer circuit 3 may be employed in which the aforementioned estimated address is previously encoded and the thus encoded signal is compared with the reproduced address with an appended error check code and, when the symbol or bit error is within a prescribed tolerable range, the estimated address is recognized to be the desired address.

0021The method of seeking the target address based on the reference address will now be explained. Figure 4 is a block diagram showing the apparatus for address seeking in accordance with a modified embodiment of the present invention. In this figure, the parts corresponding to those shown in Figure 1 are indicated by the same reference numerals. The signals can be reproduced by an optical pick-up head 2 from the optical disc 1, as an example of the recording medium. The signals from the pick-up head 2 are supplied to the data reproducer 4 so that the data recorded on the optical disc 1 are supplied at the output terminal 8. The signals from the pick-up head 2 are also supplied to the address reproducer circuit 3 to be turned into corresponding digital signals. The identification part of the thus read digital signals (playback address ADPB and correction code ECCPB) is supplied to the comparator circuit 5. To this comparator circuit 5, there are also supplied the address of the block or sector that has been estimated by an arithmetic operation based on the fixed reference address ADre previously read from the optical disc 1, and an error correction code ECCes obtained upon encoding or error correction coding of the estimated address ADes by the ECC encoder 7.

0022As will be explained in greater detail below, the address of the sector to be sought by the pick-up head 2 is estimated by an arithmetic operation carried out in a current address computing circuit 14 on the basis of the sector address ADre previously read from the optical disc 1 and determined to be valid. The estimated address ADes to be sought is subjected to encoding to find the error correction code ECCes. The set of the estimated address ADes and the code ECCes are compared bit-by-bit with the set of the playback address ADPB and the correction codes ECCPB obtained upon actual reproduction. When the symbol or bit error is within a prescribed range of the error correction capability of this error correcting code ECC, the address is determined to be valid.

0023A description will now be given referring to Figures 4 and 5. Figure 5 shows a plurality of sectors SEC1, SEC2, SEC3, .... contiguously formed on the optical disc 1. The addresses of the sectors SEC1, SEC2, SEC3, .... are indicated by AD1, AD2, AD3, .... while the corresponding error correction codes are indicated by ECC1, ECC2, ECC3, .... Although the sector addresses and the corresponding error correction codes are shown to be formed only once for simplicity, they may also be formed by multiple recording, that is, they may be written a prescribed number of times, as shown in Figure 3. The error correction codes may also be replaced by an error detection code.

0024Turning now to the aforementioned reference address ADre, an address of a desired sector, such as an address AD1 of the section SEC1 shown in Figure 5, is read by the address reproducer circuit 3 shown in Figure 4. This address is decoded by an ECC decoder 26 using the error correction code ECC1 to find a reference address ADre having a higher reliability. This reference address is supplied from the ECC decoder 6 and is transmitted to the current address computing circuit (actually an address estimating circuit) 14 through a select terminal <u style="single">b</u> of a changeover switch 18. Then, when reproducing the address of the sector SEC2 next to or contiguous to the sector SEC2, the address of the sector SEC2 to be sought is computed or estimated on the basis of the reference address ADre in the address computing circuit 14 by the arithmetic operation of ADre+1. The thus estimated address is then supplied to the ECC encoder 7 where it is subjected to error correction coding or encoding to derive the error correction code ECCes to be appended to the estimated address. The set of the estimated current address ADes (=ADre+1) and the code ECCes are compared in the comparator circuit 5 to the set of the current address AD2 obtained on actual reproduction and the error correction code ECC2. When these two sets of the addresses and codes are coincident within a prescribed range of the error correction capability of the error correction code, the comparator circuit 5 supplies a coincidence signal at the output terminal 9. In accordance with the coincidence output signal, the address output signal is issued as a valid address via an output terminal 13 from the address computing circuit 14. The valid address output signal from the output terminal 13 is transmitted through, for example, an input terminal 17 to a select terminal <u style="single">a</u> of the changeover switch 18, so as to be transmitted to the current address computing circuit 14 as the aforementioned reference address to be used for seeking the next sector address.

0025The arithmetic operation to be performed for address estimation in the address computing circuit 14 will now be explained. When the continuous sector data are sequentially reproduced sector by sector by address seeking as described above, the estimated address ADes can be computed by adding an addition data "1" from an input terminal 16 to the reference address ADre each time a sector count signal based on the disc rotation detection pulses or sector sync pulses obtained from the pick-up head 2 is supplied to an input terminal 15. When the address signals are to be reproduced at intervals of two or more sectors, such as <u style="single">n</u> sectors, the addition data to be supplied to the input terminal 16 is set to "n" and the additive operation is performed each time <u style="single">n</u> sectors are counted, so that the estimated address will be transmitted to the comparator circuit 5.

0026The comparison operation to be performed in the comparator circuit 5 may, for example, be a bit-by-bit comparison to find the number of the faulty symbols or bits. In case the address part is composed of twenty-four bits and the error correction code part is composed of twenty-four bits, the sum total of the bits being thus forty-eight, and the number of faulty bits or symbols is not more than <u style="single">a</u> (for example, a=4), the block or sector that is currently reproduced is determined to be the block of the target address ADt. In other words, a check is made of whether the aforementioned faulty bit or symbol number is within the allowable range or threshold value in order to check for possible address coincidence. In this manner, the reproduced address can be checked at high speed and with a high operational reliability compared to a conventional decode processing operation that requires in general a complicated algorithm and more hardware.