In the art of information processing, efforts are being made to increase the density of the processing for reading out information from an information storage medium, the transfer speed, and the like. When information is subjected high-density and high-speed processing, it is important to ensure the reliability of the information. There is a well-known error correcting technique using error correcting codes in order to increase the reliability of information.
In this technique, information data and other predetermined data for correcting errors within the former (hereinafter referred to as "error detection code(s)") are recorded in the information storage medium in advance.
When the information recorded on the information storage medium is to be reproduced, the data and the error correcting codes read out from the storage medium are processed in a predetermined manner so as to detect the position of any error within the read-out data and obtain information for correct the datum at the error position. An apparatus for correcting errors within the data obtained by the above mentioned processing, namely, for decoding, is referred to as an error correcting apparatus.
An error correcting apparatus provided in a system for reading information recorded on a compact disc read only memory (CD-ROM: developed by Sony Corporation and Phillips Consumer Electronics B.V. and standardized in 1985), which is a type of optical disk, carries out two methods of error correction.
The first error correction which is used for correcting information recorded on a music compact disk (CD) uses the Cross Interleave Reed Solomon Code (CIRC). The error correction using the CIRC includes two stages of error correction, one of which is called C1 correction and the other of which is called C2 correction.
In the step of encoding, error correcting codes of 4 bytes used for the C1 correction and those of 4 bytes used for the C2 correction are added to data of 24 bytes to be recorded on the CD-ROM. The C1 correction is carried out with respect to the data of 24 bytes, while the C2 correction is carried out with respect to data of 28 bytes consisting of the data of 24 bytes and the error correcting codes for the C1 correction of 4 bytes.
In the error correction of the CD-ROM, in order to improve the reliability of the information, Layered Error Corrected Code (LECC) is used as the second error correction. The data recorded on the CD-ROM in mode 1 are managed in logical data units of a predetermined length called sectors. One sector is constituted of 12 bytes of synchronization data, 4 bytes of header data, 2048 bytes of subsequent user data, and 288 bytes of auxiliary data. The auxiliary data consists of 4 bytes of error detecting codes (EDC) other than the LECC, 8 bytes of zero data, 172 bytes of error correcting codes for error correction by a parity P based on the LECC, and 104 bytes of error correcting codes for error correction by a parity Q based thereon.
In the second error correction, a calculation using the EDC is carried out with respect to the 2068 bytes of data from the header data to the EDC. further, in the error correction using the LECC, two stages of error correction are carried out. The error correction by the parity P is carried out with respect to the 2336 bytes of data from the header data to the error correcting codes for the parity P, while the error correction by the parity Q is carried out with respect to the 2340 bytes of data from the header data to the error correcting codes for the parity Q.
In the error correction using the LECC, error data are detected and corrected every byte. The number of correctable error data can be increased by repeating the error correction by parities P and Q one after another. In other words, the error correction capability can be increased by increasing the repetition number of the error correction by parities P and Q.
Since the above mentioned calculation concerning the error correcting codes is complicated, the error correcting apparatus is generally constituted by a dedicated hardware circuit which can operate at a high speed to complete the calculation within a predetermined time period. However, the configuration of a hardware circuit which can operate at a high speed is complicated, which makes it large and costly. The time required to carry out the error correction increases as the error correction capability is increased, while the structure of the circuit becomes more complicated as the processing time is reduced.
When the information recorded on the CD-ROM is reproduced, the probability of random errors other than those due to blemishes on the CD-ROM is approximately the minus 6th to minus 4th power of ten (10.sup.-6 -10.sup.-4) in terms of byte error rate.
Further, even if the uncorrected data contain no errors, the conventional error correcting apparatus using the hardware circuit carries out the calculation concerning the error correcting codes of the data or the like. Accordingly, it is difficult to reduce the processing time.
In view of the above, Japanese Patent Application Laid Open (KOKAI) No. 5-218883 discloses an error correcting apparatus which, for reducing the processing time, first of all detects errors within the uncorrected data using an algorithm whose calculation is easy, then carries out the error correction using the LECC or the like only if an error exists. The error correcting apparatus comprises error correcting means for receiving uncorrected data and error correcting codes and for correcting errors within the received data, mis-correction detecting means for receiving the result generated by the error correcting means and for detecting mis-correction therein, storage means for storing the uncorrected data, error detecting means for detecting errors within the uncorrected data, and select means for selecting either the output of the mis-correction detecting means or that of the storage means based on the result of the error detecting means and outputs selected one as corrected data.
In this apparatus, the error detecting means determines whether or not errors exist in the uncorrected data, and if it is determined that no error exists, the select means outputs as the corrected data the uncorrected data stored in the storage means. In accordance with this apparatus, the processing time can be reduced if the uncorrected data contain no errors. Alternatively, if the uncorrected data contain errors, since the error correction is carried out by the error correcting means, it takes substantially the same time as the processing time required to obtained the corrected data by the conventional error correcting apparatus.
However, the above mentioned error correcting apparatus has certain disadvantages.
For example, when the information recorded on the CD-ROM is reproduced, generally it is continuously read at the rate of 1 sector per 13.3 millisecond. Since the above-mentioned error correcting apparatus cannot read the uncorrected data therein during the period of the error correction, the error correction has to be completed within 13.3 milliseconds at the longest.
If the time required to correct errors within the data of one sector (the sector data set) is greater than the time required to read the information corresponding to a subsequent sector data set, the CD-ROM drive apparatus has to suspend reading of the information recorded on the CD-ROM, and after the error correction is completed, has to initiate reading of the information recorded on the CD-ROM again. Actually, when the CD-ROM drive apparatus suspends reading of the information, it continues to rotate the CD-ROM (in pause status).
However, when the status of the CD-ROM drive apparatus changes from pause to reproduction, a period of time is required for accurately directing a light beam from an optical pickup onto a target track of the CD-ROM, moving the optical pickup into the vicinity of the target track, or the like. Accordingly, a time loss occurs relative to the continuous reading of the information. As a result, it is impossible to reduce the processing time.
In addition, the above problem may also occur if the information is read out from the CD-ROM at a higher speed, such as a double speed or a triple speed, or the error correcting capability is increased to enhance the reliability of the information. It may be possible to solve this problem by using an error detecting circuit operating at a higher speed and having higher error correcting capability. However, it is difficult to design such an error detecting circuit without increasing circuit size and cost.
The above mentioned problem may also occur when information is read out from types of disks other than the CD-ROM, such as optical disks.
The object of the present invention is to provide an error correcting apparatus which can carry out the error correction without suspending reading of the information recorded on the CD-ROM.