Optical disc

Abstract: An optical disc of a type having at least one index region for each track. The length of each of the index regions is chosen to be smaller than one frame so that information can be recorded on the index region in multiplex recording fashion.

Description: The present invention relates to a generally disc-shaped information carrier medium such as an optical disc or a magneto-optical disc, and, more particularly, to the format of an index region of the information carrier medium, such as e.g. described in Supplement IERE Conference Proceeding No 43, 24-27th July 1979 p.1-9. For the purpose of the present invention, the term "optical disc" hereinafter referred to and used in the appended claims is to be understood as including not only a compact optical disc, but also a magneto-optical disc. In the optical disc now in use, it is well known that at least one index region in which an index such as the track number is provided for each track on the optical disc. According to the prior art, only the index is recorded in the index region and, therefore, the provision of the index region correspondingly reduces the information storage capacity of the optical disc as a whole. In addition, where the substantial amount of information involving a long-time recording or playback such as music and video information is recorded on the optical disc, the use of a memory buffer for the index region is necessitated, which buffer is required to have a relatively large storage capacity, making it impossible, or impractical if not impossible, to record the information to record the information involving the long-time recording or playback. In particular, the problem associated with the recording of the information involving the long-time recording or playback has provided an obstruction to the development of a digital audio disc or video disc having a capacity sufficient to accommodate such information. In order to eliminate the above discussed problems, it can be contemplated to record the information on the index region in multiplex recording fashion. However, it has been found that the multiplex recording of the information on the index region of the optical disc often constitutes a cause of considerable reduction of the reproducibility of a signal being reproduced and cannot provide support for an error correction. The present invention has been devised with a view to substantially eliminating the above discussed problems. According to one aspect, the present invention provides an optical disc on which information is recorded in a plurality of tracks as a plurality of linear frames for each track, the disc having at least one preformed linear index region associated with each track, characterised in that said index regions are coextensive with frame regions each said index region having a length which is less than that of each said frame. According to another aspect, the invention provides a method of recording information on an optical disc in a plurality of tracks as a plurality of linear frames for each track, the disc having at least one preformed linear index region associated with each track, wherein each frame is recorded on the disc in such manner that in the reproduction of said information the content of the frame is divided between two output frames, characterised in that said index regions are coextensive with frame regions and that each said index region has a length which is less than that of each said frame, whereby errors in the reproduced information of any one track due to the presence of a said index region are confined to at most three output frames. By adopting this method, since each of the index regions in the optical disc has a length smaller than one frame, when information recorded on the optical disc is decoded, information of one frame on the optical disc is, after the decoding, dispersed into two frames. Therefore, in the event of an error occurring in the information recorded on the index region in multiplex recording fashion, since the length of the index region is within one frame, the number of frame errors after the decoding is usually within the two frames and, even if the index region extends over the two frames on the optical disc, the number of the frame errors after the decoding will be within three frames. In this way, since the number of the frame errors per track increased as a result of the multiplex recording of information on the index region is within the three frames, reduction in reliability of the reproduced signal is slight and such an increase in number of the frame errors does not hamper the capability of correcting with an error correction. The features of the present invention will become clear from the following description of a preferred embodiment thereof, with reference to the accompanying drawings, in which: Fig. 1(a) is a diagram used to explain the format of a track on an optical disc; Fig. 1(b) is a diagram showing the details of an index region; Fig. 2 is a diagram used to explain a decoding circuit for decoding information recorded on the optical disc; Fig. 3 is a diagram used to explain the arrangement of frames; Figs. 4(a) to 4(c) are diagrams used to explain the arrangement of the frames after the decoding; and Fig. 5 is a graph showing the relationship between the length of the index region and the frame error rate. Figs. 1(a) and 1(b) illustrate one example of the format of each track on an optical disc according to the present invention. As the optical disc, a magneto-optical disc was employed which was formed by depositing an AlN film of 80 nm, a GdTbFe amorphous film of 20 nm, an AlN film of 25 nm and an Al film of 50 nm on a glass substrate and radiating a laser beam from the glass substrate side to effect a recording at Curie point, and is of a type wherein reproduction of information recorded thereon is carried out by radiating a double polarized laser beam to detect the rotation of a polarizing plane of reflected light by the magneto-optical effect. As shown in Fig. 1(a), each track 41 is provided with at least one index region 42 on which indexes such as the track number, i.e., the number allocated to the particular track 41 and others, are recorded. As shown in Fig. 1(b), the index region 42 is composed of a marking area 42a indicative of the heading of the index region 42, a track number area 42b indicative of the track number in which such index region 42 is included, and a CRC (cyclic redundancy check) area 42c for the detection of a bit error in the track number area 42b. The index region 42 is recorded on the optical disc in the form of a series of indentations formed on the glass substrate by the use of an etching technique during the manufacture of the optical disc. Accordingly, although it is possible to record information on the index region 42 in multiplex recording fashion or to reproduce or erase the information so recorded on the optical disc, it is not possible to erase the index region 42 itself. A decoding portion having an error correcting capability based on a cross-interleaved Solomon system (hereinafter referred to as "CIRC system") which is an error correcting method used in connection with a compact optical disc is shown in Fig. 2. One frame on the optical disc includes, as indicated by E in Fig. 2, 24 symbol information indicated by codes 0 to 11 and 16 to 27, and 8 symbol parities for the error detection and correction as indicated by codes 12 to 15 and 28 to 31 and, when this is decoded, 24 symbol information shown by F in Fig. 2 can be obtained as corrected if the error exists. It is to be noted that each symbol consists of 8 bits. When the decoding has taken place in the decoding portion shown in Fig. 2, the symbol included in each frame on the optical disc is dispersed into two frames. By way of example, symbols A₀, A₁, A₂, ... of one frame A on the optical disc as shown in Fig. 3 are dispersed into two frames I and II shown in Figs. 4(a) and 4(b), respectively, and symbols B₀, B₁, B₂, ... of one frame B on the optical disc as shown in Fig. 3 are dispersed into two frames II and III as shown in Fig. 4(b) and 4(c). Accordingly, if an error of only one symbol occurs in one frame on the optical disc, an error occurs in one frame after the decoding, and if an error of 2 to 32 symbols occurs in one frame on the optical disc, an error occurs usually in two frames after the decoding. Even though the length of the index region 42 is chosen to be less than that of one frame, it may happen that this index region 42 may extend over the two frames on the disc and, in such a case, if an error occurs in the information at the index region 42, an error will occur in three frames, at maximum, after the decoding. In this way, if the length of the index region 42 is chosen to be within one frame, even though an error occurs in the information recorded on the index region 42 in multiplex recording fashion, the maximum number of frame errors after the decoding will be three and merely corresponds to 0.2% of the average number of frames on each track, which average number is about 1500 (line velocity: 1.3 m/s, radial position of disc: 41.5 mm). Hereinafter, the CIRC system will be discussed. With this system, most errors occurring in the optical disc can be corrected, and, in particular, as far as a burst error (continuous errors) is concerned, correction can be made of up to 450 symbols (about 14 frames). Also, with respect to the continuous errors greater than that, correction is possible to the error occurring in 1500 symbols. Accordingly, as hereinbefore discussed, if the number of frame errors increased as a result of the multiplex recording of the information on the index region 42 is merely three frames, error correction can be effectively carried out and reliability of the reproduced signal will not be lowered. A result of measurement of the frame error rate exhibited by the actual error of the optical disc wherein the length of the index region 42 is varied is shown in Fig. 5. In Fig. 5, where the index region 42 is of a length greater than 8 frames, uncorrectable frames were found. A modulation system for the index region 42 will now be discussed. As the information modulating system, EFM system is generally used in association with the compact optical disc. However, in the case of the modulating system for the index region 42, the employment of a system different from the information modulating system is preferred in order to minimize the error detection. Also, in order to accomplish writing of the same indexes with a minimized number of bits, a modulating system of high code rate is preferred for the modulation system for the index region 42. In view of the foregoing, a so-called 8/10MNRZI system generally used in the digital audio tape recorder is preferred because it satisfies all of the above discussed conditions. When the index region 42 consisting of one byte of the marking area 42a, 2 bytes of the track number area 42b and 2 bytes of CRC area 42c is recorded with the use of the 8/10MNRZI system, the recording bit length is 50 micrometres and, even if the minimum bit length is 1 micrometre, about 50 micrometres will suffice. This value is sufficiently smaller as compared with the shortest value of one frame which is about 163 micrometres (in the case of the line velocity being 1.2 m/s). It is to be noted that the 8/10MNRZI system is well known and disclosed in, for example, the Japanese Laid-open Patent Publication No. 60-93857 and, therefore, the details thereof will not be herein reiterated. It is to be noted that, although in the foregoing embodiment the present invention has been described as applied to the magneto-optical disc, the present invention can be equally applicable to an optical disc of a phase variable type (amorphous phase ↔ crystalline phase) wherein a recording medium is formed by adding a slight amount of germanium or tin to a low oxide such as tellurium. From the foregoing description of the present invention, it has become clear that the optical disc herein disclosed is of a type having at least one index region for each track, which region has a length chosen to be smaller than one frame so that information can be recorded on the index region in multiplex recording fashion. While the length of each index region is chosen to be within one frame on the optical disc, when the information on the optical disc is decoded, the information of one frame on the optical disc is, after the decoding, dispersed into two frames and, therefore, in the event that an error occurs in the information recorded on the index region in multiplex recording fashion, the number of frame errors after the decoding is usually within two frames. Also, even though the length of the index region may extend over the two frames, the maximum number of frame errors after the decoding will be three. In this way, since the maximum number of the frame errors per track increased as a result of the multiplex recording of information on the index region is three frames, reduction in reliability of the reproduced signal is slight and such an increase in number of the frame errors does not hamper the capability of correcting with an error correction. Accordingly, since it is possible to record information on the index region in multiplex recording fashion without the reliability of the reproduced signal being lowered, the capacity of the optical disc is substantially increased enough to permit the information involving a long-time playback to be recorded.

Claims: An optical disc on which information is recorded in a plurality of tracks (41) as a plurality of linear frames (A, B, C) for each track, the disc having at least one preformed linear index region (42) associated with each track, characterised in that said index regions are coextensive with frame regions, each said index region (42) having a length which is less than that of each said frame. A method of recording information on an optical disc in a plurality of tracks (41) as a plurality of linear frames (A, B, C) for each track, the disc having at least one preformed linear index region (42) associated with each track, wherein each frame is recorded on the disc in such manner that in the reproduction of said information the content of the frame (A; B) is divided between two output frames (I, II; II, III), characterised in that said index regions are coextensive with frame regions and that each said index region (42) has a length which is less than that of each said frame, whereby errors in the reproduced information of any one track due to the presence of a said index region are confined to at most three output frames (I, II, III).

Inventors: FUJI HIROSHI, FUJIWARA TSUNEO, NUMATA TOMIYUKI, TERASHIMA SHIGEO, TSUJI KENTAROH, FUJI, HIROSHI, FUJIWARA, TSUNEO, NUMATA, TOMIYUKI, TERASHIMA, SHIGEO, TSUJI, KENTAROH
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B20
IPC codes: G11B27, G11B7, G11B20, G11B11