Optical recording element and optical data retrieval system for use with the same

Abstract: In the optical recording element of the invention, alignments of long thin pits inclined in various directions with respect to the track direction are formed on the tracks and information is recorded therein according to the different inclines of the pits, thereby performing higher-density and larger-volume recording without reducing the spacing between each pit and the track pitch. The invention also discloses an optical data retrieval system for use with the above optical recording element, which scans the pit alignments with a laser beam spot, detects the inclines of the pits by obtaining the distribution of the intensity of reflected light which is caused by light diffraction at the pits, and plays back information in accordance with the inclines. Therefore, not only can the substantially same structure as that of a standard type optical data retrieval system be applied to the invention without making any complicated change, but also cross-talk is less likely to be produced in the read out signals.

Description: FIELD OF THE INVENTION The present invention relates to optical recording elements wherein information is stored by pits formed on the tracks, and optical data retrieval systems capable of at least playing back the information by projecting a light beam onto the optical recording element. BACKGROUND OF THE INVENTION Optical disks functioning as an optical recording element from which information is played back with a light beam have been already known as a high density and high storage capacity recording medium. Of these optical disks, optical disks used only for the playback of information such as optical ROM disks are particularly widely known. In an optical ROM disk, application software for a computer and various data are already stored in the form of pit alignments on the disk. A data retrieval system for use with such an optical ROM disk is designed such that the presence/absence of a pit is detected by scanning the pit alignments with a laser beam which is used as a light beam; the presence/absence of a pit is electrically represented by a binary signal; and stored information is played back in accordance with the binary signal. More concretely, as shown in Fig. 9(a), when a laser beam spot scans the alignment of pits 42 formed on a track 43 on an optical disk 41, the intensity of reflected light decreases as the laser beam enters the pit 42 due to light diffraction. The electrical output of a read-out signal is changed in accordance with the presence/absence of the pits 42, namely, it decreases when the laser beam scans the pit 42, and increases when the laser beam scans the flat area where no pit 42 is formed, as shown in Fig. 9(b). The output of the signal is converted to a binary code with a predetermined threshold value to obtain a binary signal as shown in Fig. 9(c). In the conventional optical disk 41, information recorded by the pit 42 is represented by 0 or 1 and therefore the information volume thereof is not more than 1 bit. In order to enable optical disks to perform higher-density recording and have higher storage capacity, the length of the smallest pits has to be shortened to reduce the spacing between each pit, or the track pitch has to be reduced. This, however, requires an improvement in the frequency characteristic of the optical head to a considerable extent and causes more cross-talk in reading out signals. In other words, the use of an optical disk in which the pit spacing and track pitch are reduced in order to achieve higher-density and large-volume recording, has the disadvantage of overloading the optical disk data retrieval system used for driving such a disk. From EP-A-0 082 176, there is known a method of preformatting a disk, in which a header pattern includes elongate pits which extend alongside and parallel to a track centre-line and elongate pits which extend across and perpendicularly to the track centre-line. An optical disk recording/playback apparatus is known from GB-A-2 058 434. SUMMARY OF THE INVENTION It is one of the objects of the present invention to provide an optical recording element capable of performing high-density and large-volume recording without reducing the pit spacing and track pitch. It is another object of the present invention to provide an optical data retrieval system (apparatus) capable of at least playing back information with the use of the above optical recording element. In a first aspect, this invention provides an optical recording element comprising, on the tracks thereof, alignments of elongate pits inclined in various directions with respect to the track direction, characterised in that the pits are located on the track centre-line and in that data information is recorded on the element by different inclinations of the pits being representative of data information. In a second aspect, this invention provides an optical data retrieval apparatus for retrieving data information recorded according to pits on the tracks of an optical recording element, characterised in that the apparatus is adapted for use with an optical recording element according to the first aspect, the apparatus including means for reading the recorded information by discriminating between said various directions of inclination of said elongate pits with respect to said track direction. According to the invention, since information is not recorded by the presence/absence of a pit but recorded by the difference in the inclinations of the pits with respect to the track direction, information volume per pit increases. This allows the optical recording element to perform higher density and larger-volume recording, without shortening the length of the pits to reduce the spacing therebetween and the track pitch. Also, since the pit alignments are scanned with a laser beam spot and the inclinations of the pits are detected from the distibution of the intensity of reflected light, the substantially same structure as that of a standard type data retrieval system can be adapted without making any complicated change. Furthermore, the pit spacing and track pitch are not reduced in the optical recording element. Hence, there is no need to drastically improve the frequency characteristic of the optical head and cross-talk is less likely to occur. For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 through 8 show one embodiment of the present invention. Fig. 1 is a view showing an alignment of pits formed on an optical disk which functions as an optical recording element. Fig. 2 is a plan view of the optical disk. Fig. 3 is a block diagram of a pit recognition circuit. Fig. 4 is a schematic block diagram of an optical disk data retrieval system used as an optical data retrieval system. Figs. 5(a), 6(a), 7(a) and 8(a) are diagrammatical views of pits each of which has a predetermined inclination with respect to a track direction. Figs. 5(b), 6(b), 7(b) and 8(b) are views showing the distribution of light intensity on a photodetector. Figs. 5(c), 6(c), 7(c) and 8(c) are views showing code preparation. Fig. 9 shows a prior art and Fig. 9(a) is a diagrammatical view of an alignment of pits. Fig. 9(b) is a waveform chart showing the waveform of an output signal read out by a photodetector. Fig. 9(c) is a waveform chart showing the waveform of a binary signal corresponding to the output signal shown in Fig. 9(b). DESCRIPTION OF THE EMBODIMENTS Referring now to Figs. 1 through 8, one embodiment of the present invention will be explained hereinbelow. An optical disk 1 functioning as an optical recording element is e.g. 90 mm in diameter and has e.g. about 10,000 spiral tracks 2 on the surface thereof as shown in Fig. 2. Each track 2 (one round of track) is divided into 22 sectors 3 for example, and the sector 3 is treated as a unit in recording information. As shown in Fig. 1, each sector 3 comprises a data area 3a for storing 8 bytes of information and a servo area 3b in which information equivalent to 2 bytes, used for the generation of a channel clock signal and the control of the servo mechanisms in the data retrieval system are stored. The servo area 3b is provided with pits 5a, 5b and 5c as well as mirror section 4. The pits 5a and 5b are used for controlling a tracking servo mechanism and are diagonally placed with the track 2 therebetween, being equidistant from the centre of the track 2 in the radial direction of the optical disk 1, in order for a laser beam spot not to be irradiated to them at the same time. This arrangement allows a laser beam spot to accurately track the elaborately formed tracks. To be more specific, the levels of the detection signals based on the reflected lights of the laser beam from the pits 5a and 5b are controlled to be equal, whereby the laser beam spot is prevented from being out of the track 2 in the radial direction of the optical disk 1 and accurate tracking can be thus performed. On the other hand, the pit 5c is formed on the centre of the track 2 and used for the generation of a channel clock signal. The output signal read out from the pit 5c in each servo area 3b is entered in a circuit to which PLL (Phase Locked Loop) method is adapted, and a channel clock signal is generated. The mirror section 4 is a flat area used for controlling a focus servo mechanism, where no pit exists. An objective lens 19 (see Fig. 4) is moved in a focus direction by the focus servo mechanism so that a laser beam is always converged on the recording surface of the optical disk 1. Information to be played back is stored in the form of an alignment of pits 7 inclined in various directions with respect to the track direction, in the data area 3a formed in each sector 3. In this embodiment, 4 types of pits are employed, i.e. pits inclined at 0° to the track direction (Pa); pits inclined at 90° to the same (Pb); pits inclined at about 45° to the same in a counterclockwise direction (Pc); and pits inclined at about 45° to the same in a clockwise direction (Pd). Each type of pits has a fixed code. For instance, the pits Pa are represented by "00", the pits Pb "11", the pit Pc "01" and the pits Pd "10". Since one pit has two pieces of information combined together, each information being represented by 0 or 1, the information volume per pit is 2 bits in this embodiment. The optical disk data retrieval system 11 functioning as an optical data retrieval system comprises, as shown in Fig. 4, a spindle motor 12 for rotatively driving the optical disk 1 at a fixed angular velocity for example. The spindle motor 12 is connected to a spindle control circuit 13 which controls the spindle motor 12 in accordance with a reference signal entered therein. Under the optical disk 1 is disposed an optical pick-up 14 which functions as an optical pick-up device and can move in the focus direction and track direction. In the optical pick-up 14, a laser beam emitted from a semiconductor laser 15 is directed to a collimating lens 16 to produce a parallel beam. The parallel beam is shaped by a shaping prism 17 and then projected to an objective lens 19 via a polarized beam splitter 18. Thereafter, the beam is converged by the objective lens 19 to produce a laser beam spot irradiated on the optical disk 1. The laser beam reflected at the optical disk 1 (hereinafter referred to as a reflected beam) is projected back to the objective lens 19 and the polarized beam splitter 18. The reflected beam then passes through another polarized beam splitter 20 to be split off. One of the reflected beams thus split is guided into a photodetector 23 after passing through a lens 21 and a cylindrical lens 22. The output from the photodetector 23 is entered into a focus servo circuit 29. On the other hand, the other reflected beam is guided into a photodetector 25 via a lens 24. The photodetector 25 has a rectangular light receiving surface which is equally divided into eight light receiving areas d1 to d8 with parting lines (i.e. two diagonal lines and two lines each extending between the middle points of the opposite sides) so that those light receiving areas d1 to d8 are equal in size. The photodetector 25 is connected to a total signal generation circuit 26 which generates a total signal by summing up the outputs of the light receiving areas d1 to d8. The total signal generation circuit 26 is connected to a channel clock signal generation circuit 27 which functions as the aforesaid circuit of PLL method. The channel clock signal generation circuit 27 generates a channel clock signal based on the total signal for the pit 5c formed on the servo area 3b and this channel clock signal is sent to a tracking servo circuit 28, a focus servo circuit 29 and a pit recognition circuit 30 functioning as pit recognition retrieval means. The tracking servo circuit 28 generates and releases a control signal to an objective lens actuating mechanism 14a upon receipt of the channel clock signal from the channel clock signal generation circuit 27 so that total signals each corresponding to the pits 5a and 5b formed in the servo area 3b are equal. Likewise, the focus servo circuit 29 releases a control signal to the objective lens actuating mechanism 14a upon receipt of the channel clock signal, the control signal being generated by astigmatism according to the output of the photodetector 23 which corresponds to the mirror section 4 formed in the servo area 3b. The pit recognition circuit 30 is connected to the photodetector 25. As shown in Fig. 3, the pit recognition circuit 30 comprises circuit members such as adders the number of which is the same as the number of types of the pits 7 classified according to the inclinations thereof (in this embodiment, four adders 31 to 34 are employed); the same number of A/D converters; (in this embodiment, four A/D converters 35 to 38 are employed); and a maximum value detection circuit 39; and a code data generation circuit 40. Incidentally, when the distribution of reflected light intensity is detected in a combination of two adjacent light receiving areas, eight combinations of two adjacent light receiving areas (d1-d2, d2-d3, d3-d4, d4-d5, d5-d6, d6-d7, d7-d8, d8-d1) are available. However, in order to obtain the overall distribution of reflected light intensity, the distribution of reflected light intensity in four successive combinations (in this embodiment, the combinations d1-d2, d2-d3, d3-d4 and d4-d5 are selected) may be detected for the following reason. That is, the centre of the laser beam spot is controlled by the tracking servo mechanism to substantially coincide with the centre of the pit 7, the distribution of reflected light intensity is symmetrical about the centre of the light receiving surface of the photodetector 25. The adder 31 generates a sum signal d12 based on analog signals varying in accordance with the amount of light received at the light receiving areas d1 and d2 which are adjacent to each other, while the adder 32 generates a sum signal d23 based on analog signals released from the light receiving areas d2 and d3. The adder 33 generates a sum signal d34 based on analog signals from the light receiving areas d3 and d4, and the adder 34 generates a sum signal d45 based on analog signals from the light receiving areas d4 and d5. The sum signals d12, d23, d34 and d45 are entered together with the channel clock signal into the A/D converters 35, 36, 37 and 38 respectively. When the channel clock signal generated synchronously with the reading out of information stored in each sector 3 is entered, the A/D converters 35 to 38 respectively converts the sum signals d12, d23, d34 and d45, which are detected from light reflected at the pit 7, to digital signals S12, S23, S34 and S45 in order to release to the maximum value detection circuit 39. The maximum value detection circuit 39 selects a digital signal having a maximum value from the digital signals S12, S23, S34 and S45. The maximum value signal thus selected is converted to the predetermined code by a code data generation circuit 40. For instance, the digital signal S12 is converted to the code "01". Likewise, the digital signal S23, S34 and S45 are converted to the codes "11", "10" and "00" respectively. The coded signal is entered, as shown in Fig. 4, to a demodulation circuit 41 and then to an error correction circuit 42 from which a read out signal is released. In the foregoing arrangement, when a laser beam spot is projected to the pit Pa as shown in Fig. 5(a), the distribution of light intensity on the light receiving surface of the photodetector 25 is as shown in Fig. 5(b). That is to say, light intensity increases in the region where light reflected at the outside of the pit Pa is received (i.e. the dark shaded part in Fig. 5(b)) and light intensity decreases in the regions where light reflected within the pit Pa is received (the hatched part in Fig. 5(b)), because a laser beam is diffracted at the pit Pa. In this case, among the combinations of two adjacent light receiving areas (i.e. the combinations of light receiving areas d1-d2, d2-d3, d3-d4, and d4-d5), the combination of the adjacent light receiving areas d4 and d5 has the highest light intensity, because the area ratio of the total of the dark shaded parts to the total of the hatched parts [i.e. (the total of the area where reflected light from the outside of the pit Pa is received)/(the total of the area where reflected light from the pit Pa is received)] in the areas d4 and d5 is higher than those of any other combinations. Accordingly, the output of the sum signal d45 for the detection signals from the light receiving areas d4 and d5 is maximum. Therefore, the digital signal S45 is detected as a signal having a maximum value at the maximum value detection circuit 39 and then entered to the code data generation circuit 40. In the code data generation circuit 40, the digital signal S45 is coded to "00" as shown in Fig. 5(c) and the code "00" is sent to the demodulation circuit 41. When a laser beam spot is projected to the pit Pb as shown in Fig. 6(a), the light intensity in the adjacent light receiving areas d2 and d3 is the highest, where (the total of the area where reflected light from the outside of the pit Pb is received)/(the total of the area where reflected light from the pit Pb is received) ratio is maximum. The output of the sum signal d23 for the signals detected at the light receiving areas d2 and d3 is maximum. Therefore, the digital signal S23 is selected as a signal having a maximum value in the maximum value detection circuit 39 and sent to the code data generation circuit 40. In the code data generation circuit 40, the code "11" is generated as shown in Fig. 6(c). Likewise, when a laser beam spot is projected to the pit Pc as shown in Fig. 7(a), the output of the sum signal d12 is maximum as shown in Fig. 7(b), and the code "01" is generated as shown in Fig. 7(c) according to the digital signal S12. When a laser beam spot is projected to the pit Pd as shown in Fig. 8(a), the sum signal d34 is maximum as shown in Fig. 8(b) and the code "10" is generated as shown in Fig. 8(c), according to the digital signal S34. As described above, four types of inclination of the pits 7 with respect to the track direction are respectively converted to a predetermined code constructed by two bits, i.e., 0 and 1, so as to be read out as information. With the use of the optical recording element and optical data retrieval system of the present invention, not only is information volume doubled compared to the prior art, but also the transmission speed at the time of playback is highly improved as the number of bits increases. In the foregoing embodiment, there are provided in the data region 3a formed on the track 2 of the optical disk 1, four types of long thin pits 7, that is, pits inclined at 0°, 90° and substantially 45° in a clockwise direction and counterclockwise direction with respect to the track direction, and according to the number of types of pits, the light receiving surface of the photodetector 25 is equally divided into eight divisions. Further, four adders 31 to 34 and four A/D converters 35 to 38 (the numbers of adders and A/D converters are equal to the number of types of the pits) are provided in the pit recognition circuit 30. However, the invention is not limited to the above arrangement. In the above-described arrangement, information volume per pit increases since information is not recorded by the presence/absence of a pit but recorded by the different inclinations of pits. This allows the optical recording element to perform higher-density and larger-volume recording, and highly improves the transmission speed at the time of playback, without shortening the length of the pits to reduce the spacing therebetween and reducing the track pitch. In the optical data retrieval system, a laser beam spot is irradiated on the pit formed on the optical recording element and the distribution of the reflected light intensity is obtained, thereby detecting the inclination of the pit, so that the substantially same structure as that of a standard type data retrieval system can be adapted without making any complicated change. Furthermore, the pit spacing and track pitch in the optical recording element are not reduced, with the result that there is no need for a drastic improvement in the frequency characteristic of the optical head and cross-talk is less likely to occur. The invention being thus described, it may be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention. There are described above novel features which the skilled man will appreciate give rise to advantages.

Claims: An optical recording element (1) comprising, on the tracks (2) thereof, alignments of elongate pits (7) inclined in various directions with respect to the track direction, characterised in that the pits (7) are located on the track centre-line (2) and in that data information is recorded on the element (1) by different inclinations of the pits (7) being representative of data information. An optical recording element according to claim 1 and in the form of a disk, wherein one round of track (2) is divided into a plurality of sectors (3) and the sector is treated as one unit of recording information. An optical recording element according to claim 2, wherein the sector (3) comprises a data area (3a) for storing said data information and a servo area (3b) for storing servo control information for reproduction of the data information, the data area (3a) comprising said inclined pits (7). An optical recording element according to claim 3, wherein said data area (3a) stores eight bytes of said data information and said servo area (3b) stores two bytes of said servo control information. An optical recording element according to claim 3 or claim 4, wherein the data information is stored in the data area (3a) by four types of elongate pits (7) having directions of inclination different from each other. An optical recording element according to claim 5, wherein said pits (Pa,Pb,Pd,Pc) are inclined at 0°, 90°, substantially 45° in a clockwise direction and substantially 45° in a counterclockwise direction, with respect to the track direction. An optical recording element according to any one of claims 3 to 6, wherein the servo area (3b) comprises: a pit (5c) formed on the centre of the track and used, in reproduction, for the generation of a channel clock signal; a pair of pits (5a,5b) which are used, in reproduction, for controlling a tracking servo mechanism, and diagonally placed with the track therebetween, being equidistant from the centre of the track in the radial direction of the optical disk so that a laser beam spot is not irradiated onto them at the same time; and a mirror section (4) which is a flat area used, in reproduction, for controlling a focus servo mechanism and has no pit therein. An optical recording element according to any one of the preceding claims, wherein each of said elongate pits (7) records a respective two-bit code of said data information. An optical data retrieval apparatus for retrieving data information recorded according to pits on the tracks of an optical recording element, characterised in that the apparatus is adapted for use with an optical recording element (1) defined by any one of claims 1 to 8, the apparatus including means (25,30,41) for reading the recorded information by discriminating between said various directions of inclination of said elongate pits (7) with respect to said track direction (2). An optical data retrieval apparatus according to claim 9, comprising: pit recognition retrieval means (30) for retrieving the data information, operable to detect the inclinations of said pits by obtaining the distribution of the intensity of light reflected from each pit (7) of the optical recording element on which a laser beam spot is irradiated. An optical data retrieval apparatus according to claim 10, comprising an optical pick-up device (14) which includes: a first photodetecting means (23) for generating a control signal used for controlling a focus servo mechanism (29) upon receipt of reflected light; a second photodetecting means (25) for generating detection signals used for the playback of data information; and an optical system (18,20) for projecting a laser beam to produce a said spot on the optical recording element and guiding reflected light therefrom to both the first and second photodetecting means. An optical data retrieval apparatus according to claim 11, wherein the second photodetecting means (25) comprises a light receiving surface equally divided into a plurality of light receiving areas so that those light receiving areas are equal in size. An optical data retrieval apparatus according to claim 12, wherein the second photodetecting means (25) has a rectangular light receiving surface equally divided by parting lines into eight light receiving areas (d1 to d8) of the same shape. An optical data retrieval apparatus according to claim 12 or claim 13, further comprising: a total signal generation circuit (26) for generating a total signal by summing up the detection signals generated by the second photodetecting means (25) which vary in dependence upon the amount of light reflected at each said pit (7) of the optical recording element; a tracking servo circuit (28) for generating and releasing a control signal for controlling a tracking servo mechanism in the optical pick-up device (14); a focus servo circuit (29) for generating and releasing a control signal for controlling a focus servo mechanism in the optical pick-up device (14); and a channel clock signal generation circuit (27) for generating a channel clock signal synchronously with the playback of information stored in each sector (3) of the optical recording element and releasing the channel clock signal to a pit recognition circuit (30) functioning as said pit recognition retrieval means, said tracking servo circuit (28), and said focus servo circuit (29). An optical data retrieval apparatus according to claim 14, wherein the tracking servo circuit (28) releases a control signal to the optical pick-up device upon receipt of a channel clock signal from the channel clock signal generation circuit (27) so that the levels of total signals respectively generated from the pair of pits (5a,5b) formed in said servo area (3b) of the optical recording element are equal, thereby controlling the tracking servo mechanism. An optical data retrieval apparatus according to claim 14, wherein the focus servo circuit (29) generates a control signal by astigmatism and releases it to the optical pick-up device (14), upon receipt of a channel clock signal from the channel clock signal generation circuit (27), so as to control the focus servo mechanism, the control signal being generated based on the output of the first photodetecting means which corresponds to said mirror section (4) of the optical recording element. An optical data retrieval apparatus according to claim 14, wherein the pit recognition circuit (30) comprises: a plurality of adders (31 to 34) for generating a sum signal from analogue signals which vary in accordance with the respective amounts of reflected light received at two adjacent ones of said plurality of light receiving areas of the second photodetecting means (25); as many A/D converters (35 to 38) as the adders, each of which A/D converters is connected to the associated adder and converts the sum signal to a digital signal upon receipt of a channel clock signal from the channel clock signal generation circuit (27); a maximum value detection circuit (39) for selecting a said digital signal having a maximum value from the plurality of digital signals entered from the A/D converters; and a code data generation circuit (40) for converting the selected digital signal having a maximum value to a code which is prefixed for every digital signal. An optical data retrieval apparatus according to claim 17, wherein the number of said adders and the number of said A/D converters are respectively equal to the number of types of inclination direction of said elongate pits (7) of the optical recording element. An optical data retrieval apparatus according to claim 17, wherein the maximum value detection circuit (39) selects a digital signal, as a signal having a maximum value, the digital signal being generated by converting a sum signal of the detection signals obtained from the two adjacent light receiving areas where the ratio of (the total of the area where reflected light from the outside of the pit is received) to (the total of the area where reflected light from the pit is received) is the highest. An optical data retrieval apparatus according to claim 14, adapted for use with the optical recording element defined by claim 3, or any claim dependent thereon, wherein the channel clock signal generation circuit (27) generates a channel clock signal by PLL method from the total signal of the pit formed on the centre of the track in said servo area (3b) of the optical recording element.

Inventors: DEGUCHI TOSHIHISA, MATOBA HIROTSUGU, TERASHIMA SHIGEO, YAMAOKA HIDEYOSHI, DEGUCHI, TOSHIHISA, MATOBA, HIROTSUGU, TERASHIMA, SHIGEO, YAMAOKA, HIDEYOSHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B7
IPC codes: G11B7