Method and apparatus for recording and reproducing information

Abstract: A method for recording, reproducing and erasing information by using a recording film of either crystalline-amorphous phase-transition type or magneto-optic type. By modulation of the laser power between three different levels, it is possible to record, reproduce, erase or directly over-write (erase and re-record in one pass) with the same beam. The method is not limited to a light beam, but may also use another energetic beam, such as an electron beam or ion beam.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for recording, reproducing and erasing information by irradiating an energy, beam such as a light or electron beam onto an information recording medium, and more particularly to a method for recording, reproducing and erasing information by a single beam spot by using a phase change and an information recording medium which is effective for use in the above method. A prior art method for recording, reproducing and erasing information in a phase change type optical disk is shown in JP-A-59-71140 in which recording is done by irradiating a small circular light spot formed by fully focusing a light beam spot for a short time period to render a recording film to a complete amorphous state by quick heating and cooling, and erasing is done by irradiating an elliptical light spot extending along a track to keep the recording material at a crystallization temperature for a relatively long time so that the amorphous state recorded area is returned to a complete crystalline state. Thus, two different beam spots are used for recording and reproducing. A second prior art method is disclosed in JP-A-56-148740 in which a single light beam spot is used to erase recorded information in one or a plurality of revolutions of a disk medium and record information in the next revolution. A third prior art method is disclosed in JP-A-56-145530 in which a light which is intensity modulated between at least two power levels is irradiated to simultaneously record and erase information. In the first method, since two beam spots are used, two light sources are necessary and an optical system is complex. In the second method, it is necessary to rotate the disk medium a number of revolutions to update the information, and hence it takes a long time. In the third method, the above problems are resolved but it is necessary to completely crystallize the recording film prior to recording, and tracking at a reproduction power level, automatic focusing and track or sector address confirmation prior to recording are not effected. Further, since an initial power level is not always an erase power level, a leading portion of the previously written information will not be updated if a start position of updating is deviated. On the other hand, a recording thin film for the phase change type optical disk is disclosed in JP-B-47-26897 which also describes various thin films such as Te-Ge, As-Te-Ge and Te-O. JP-B-54-41902 discloses various compositions such as Ge 10 Tl 5 Sb 5 Se 70 and Ge 20 Bi 10 Se 70 . JP-A-57-24039 discloses thin films of Sb 25 Te 12 5 Se 62.5 , Cd 14 Te 14 Se 72 , Bi 2 Se 3 , Sb 2 Se 3 , In 20 Te 20 Se 60 , Bi 25 Te 12.5 Se 62.5 , CuSe and Te 33 Se 67 . However, in the prior art, when the rewritable phase change type recording thin film is used, a crystallization rate is low, a semiconductor laser beam is not well absorbed and a sensitivity is low, a reproduced signal intensity is not sufficient, a stability of the amorphous state is low or resistance to oxidation is insufficient. The JP-A-56-145 530 discloses the method for rewriting information described in the preamble of claim 1. This method includes the use of a beam energy for rewriting having two different power levels. SUMMARY OF THE INVENTION It is the object of the present invention to provide a method for recording, reproducing and erasing phase change type information, which can resolve the problems encountered in the prior art method, can record, reproduce and erase information by a simple optical system and can record new information while erasing recorded information in a short time. The above object is achieved by the method defined in claim 1. The power of an energy beam such as a light, electron or ion beam is changed between power levels capable of causing a state change (phase change) during recording of information. More specifically, information may be updated (erasing and rewriting) during one passage of the spot on a recording film. The reproduction level of the updating light spot may be used only for tracking and auto-focusing. The power may be rendered zero from the reproduction level for a very short time without affect to the tracking and auto-focusing, and such is within the scope of the present invention. There are many states of the recording thin film continuously or discontinuously between two states, for example, between perfect amorphous state and perfect crystalline (polycrystalline) state. Accordingly, by changing a power of an energy beam between two levels, binary or digital recording is attained, and by changing it among three or more levels, analog or multi-level recording is attained. In the parent application EP 87 106 312.9 for the present divisional application, the lowering of the energy level from the high level to a level lower than said medium level has been patented. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a change of a laser power in a first embodiment of the present invention, Fig. 2 shows a change of a laser power in a second embodiment of the present invention, Fig. 3 shows an optical record/reproduce/ erase apparatus used in the embodiments of the present invention, Fig. 4 shows a reproduced signal waveform in the first embodiment and a method for shaping the waveform, Fig. 5 shows a method for combining rewriting waveforms in the first embodiment, Fig. 6 shows a principle of multi-value recording in the second embodiment, and Fig. 7 shows a change of a laser power in a third embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of recording, reproducing and erasing information in accordance with the present invention enables so-called overwriting by which new information is recorded while an existing record is erased. Thus, information may be updated (erased and rewritten) while an energy beam spot scans a recording film in one run. For example, a disk is rotated while a recording track (between tracking grooves) is rendered almost amorphous and an area between tracks is rendered crystalline, and information is reproduced by a single laser or electron beam spot. When the beam spot reaches a rewrite position, the power is changed between a plurality of power levels other than a reproduce power level (continuous oscillation) (usually between a record level higher than the reproduce power level and an erase level) to rewrite information. The record level and erase level are defined as laser powers which are optimum for recording and erasing, respectively, to continuously maintain the power. The laser power may be changed by superimposing to a waveform whose power pulsively rises from a reproduce level to a record level in a conventional write-once optical disk, a waveform whose power rises from a power level which is between 0 level to the reproduce level in a rewrite area to an erase level (that is, producing a higher one of those powers). By controlling the power of the superimposing waveform, the rewritable optical disk and the write-once optical disk may be recorded and reproduced by one apparatus. When the power is changed between the two power levels, that is, record power level and erase power level to rewrite information, the decrease of cooling rate due to residual heat of the recording film can be prevented by lowering the power than the lower power level after the higher power level has been reached. Where a reciprocal phase change type recording material is used and the near-amorphous state is used as a record state while near-crystalline state is used as an erase state and information is recorded with a higher power than the erase level, the power is raised from the reproduce power level to the erase power level and then the power is pulsively raised from the erase level. Thus, the problem encountered in the third prior art is resolved. In the pulsive power reduction for erasing, it is preferable to significantly reduce the power in an initial stage, so that a temperature suitable for erasing is quickly attained. In the pulsive power rise for recording, it is preferable to significantly raise the power in an initial stage to raise the power above the record power level and then lower the power to the record power level. During the irradiation of the laser beam of the lower power (erasing), the beam spot on the disk medium is preferably enlarged by utilizing wavelength change due to mode change by laser power control, reduction of high frequency superposition or control of amount of return light, or color astigmatism of a lens, or by changing a light path length by utilizing a beam deflector to defocus the beam spot, so that the irradiation time is gained and imperfection of erase information is prevented. Which of two or more states of the recording film are to be selected as the record state and erase state is arbitrary. For example, a low reflection state by a high power laser beam irradiation may be selected as the record state or it may be selected as the erase state. Since it is preferable to shorten the irradiation time of the high power laser beam in order to avoid deformation of the recording film, it is preferable to select the high power laser beam irradiation state as the record state. In the present invention, when information is recorded by utilizing the reciprocal phase change between the crystalline state and the near-amorphous state (low regularity of atom arrangement or low crystallization) of the recording film, the generation of noise due to ununiform crystallization of an area between tracks when the information is repeatedly rewritten can be prevented by previously rendering the area between the record tracks the crystalline state. If a disk rotation speed is high, a point on the disk may not be completely crystallized because of insufficient irradiation time even if the beam of crystallization energy level is irradiated. If the beam of the crystallization level is again irradiated in the rewriting mode, the crystallization proceeds and the reflection coefficient of the crystallized area is not uniform but varies from point to point. In such a case, it is preferable to set a comparator level to a voltage corresponding to a reflection coefficient of a point of the crystallized area which is closest to a reflection coefficient of a point of other state (near-amorphous state), or a reflection coefficient which is intermediate of said reflection coefficient and a reflection coefficient of a point of the near-amorphous state (if it has a range, a reflection coefficient of a point closest to the crystalline state) so that a signal voltage of the crystallization level is clipped to such a level. In this case, if the comparator level is set to an intermediate voltage between a voltage corresponding to the reflection coefficient of the crystalline state and a voltage corresponding to a reflection coefficient of the near-amorphous state, or a voltage closer to the voltage corresponding to the reflection coefficient of the near-amorphous state, the reproduction of information is assured. If the comparator level is set to a voltage between the above intermediate voltage and a voltage of the crystalline state closest to the intermediate voltage, the original waveform can be exactly reproduced. Other waveform shaping devices may be used to shape the waveform. When information is recorded by the phase change, it is one approach to previously crystalize the entire surface of the recording film. When the substrate is made of an organic material, the substrate should not be heated to a high temperature and another crystalization method should be employed. In such a case, irradiation of a laser beam focused to a spot diameter of less than 2 µm, irradiation of UV ray and heating, irradiation of light from a flash lamp such as xenon lamp or Hg-vapor lamp, irradiation of light by a large light spot from a high power gas laser, or irradiation of a laser beam and heating may be preferably used. When the irradiation of light from the gas laser is used, the light spot diameter (half-value width) is preferably between 5 µm and 5 mm in order to attain a high initial crystalization efficiency. Information may also be recorded by crystallizing the amorphous recording thin films or by changing crystal grain sizes in polycrystalline thin film. In the recording film used in connection with the present invention, it is not necessary to utilize the change between the amorphous state and the crystalline state for the recording of information, but an optical property may be changed by any change of atom arrangement (widely interpreted phase change). For example, a change between polycrystalline states of different average grain sizes of crystals may be utilized for the recording. The recording member may be used not only in a disk form but also in other forms such as tape and card. It is effective to both the rewriting by a single light beam spot and the rewriting by a multi-light beam spot. In order to prevent reduction of a CN ratio due to repetitive recording and erasing of information, an area of transition may be continuously irradiated by a light for unification at a time when the reflectivity of some part on the recording track has changed to a predetermined level by the crystallization of the recording film or when at least one of signal level and noise level has changed to a predetermined level. It is preferable to melt a portion of the recording film for unification. The information recorded prior to the continuous light irradiation is temporarily shifted to another place (device) before melting, and it is rewritten after melting. When the above process is carried out in rewriting information, information to be newly written is temporarily stored in another place. A reflectivity of a predetermined portion of the recording film is always monitored so that the change can be detected as soon as possible. It may be automatically carried out at every predetermined number of times of rewriting, every predetermined number of times of reading or predetermined storage time. By irradiating the continuous light which causes melting of the recording film or a sufficient phase change, the irradiated area is rendered into an uniform state, for example, a perfect amorphous state or a saturated crystalline state. This means that non-uniformity of crystallization or deformation which affects to signal level or noise level is avoided. If any non-uniformity of reflectivity again increases by rewriting or maintenance of the recording film at a high temperature, the above process is repeated so that the recorded information can be stably maintained for an extended period of time. Regardless of the above process, a signal-to-noise ratio of the reproduced signal is determined by a ratio of a higher level and a lower level (which are called high level and middle level and represent heights from zero power level) when the laser power is changed in the record mode. The middle level is preferably between 55% and 90% of the high level, and more preferably between 65% and 85%. If the power waveform is a sine waveform in which a time at which the power level stays at a level is short, an average value in one quarter of a shortest period of vertical movement of the waveform including a lowest portion of the waveform, and an average value in one quarter of the shortest period of vertical movement including a highest portion of the waveform are set as the middle level and high level, respectively. This method can be applied to the recording film which utilizes the change of reflectivity due to the phase change between the amorphous state and the crystalline state, as well as the recording film which records, reproduces and erases information by utilizing the change of reflectivity due to the change of atom arrangement (widely interpreted phase change) between crystalline states or amorphous states such as between a high crystalline state (high regularity of atom arrangement) and a low crystalline state (including polycrystalline states of different average grain sizes of crystals or different types of crystals), and to an magneto-optic recording film. Embodiments Embodiments of the present invention are now explained in detail with reference to the drawings. Embodiment 1 Fig. 1 shows a time transition of a laser power P in a method of recording, reproducing and erasing information in accordance with the present invention. A recording track of an optical disk medium having a diameter of 130 mm and an In-Se recording film which enables rewriting of information by a phase change between a crystalline state and a near-amorphous state, is rendered near-amorphous state in the record tracks, and an area between the record tracks is rendered crystalline state. The record track may also be rendered the crystalline state. In this case, the reflectivity of the crystalline state area is preferably substantially equal to a reflectivity of an erased area. The area between the record tracks is rendered completely crystalline state so that the area is prevented from gradually ununiformly crystallized by diffusion of heat to the area between the tracks when the information is rewritten resulting in mixture of pre-recorded information with a reproduced signal of new information (a kind of residual signal), or the increase of noise is prevented. The optical disk medium is rotated and a laser beam (continuous light) of a reproduce power level 101 is irradiated with a single light beam spot along the track to perform auto-focusing and tracking. When the beam spot reaches a rewriting position (A), the power level is raised to a crystallization level (erase level) 102. When the beam spot reaches a new record position (B), the power level is pulsively raised to an amorphous power level (record level) 103. Then, the laser power is lowered to the crystallization level 102. When the cooling rate is reduced by residual heat, the laser power is lowered to a level below the crystallization level (shown by a broken line between C and D) from the amorphous level and then the laser power is raised to the crystallization level. Thereafter, the recorded information is erased. In this case, an average laser power between C and D is preferably between 0 level and a power level immediately below the crystallization level 102, and more preferably between 1/4 and 3/4 of the crystallization power level. By repeating the above laser power cycle, the information is overwritten. For example, when the rotation speed of the disk was 2400 rpm and a carrier of 1.8 MHz was recorded, a carrier-to-noise (C/N) ratio measured at a band width of 30 KHz was approximately 50 dB. The same position was rewritten with a carrier of 0.9 MHz and a signal of 0.9 MHz was reproduced. A C/N ratio was again approximately 50 dB. The amorphous level (103) laser power was approximately 15 mW, the crystallization level (102) laser power was approximately 7 mW, and the reproduce level (101) laser power was approximately 1 mW. In the above laser power cycle, the information may be recorded by returning the power level from the amorphizing level (103) to the crystallizing level (102), but when the cooling rate is lowered by the residual heat, the C/N ratio or S/N ratio of the reproduced signal may be degraded. By applying a peak power shown by a broken line immediately after the laser power has been raised from the crystallization level 102 to the amorphous level 103, the amorphizing process of the recording film may be accelerated. The magnitude of the peak power may be higher than the amorphizing level 103 by upto a difference between the amorphizing level 103 and the crystallization level 102, and preferably between 1/4 to 3/4 of the level difference. In order to prevent information from being failed to erase, the light beam spot is defocused in the erase mode. This is done by using a focusing lens having a color astigmatism to utilize the fact that the wavelength varies with a change of raser oscillation mode due to a change of laser power. If the beam spot is focused in the record mode, it is defocused when the laser power is at the erase level lower than the record level. Fig. 3 shows a construction of an optical record/reproduce/erase appratus used in the embodiment of the present invention. The optical system is essentially identical to an optical system for a commercially available write-once optical disk. Numeral 301 denotes an optical disk substrate, numeral 302 denotes a recording film, numeral 303 denotes a protective film, numeral 304 denotes a focusing lens, numeral 305 denotes a golvano-mirror, numeral 306 denotes a quarter wavelength plate, numeral 307 denotes a polarization prism, numeral 308 denotes a prism, numeral 309 denotes a coupling lens, numeral 310 denotes a semiconductor laser, numeral 311 denotes a filter, numeral 312 denotes a convex lens, numeral 313 denotes a half-mirror, numeral 314 denotes a cylindrical lens and numeral 315 denotes a knife edge. Since a reflected light from the recording film 302 increases in the record or erase mode to cause malfunction of the servo system, the reflected light passes through the filter 311 during this mode before the reflected light is directed to an auto-focusing detector 316 and a tracking and signal reproducing detector 317 so that an incident light intensity is reduced. Instead of inserting the filter, the output voltage of the detector may be attenuated during these modes. As described above, when the rotating speed of the disk is high, the irradiation time at a point on the disk is too short to completely crystallize the film. When the light of the crystallization level is again irradiated in the rewriting mode, the crystallization is proceeded. As a result, the reflectivity of the crystallized area is not uniform and the reproduced output voltage V varies from point to point as shown in Fig. 4(a). Accordingly, a comparator level 403 is set between a reproduced output voltage 401 of the amorphous area and a reproduced output voltage 402 of a least crystallized area to clip the waveform to produce a waveform shown in Fig. 4(b). When the comparator level is set at an intermediate level of the voltage 401 and 402 or closer to the amorphous area output voltage, the detection is assured. If it is set closer to the crystallized area output voltage, the reproduced waveform is more exact to the original waveform. Either one of the crystallization state and amorphous state may be used for recorded state or erased state, respectively. However, since the amorphizing laser power is higher, the time at the amorphizing power level should be shorter than the time at the crystalizing power level. In the commercially available optical disk apparatus, the recording pulse emission time is shorter and hence the amorphous state is preferably used for the recording. As shown in Fig. 5, a current (hatched area 505) corresponding to a difference between the reproduce level and the crystallizing level of the rewriting area is superimposed to a laser drive pulse current (hatched area 504) of the commercial write-once optical disk apparatus to produce a current waveform shown by thick solid line. In this manner, a rewritable optical disk may be used. In Fig. 5, the top of the waveform is limited to prevent the amorphous level of the combined waveform from being too high. By lowering the level of the superimposing current 505 or even reducing it to zero, the information of the write-once optical disk can be recorded or reproduced by the rewritable optical disk apparatus. Embodiment 2 Fig. 2 shows a time transition of a laser power P in another method for recording, reproducing and erasing information. In this embodiment, a record level 202 is lower than an erase level. The recording film, auto-focusing and tracking used in the present embodiment are identical to those of the Embodiment 1. When the beam spot reaches a rewriting place (a), the laser power is abruptly raised to the erase power level 203 and held at that level. During this period, the information is erased. When the beam spot reaches a new recording place (b), the laser power is pulsively lowered to the record power level 202 to record information. The information may be recorded by only lowering the power to the record level 202 as described above, but if the laser power is lowered below the record level 202 as shown by a broken line between (b) and (c) at the beginning of lowering the power and then gradually raised to the record level 202, a time during which the recording area is at a recordable temperature is extended so that the recording is assured. The power level below the record level 202 is preferably just below the record level 202, and more preferably between 1/4 to 3/4 of the record level 202. Then, the laser power is raised to the erase level 203. As shown by a broken line between (c) and (d) in Fig. 2, the power is raised above the erase level 203 and then lowered to the erase level 203 so that the erasing is assured. The average power between (c) and (d) may be the erase power level 203 plus a difference between the erase level 203 and the record level 202 and preferably 1/4 to 3/4 of such difference. The method for preventing the failure of erasing information is same as that of the Embodiment 1, and the information is recorded, reproduced and erased in the present embodiment by using the optical system of the Embodiment 1 shown in Fig. 1. The change of the laser power P when multi-level recording is carried out in the present embodiment is shown in Fig. 6a and a reproduced voltage waveform V is shown in Fig. 6b. Numeral 601 denotes a reproduce level, numeral 602 denotes a record level by crystallization, and numeral 603 denotes an erase level. In Fig. 6, the number of record states is three, but by increasing the number of levels, multi-level recording is attained, and by continuously changing the level, analog recording is attained. Since the change of power level and the change of reproduced voltage are normally not linearly related, at least one of the recording waveform and the reproduced waveform must be converted in order to attain a desired reproduced waveform. Embodiment 3 The present embodiment relates to a method effective to prevent reduction of a C/N ratio due to repetitive recording, reproducing and erasing of information. Fig. 7a shows a reproduced output waveform of pre-rewrite information, Fig. 7b shows a time transistion of a laser power when recorded information is rewritten, and Fig. 7c shows a change of reflectivity of the recording film (change of reproduced output volage). Before recording, the recording film may be either in the amorphous state or in the crystalline state. As shown in Fig. 7b, the laser power is raised from a low reproduce level 901 to a high amorphizing level 903 and held thereat. If information is present during this period, it is erased. When the laser beam reaches a new information recording point, the laser power is pulsively lowered to an intermediate crystallization level 902. After recording, the laser power is again raised to the high amorphous level 903 and the preexisting information is erased. By repeating the above process, information is recorded. AT a point where the medium level laser power was irradiated, the recording film is in the near-amorphous state, and at a point where the middle level laser power was irradiated, the recording film is in the crystalline state. After the power has been raised from the low level to the middle level, the power may be changed between the middle level and the high level. Either one of the crystalline state and the amorphous state may be selected as the record state or erase state, information "1" or "0", or high level or low level. The middle laser power level is preferably between 55% and 90% of the high level, and more preferably between 65% and 85%. If the middle level power is too high or too low, sufficient phase change is not attained during the irradiation. In the preferable range, the S/N ratio was 3 dB lower than that in the more preferable range, and the S/N ratio was more than 6 dB lower in other range. This trend is essentially same for other recording film. When a recording period is short, complete crystallization may not be attained. If such record/erase process is repeated many times, the number of times of irradiation by the laser beam and the power level of the laser beam vary from place to place. As a result, the reflectivity may be non-uniform as shown in Fig. 7 depending on the degree of crystallization, type of crystal structure and degree of deformation. Thus, the C/N ratio may be lowered. The reflectivity may also vary during reading of information or when the recording film is placed in a high temperature atmosphere. Numeral 904 denotes a normal level of reflectivity. Such a change of reflectivity is monitored by a low level laser power during the rewrite mode, and if the reflectivity changes beyond a preset level 905, the information for that area is read and recorded at another area on the same disk, or written into another recording medium such as a memory IC. Then, a continuous light of a level to melt the recording film (amorphous level 903) as shown in Fig. 7d is irradiated to render the recording film completely amorphous. By such continuous light irradiation, the reflection level of the initial amorphous state is restored as shown in Fig. 7c. Then, the previously recorded information is overwritten. When the change of reflectivity is detected in the rewrite mode, the information is not refuged to the other recording medium and the new information is recorded after the continuous light irradiation. By repeating the above process once or more each time the reflectivity change is detected, the recorded information can be stably maintained for an extended time period. It is preferable to always monitor the reflectivity change. The above process may be automatically carried out at every predetermined number of times of rewriting, reading or reserving. Instead of continuous light irradiation at the amorphous power level, continuous light irradiation at the crystallization power level may be used. When the crystallization rate of the recording film is very high, the cooling rate of only the falling portion of the recording pulse light is fast enough to amorphize. In such a case, a continuous light irradiation causes crystalization of the entire recording film (In this case, a portion in which continuous light irradiation finished is not crystallized). The change by the crystallization may appear not on the track but between tracks. In such a case, the change is covered by the read light spot and a noise is detected. In this case, a target position of tracking may be intentionally shifted or a continuous light may be irradiated by another light beam to the area between tracks (which may be on the tracking groove or between grooves) to unify the change.

Claims: A method for rewriting information on a recording medium which undergoes a phase change when irradiated with an energy beam, wherein the beam energy at the beginning of the formation of a recording mark is raised to a high level (103) and at the end of the formation of a recording mark reaches a medium level (102), these steps being repeated to record a plurality of marks, characterised in that a low level (101) of beam energy is used before the rewriting and the beam energy at the beginning of the rewriting of said recording mark is raised from said low level (101) to said medium level (102) and then to the high level (103). The method of claim 1, wherein the energy of the beam at the end of the formation of a recording mark is lowered from said high level (103) to a level lower than said medium level and then returned to the medium level (102). The method of claim 1, wherein one of said medium and high energy levels renders the recording medium to a relatively highly crystallized state. The method of claim 3, wherein the other of said medium and high levels renders the recording film to a relatively lowly crystallized state. The method of claim 1, wherein the high level is made to vary in accordance with the information signal by superimposing a waveform (504) which varies between a level within a range from zero level to a reproduce level and a level other than said level to a waveform (505) which is substantially constant at a level between a level within a range from zero level to the reproduce level and a level other than said level. The method of claim 1, wherein, when the energy level is raised from the medium level (102) to the high level (103), the level is raised beyond the high level and then lowered to the high level. The method of claim 1, wherein the irradiation time of the energy beam at the medium level is longer than the irradiation time of the energy beam at the high level.

Inventors: HORIGOME SHINKICHI, MIYAUCHI YASUSHI, NISHIDA TETSUYA, OHTA NORIO, TERAO MOTOYASU, HORIGOME, SHINKICHI, MIYAUCHI, YASUSHI, NISHIDA, TETSUYA, OHTA, NORIO, TERAO, MOTOYASU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1987
Main classification: G11B7
IPC codes: G11B9, G11B11, G11B7