Digital data reproducing apparatus

Abstract: A digital data reproducing apparatus and method which allows picture reproduction for a long period of time without deterioration in picture quality is described. Data are reproduced from an optical disk (16) by a pickup (41) and are transferred to and stored into a ring buffer memory (45). Data read out from the ring buffer memory (45) are transferred to and stored into a video code buffer (21) of a decoder section (31). The data stored in the video code buffer (21) are decoded in the decoder section (31) and displayed on a display (29). A track jump judging circuit (47) produces, when the amount of data stored in the ring buffer memory (45) reaches a predetermined value set in advance, a track jumping instruction to cause the pickup (41) to perform track jumping back by one track distance.

Description: This invention relates to a data reproducing apparatus and method. More particularly, but not exclusively, this invention relates to data reproduction from a disk on which moving picture image data compressed to a variable degree are digitally recorded. When moving pictures are digitally recorded and reproduced, since a very large amount of data are involved, a technique of compressing data is used. Figure 1 shows the construction of an exemplary apparatus which records and reproduces moving pictures in a compressed form. Referring to Figure 1, a video signal outputted from a video camera 1 is converted from an analog signal into a digital signal by an analog to digital (A/D) converter 2 and is stored into a frame memory 3. The data stored in the frame memory 3 are read out from the frame memory 3 and inputted to a DCT (discrete cosine transform) circuit 5. The DCT circuit 5 DCT processes the input data. Data outputted from the DCT circuit 5 are inputted to and quantized by a quantizing circuit 6 and are then inputted to a VLC (variable length coder) circuit 7, at which they are converted into variable length codes, such as Huffman codes. The variable length codes from the VLC circuit 7 are supplied to and stored into a video code buffer 8. The data quantized by the quantizing circuit 6, when they are data of an I picture (intra-coded image) or a P picture (forward predictive coded image), are supplied to and inversely quantized by an inverse quantizing circuit 10. The data inversely quantized by the inverse quantizing circuit 10 are inputted to an IDCT (inverse discrete cosine transform) circuit 11, at which they are IDCT processed. Output data from the IDCT circuit 11 are, by way of an adder 12, supplied to and stored into a frame memory 13. Meanwhile, a movement detecting circuit 14 detects a movement of an image stored in the frame memory 3 and outputs a movement vector to the VLC circuit 7 and a movement compensating circuit 15. The movement compensating circuit 15 compensates for a movement of the data stored in the frame memory 13 corresponding to the movement vector and outputs the thus compensated data to a subtractor 4 and the adder 12. The subtractor 4 subtracts the data inputted thereto from the movement compensating circuit 15 from the data inputted thereto from the frame memory 3. Consequently, a P picture is produced using as a predictive image (a reference image with reference to which a difference should be taken) an I picture or a P picture which is positioned forwardly in time and decoded already, or a B picture (bi-directional predictive coded image) is produced using as predictive images three images including an I picture or a P picture which is positioned forwardly in time and decoded already, another I picture or another P picture which is positioned rearwardly in time and decoded already and an interpolation image produced from the two pictures. An I picture is produced when only data outputted from the frame memory 3 are supplied to the DCT circuit 5 without making use of data from the movement compensating circuit 15. The adder 12 adds the movement compensated data received from the movement compensating circuit 15 and the data supplied from the IDCT circuit 11 to produce a decoded image of an I picture, a P picture or a B picture and supplies the thus produced image to the frame memory 13 so that it is stored into the frame memory 13. This means that image data obtained by decoding the same data as the data which have been quantized by the quantizing circuit 6 and supplied to the video code buffer 8 by way of the VLC circuit 7 are stored in the frame memory 13. As a result, it is possible to obtain data of a P picture or a B picture making use of the data stored in the frame memory 13. A rate controller 9 monitors the amount of data stored in the video code buffer 8 and adjusts the quantizing step size of the quantizing circuit 6 so that the stored amount does not overflow or underflow. Consequently, the bit rate Rv at which data are supplied from the VLC circuit 7 to the video code buffer 8 is varied so that an otherwise possible overflow or underflow of the video code buffer 8 is prevented. The data stored in the video code buffer 8 in the above manner are transferred at a fixed transfer rate to and written onto an optical disk 16. The encoder section of the recording and reproducing apparatus is constructed in such a manner as described so far. In the decoder section, data reproduced from the optical disk 16 are transferred at a fixed transfer rate to and stored into a video code buffer 21. Data read out from the video code buffer 21 are supplied to an IVLC (inverse variable length coder) circuit 22, at which they are IVLC processed. After completion of such IVLC processing of the input data, the IVLC circuit 22 supplies the data to an inverse quantizing circuit 23. Then, the IVLC circuit 22 outputs a code request to the video code buffer 21 to request transfer of new data. When such code request is received, the video code buffer 21 transfers new data to the IVLC circuit 22. The transfer rate Rv is set to a value equal to the bit rate at which data are transferred from the VLC circuit 7 to the video code buffer 8 in the encoder section so that the video code buffer 21 does not overflow or underflow when data are transferred at a fixed transfer rate from the optical disk 16 to the video code buffer 21. In other words, the bit rate in the encoder section is set so that the video code buffer 21 in the decoder section does not overflow or underflow. The inverse quantizing circuit 23 inversely quantizes the data supplied from the IVLC circuit 22 in accordance with data of the quantizing step size supplied thereto from the IVLC circuit 22. The quantizing step size and a movement vector which is supplied from the IVLC circuit 22 to a movement compensating circuit 26 are such as those which have been supplied from the rate controller 9 and the movement detecting circuit 14 to the VLC circuit 7 and recorded onto the optical disk 16 by way of the video code buffer 8 together with image data in the encoder section and then reproduced from the optical disk 16. An IDCT circuit 24 IDCT processes the data supplied thereto from the inverse quantizing circuit 23. When the thus IDCT processed data are data of an I picture, they are supplied as they are to and stored into a frame memory 27 by way of an adding circuit 25. When the data outputted from the IDCT circuit 24 are data of a P picture for which an I picture is a predictive image, data of an I picture are read out from the frame memory 27 and are movement compensated by the movement compensating circuit 26, whereafter they are supplied to the adding circuit 25. The adding circuit 25 adds the data outputted from the IDCT circuit 24 and the data outputted from the movement compensating circuit 26 to produce data of a P picture. The data thus produced are also stored into the frame memory 27. When the data outputted from the IDCT circuit 24 are data of a B picture, data of an I picture or a P picture are read out from the frame memory 27 and then movement compensated by the movement compensating circuit 26, whereafter they are supplied to the adding circuit 25. The adding circuit 25 adds the data outputted from the IDCT circuit 24 and the data received from the movement compensating circuit 26 so that decoded B picture data are obtained. The data are also stored into the frame memory 27. The data stored in the frame memory 27 in this manner are converted from digital values into analog values by a digital to analog (D/A) converter 28 and then supplied to and displayed on a display 29. In this manner, the redundancy in a frame is reduced by DCT processing and the redundancy between frames is reduced using a movement vector, and a high compression ratio is realized by a combination of these techniques. In the above-described recording and reproducing apparatus, data are transferred at a fixed rate from the optical disk 16 to the video code buffer 21. In this instance, the quantizing step size of the quantizing circuit 6 is controlled in advance in accordance with a stored amount of data in the video code buffer 8 of the encoder section to adjust the transfer rate from the VLC circuit 7 to the video code buffer 8 so that the video code buffer 21 does not overflow or underflow. In the case of the proposed Moving Picture Experts Group (MPEG) compression system, an I picture is inserted at an interval of time of about 0.5 seconds and, since the amount of data of a P picture or a B picture is much smaller than the amount of data of an I picture, the amount of data to be transferred to the IVLC circuit 22 over an interval of time of 0.5 seconds will vary periodically. However, since the video code buffer 21 is provided, if the variation of the amount of data per unit time is within the range of the capacity of the video code buffer 21, then it is possible to follow the variation of the amount of data and consequently, data may be regularly supplied to the IVLC circuit 22. However, in a case, for example, when a plurality of complicated screens are to be encoded successively, since the bit rate upon transfer from the VLC circuit 7 is higher, in order to prevent the video code buffer 8 from overflowing, the quantizing step size of the quantizing circuit 6 must be set to a high value, which will result in a problem that the quality of the picture is varied. One possible solution to this problem, for example, would be to fix the quantizing step size of the quantizing circuit 6 so that a code train at a variable rate outputted from the VLC circuit 7 is recorded as it is onto the optical disk 16 to achieve uniformity in picture quality. However, if such an optical disk 16 is reproduced on the above-described reproducing apparatus, then, if decoding of complicated screens continues for several seconds, the video code buffer 21 will underflow. On the other hand, if decoding of simple screens continues for several seconds, then the video code buffer 21 will overflow. In this way, correct reproduction of images cannot be reliably obtained. Another possible solution is that the average bit rate is set in advance to such a high value that the apparatus can cope, for example, with complicated screens. With this solution, however, large quantities of data are transferred even for a simple screen, and consequently, the playback or recording duration of an optical disk becomes disadvantageously low. Another problem of the above-described apparatus relates to error correction for data read out from the optical disk 16 performed by an error correcting circuit (not shown). When a reproduction signal is deteriorated, for example, by dust sticking to the optical disk 16 or the tracking servo being disturbed by mechanical vibrations from the outside, error correction becomes impossible and consequently the picture quality is deteriorated. GB-A-2 136 192 (DE-A-3 407 921) approaches the problem of disturbances of the tracking servo by increasing its bandwidth. GB-A-2 136 192 (DE-A-3 407 921) discloses an apparatus for reading a disc-shaped record carrier with a spiral information track in which an optically readable digitally coded information signal is recorded with a spatial bit frequency which is independent of the radius of the track turn. The scanning speed of the information track is selected to be a factor n higher than the scanning speed normally employed for the reproduction for the recorded information. In this way the bandwidth of a servo system controlling tracking can be increased and its response time reduced. By means of a memory device information blocks being read and comprising a fixed number of data bits are time-expanded to restore the customary time relationship. The data bits are written into and read from the memory device at different fixed rates. A control unit determines the read-in cycles of the memory device for the storage of the information blocks being read and, via the radial positional system for the read spot. It determines the scanning pattern of the record carrier so that the ultimately recorded information blocks are reproduced in the same sequence and at the desired rate. The positioning system causes a group of N information blocks to be written into the memory device where N is an integer preferably equal to or greater than 2. The scanning spot then jumps back one track pitch to a position which allows the group to be read out time expanded and allows the spot to reach the start of the next group of blocks at the end of the read-out. The track jumps ensure the blocks are written into the memory device in the correct order. The capacity of the memory depends partly on the number M of information blocks and the number of bits per block in the outer track circumference of the disc. The capacity of one embodiment is 2NP where the bits of N blocks are stored in N memories. N is less than M. n is greater than [(M/N)+1]. In one example n = 5, M = 20, N = 5. Thus the capacity of the memory device is 5P or 10P i.e. much less than the capacity of the outermost track. This invention provides a digital data reproducing apparatus for reproducing digital data recorded in tracks on a disk, comprising: pickup means for picking up said digital data from said disk; error detecting means detecting an error in said digital data picked up by said pickup means and outputting an error occurrence signal; first storage means for storing the digital data therein and for reading the stored digital data therefrom at a variable rate, said storage means having a storage capacity greater than twice the amount of data recoverable from an outermost circumferential track of said disk; second storage means for storing the digital data read from the first storage means, the second storage means being operative to store said digital data at said variable rate and to read the stored digital data therefrom; decoding means for decoding said digital data read out from said second storage means; and jump controlling means for causing said pickup means to perform track jumping in accordance with said error occurrence signal. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a block diagram showing an exemplary data recording and reproducing apparatus; Figure 2 is a diagram illustrating a variation of the amount of data reproduced from an optical disk by the data recording and reproducing apparatus shown in Figure 1; Figure 3 is a block diagram of a data reproducing apparatus showing one preferred embodiment of this invention; Figure 4 is a diagram illustrating track jumping in the data reproducing apparatus shown in Figure 3; Figure 5 is a diagram illustrating construction of a sector of an optical disk for use with the data reproducing apparatus shown in Figure 3; Figure 6 is a diagram illustrating an amount of data to be written into a ring buffer memory of the data reproducing apparatus shown in Figure 3; Figure 7 is a diagram illustrating an amount of data to be written into the ring buffer memory of the data reproducing apparatus shown in Figure 3 upon track jumping; Figures 8(a) and 8(b) are diagrams illustrating manners of writing and reading of data into and from the ring buffer memory, respectively, of the data reproducing apparatus shown in Figure 3; Figures 9(a) and 9(b) are similar views but illustrating different manners of writing and reading of data into and from the ring buffer memory, respectively, of the data reproducing apparatus shown in Figure 3; Figures 10(a) and 10(b) are similar views but illustrating further different manners of writing and reading of data into and from the ring buffer memory, respectively, of the data reproducing apparatus shown in Figure 3; Figure 11 is a diagram illustrating a variation of the amount of data reproduced from an optical disk by the data reproducing apparatus shown in Figure 3; Figure 12 is a diagram illustrating writing and reading of data into and from the ring buffer memory of the data reproducing apparatus shown in Figure 3 upon recovery from an error; Figure 13 is a similar view but illustrating restricting conditions upon recovery of an error in the data reproducing apparatus shown in Figure 3; and Figures 14(a) and 14(b) are diagrams illustrating different manners of writing and reading of data into and from the ring buffer memory, respectively, of the data reproducing apparatus shown in Figure 3 upon recovery of an error. Referring first to Figure 3, there is shown in block diagram form a data reproducing apparatus according to a preferred embodiment of the present invention. It is to be noted that like elements are denoted by like reference characters to those of Figure 1. Data recorded on an optical disk 16 are reproduced by means of a pickup 41. The pickup 41 irradiates a laser beam upon the optical disk 16 and reproduces data recorded on the optical disk 16 from reflected light from the optical disk 16. A demodulating circuit 42 demodulates the reproduction signal outputted from the pickup 41 and outputs the demodulated reproduction signal to a sector detecting circuit 43. The sector detecting circuit 43 detects an address recorded for each sector of the optical disk 16 from the data inputted thereto from the demodulating circuit 42 and outputs the detected address to a control circuit 46. The sector detecting circuit 43 further outputs data in a sector synchronized relationship to an ECC circuit 44 at a subsequent stage. When the sector detecting circuit 43 fails to detect an address or finds out that detected addresses do not constitute consecutive numbers, it outputs a sector number irregularity signal to a track jump judging circuit 47. The ECC circuit 44 detects an error of data supplied thereto from the sector detecting circuit 43, corrects the error using a redundancy bit added to the data and outputs the corrected data to a track jump ring buffer memory 45 of the first-in first-out (FIFO) type. When the ECC circuit 44 cannot correct the error of the data, it outputs an error occurrence signal to the track jump judging circuit 47. The control circuit 46 controls writing into and reading out from the ring buffer memory 45 and monitors a code request signal for requesting data that is outputted from an IVLC circuit 22 by way of a video code buffer 21. The track jump judging circuit 47 monitors an output of the control circuit 46 and outputs, when track jumping is required, a track jumping signal to a tracking servo circuit 48 so that the reproducing position of the pickup 41 jumps a track. When the track jump judging circuit 47 detects a sector number irregularity signal from the sector detecting circuit 43 or an error occurrence signal from the ECC circuit 44, it outputs a track jumping signal to the tracking servo circuit 48 so that the reproducing position of the pickup 41 jumps a track. An output of the ring buffer memory 45 is supplied to a video code buffer 21 of a decoder section 31. Construction of the decoder section 31 including the video code buffer 21 through to a display 29 is similar to that of the apparatus shown in Figure 1. In the present embodiment, however, the frame memory 27 includes a pair of frame memories 27a and 27c each for storing an I picture or a P picture therein and a further frame memory 27b for storing a B picture therein. The operation of the data reproducing apparatus of the present embodiment will now be described. The pickup 41 irradiates a laser beam upon the optical disk 16 and reproduces, from reflected light from the optical disk 16, data recorded on the optical disk 16. The reproduction signal outputted from the pickup 41 is inputted to and demodulated by the demodulating circuit 42. The data demodulated by the demodulating circuit 42 are inputted by way of the sector detecting circuit 43 to the ECC circuit 44, in which detection and correction of an error of the data, if present, are performed. When a sector number (address allotted to a sector of the optical disk 16) is not detected regularly by the sector detecting circuit 43, a sector number irregularity signal is outputted to the track jump judging circuit 47. The ECC circuit 44 outputs, when data for which correction of an error is impossible are found, an error occurrence signal to the track jump judging circuit 47. Data for which data correction has been performed are supplied from the ECC circuit 44 to and stored into the ring buffer memory 45. The control circuit 46 reads an address of each sector from an output of the sector detecting circuit 43 and designates a write address (write point (WP)) of the ring buffer memory 45 corresponding to the address thus read. Further, the control circuit 46 designates, in response to a code request signal from the video code buffer 21 at the succeeding stage, a read address (reproducing point (RP)) of data written in the ring buffer memory 45, reads data from the thus designated reproducing point (RP) and supplies the data to the video code buffer 21 so as to store the data into the video code buffer 21. The data read out from the video code buffer 21 are transferred at a transfer rate Rv to the IVLC circuit 22. The IVLC circuit 22 IVLC processes the thus received data and outputs, after completion of the IVLC processing of the received data, the same to the inverse quantizing circuit 23. The IVLC circuit 22 further outputs a code request signal to the video code buffer 21 to request supply of new video data. The inverse quantizing circuit 23 inversely quantizes data thus received and outputs the inversely quantized data to the IDCT circuit 24. The IDCT circuit 24 IDCT processes the received data and supplies the same to the adding circuit 25. When the data outputted from the adding circuit 25 correspond to an I picture, they are stored into the frame memory 27a or 27c. When the data outputted from the adding circuit 25 correspond to a B picture, they are stored into the frame memory 27b. When the data outputted from the adding circuit 25 correspond to a P picture, they are stored into the frame memory 27a or 27c. Data of an I picture and a P picture stored in the frame memories 27a and 27c are supplied by way of the movement compensating circuit 26, when necessary, to the adding circuit 25 so that they are utilized for decoding of a next P or B picture. One of data stored in the frame memories 27a to 27c is selected by a switch 27d, and the selected data are D/A converted by the D/A converter 28 and then supplied to the display 29, on which they are displayed subsequently. The optical disk 16 which is rotated at a predetermined speed is sectioned into a plurality of sectors. Each sector is constituted, for example, from a header portion and a data portion as shown in Figure 5, and, for example, clock pits for producing clocks, wobbled pits for tracking and so forth are formed as pre-pits at the header portion. Meanwhile, video data and so forth are stored in the data portion. Accordingly, the data transfer rate to the ring buffer memory 45 periodically varies for each sector as shown in Figure 6. In particular, at the header portion, transfer of data is not performed, and only data from the data portion are transferred to and stored into the ring buffer memory 45. An average transfer rate of data to the ring buffer memory 45 is indicated by the line Rm in Figure 6. Referring back to Figure 3, the control circuit 46 reads out data stored in the ring buffer memory 45 and supplies them to the video code buffer 21 in response to a code request signal from the video code buffer 21. However, if processing of data of, for example, simple screens takes place successively so that the data transfer rate from the video code buffer 21 to the IVLC circuit 22 per unit time becomes low, also the data transfer rate from the ring buffer memory 45 to the video code buffer 21 is reduced. Consequently, there is the possibility that the amount of data stored in the ring buffer memory 45 may be increased until the ring buffer memory 45 overflows. To avoid this, the track jump judging circuit 47 calculates (detects) an amount of data currently stored in the ring buffer memory 45 from the writing point (WP) and the reproducing point (RP) and, when the amount of data exceeds a predetermined reference value set in advance, the track jump judging circuit 47 judges that there is the possibility that the ring buffer memory 45 may overflow and outputs a track jumping instruction to the tracking servo circuit 48. When the track jump judging circuit 47 detects a sector number abnormality signal from the sector detecting circuit 43 or an error occurrence signal from the ECC circuit 44, it calculates an amount of data remaining in the ring buffer memory 45 from the writing address (WP) and the reading address (RP) and further calculates an amount of data necessary to assure reading from the ring buffer memory 45 to the video code buffer 21 while the optical disk 16 makes one rotation from a track position at present (i.e. the time for one rotation of the optical disk 16). When the remaining data amount in the ring buffer memory 45 is sufficiently great, even if data are read out at a possible highest transfer rate from the ring buffer member 45, no underflow will take place in the ring buffer memory 45, and accordingly the track jump judging circuit 47 judges that recovery of an error is possible by reproducing the optical disk 16 at the error occurring position again by the pickup 41 and outputs a track jumping instruction to the tracking servo circuit 48. When the track jumping instruction is outputted from the track jump judging circuit 47, the tracking servo circuit 48 jumps the reproducing position of the pickup 41, for example, from a position A to another position B on the inner side by one track distance as shown in Figure 4. Then, for a period of time when the reproducing position comes from the position B to the position A again as a result of one rotation of the optical disk 16, that is, for a period of time until the sector number obtained from the sector detecting circuit 43 becomes equal to the original sector number from which track jumping was performed, the control circuit 46 inhibits writing of new data into the ring buffer memory 45, and when necessary, data already stored in the ring buffer memory 45 are transferred to the video code buffer 21. Further, even if a sector number obtained from the sector detecting circuit 43 after track jumping coincides with the sector number obtained upon track jumping, in case the amount of data stored in the ring buffer memory 45 exceeds the predetermined reference value, that is, in case there is the possibility that the ring buffer memory 45 may overflow, writing of data into the ring buffer memory 45 is not resumed and track jumping is performed again. A manner in which data are transferred to the ring buffer memory 45 when track jumping back by one track distance is performed is illustrated in Figure 7. As can be seen from Figure 7, for a period of time whilst the optical disk 16 makes one full rotation after track jumping back by one track distance until the reproducing position returns to the original reproducing position, writing of new data into the ring buffer memory 45 is not performed. Consequently, transfer of data to the ring buffer memory 45 is performed after an excess time equal to the period of time of such track jumping, and accordingly, the average transfer rate to the ring buffer memory 45 varies at values lower than Rm. In other words, Rm represents an allowable maximum average transfer rate. Here, the ring buffer memory 45 has a capacity sufficient to store therein data for at least one track (one rotation) of the optical disk 16, that is, at least a storage capacity of a maximum period of rotation of the optical disk multiplied by Rm. Consequently, when the optical disk 16 is, for example, a constant linear velocity (CLV) disk, the period of rotation is at a maximum at an outermost circumference of the optical disk 16, and accordingly, the ring buffer memory 45 at least has a storage capacity for one track (one rotation) at the outermost circumference of the optical disk 16, i.e. the storage capacity is given by the period of rotation on the outermost circumference of the optical disk 16 multiplied by Rm. Where the maximum transfer rate from the ring buffer memory 45 to the video code buffer 21 is represented by Rc, Rc is set to a value equal to or a little smaller than Rm (Rc ≤ Rm). When Rc is set to such value, a code request for transfer of data from the video code buffer 21 to the ring buffer memory 45 can be forwarded freely irrespective of a timing of track jumping. When Rc is considerably smaller than Rm, for example, when Rc is equal to approximately one half of Rm, the amount of data written into the ring buffer memory 45 is greater than the amount of data read out from the ring buffer memory 45, and consequently, the condition wherein the ring buffer memory 45 is almost full of data will continue. When the optical disk 16 is a CLV disk, the amount of data reproduced while the optical disk 16 makes one full rotation is considerably different depending upon whether data are reproduced from an inner circumference or an outer circumference of the optical disk 16. If the storage capacity of the ring buffer memory 45 is adjusted for an outermost circumference of the optical disk 16 at which the amount of data per rotation is a maximum for the optical disk 16, then considerable excess is present in the storage capacity of the ring buffer memory 45 at an inner circumference of the optical disk 16. Consequently, the probability that error recovery by jumping back, as described above, may take place is high. If the storage capacity of the ring buffer memory 45 is further increased, then the probability that error recovery can be performed is also increased, and if the storage capacity is higher than twice that at an outermost circumference track of the optical disk 16, then re-reading to attempt error recovery can always be performed independently of a remaining amount of data in the ring buffer memory 45. Subsequently, a relationship between the capacity of the ring buffer memory 45 and the transfer rate will be described. Figures 8(a) and 8(b) illustrate manners of writing and reading out of data into and from the ring buffer memory 45 when data are being read out at a fixed high bit rate from the ring buffer memory 45. Figure 8(a) illustrates a manner when the pickup 41 is reading from an outer circumference of the optical disk 16 while Figure 8(b) illustrates a manner when the pickup 41 is reading from an inner circumference of the optical disk 16. In the case of Figure 8(a), writing of data into the ring buffer memory 45 is started at an average transfer rate Rm (point A), and the ring buffer memory 45 is filled with data soon, whereupon writing is stopped (point B). Then, the pickup 41 track jumps so as to return to another track by one track distance. Meanwhile, reading out of data from the ring buffer memory 45 is started (point C), and after a time in which the optical disk 16 makes one full rotation passes, if there is a free area in the ring buffer memory 45, then writing of data is started again (point D). Such writing is stopped again (point E) when the ring buffer memory 45 becomes full, and then after a time in which the optical disk 16 makes one full rotation passes, writing of data is resumed (point F). In this instance, as the rate of reading out of data from the ring buffer memory 45 increases (as the transfer rate Rc increases), the amount of data (the shadowed area in Figure 8(a)) remaining in the ring buffer memory 45 decreases. It is to be noted that a broken line in Figure 8(a) indicates a position at which the ring buffer memory 45 becomes full. In the case of Figure 8(b) illustrating a manner of reading from an inner circumference of the optical disk 16, since the time in which the optical disk 16 makes one full rotation is shorter than that when the pickup 41 reads from an outer circumference of the optical disk 16, even after track jumping is performed (point B'), there is no free area in the ring buffer memory 45 as yet. Consequently, the pickup 41 performs track jumping again (between the points B' and D). As a result, the amount of data remaining in the ring buffer memory 45 at the point F is greater than that shown in Figure 8(a), which illustrates a manner of reading from an outer circumference of the optical disk 16. Figures 9(a) and 9(b) illustrate manners of writing and reading out of data into and from the ring buffer memory 45 when data are being read out at a fixed low bit rate from the ring buffer memory 45. In particular, Figure 9(a) illustrates a manner when the pickup 41 is on an outer circumference of the optical disk 16 on which data are recorded at a fixed linear velocity while Figure 9(b) illustrates a manner when the pickup 41 is on an inner circumference of the optical disk 16. The amount of data remaining in the ring buffer memory 45 after track jumping is greater than that of the case wherein reading out of data is performed at a high rate (Figures 8(a) and 8(b)). Figures 10(a) and 10(b) illustrate manners of writing and reading out of data into and from the ring buffer memory 45 when data are being read out from the ring buffer memory 45 at a variable rate which does not exceed Rc. In particular, Figure 10(a) illustrates a manner when the pickup 41 is on an outer circumference of the optical disk 16 on which data are recorded at a fixed linear velocity while Figure 10(b) illustrates a manner when the pickup 41 is on an inner circumference of the optical disk 16. In Figures 10(a) and 10(b), a broken line indicates reading out of data from the video code buffer 21. Meanwhile, a line representative of reading out of data from the ring buffer memory 45 is equivalent to a line representative of writing of data into the video code buffer 21. Since reading out of data from the ring buffer 45 is performed at a variable rate, a line indicative of such reading out is not a straight line as shown in Figures 8(a) to 9(b) but is a polygonal line as shown in Figure 10(a) or 10(b). In Figures 10(a) and 10(b), the lines indicative of reading out of data from the ring buffer memory 45 are the same as each other. This is because the ring buffer memory 45 absorbs a difference between times in which the optical disk 16 makes one full rotation when reading is performed at an inner circumference and an outer circumference of the optical disk 16. On the other hand, while the reading out line of data from the ring buffer memory 45 (gradient of the reading out line of data from the ring buffer memory 45) cannot exceed the maximum average transfer rate Rm, the transfer rate Rv to the IVLC circuit 22 (gradient of the reading out line (indicated by broken lines in Figures 10(a) and 10(b)) of data from the video code buffer 21) can be set to a transfer rate higher than the maximum average transfer rate Rm due to the presence of the video code buffer 21 interposed between the ring buffer memory 45 and the IVLC circuit 22. Further, Figure 11 shows a manner of a variation of an amount of reproduced data when the optical disk 16 is reproduced from the beginning to the end. It can be seen from Figure 11 that the amount of data reproduced per unit time is great in a section of complicated images, but the amount of reproduced data is small in another section of simple images. When data are being reproduced from the optical disk 16, the transfer rate of data to the ring buffer memory 45 is fixed, but since track jumping takes place when necessary, when attention is paid to the entire period from the starting point to the ending point, the average transfer rate is variable. Figures 12 and 13 illustrate recovering operations by the ring buffer memory 45 when an error appears in reading of data from the optical disk 16. Figure 12 illustrates the recovering operation when reading out of data from the ring buffer memory 45 is performed at a variable rate. For example, when an error is caused in reading out of data from the optical disk 16, for example, by mechanical vibrations to disable writing of data into the ring buffer memory 45 (point G), if the amount of data stored in the ring buffer memory 45 (shadowed area in Figure 12) is greater than an amount of data corresponding to one rotation of the optical disk 16, then the pickup 41 performs track jumping to perform reading out of data again (point H). Consequently, recovery is achieved without having an influence upon reading out from the ring buffer memory 45 at all. On the other hand, in the case of Figure 13, when an error is caused in reading out of data from the optical disk 16, for example, by mechanical vibrations to disable writing of data into the ring buffer memory 45 (point G), since the amount of data stored in the ring buffer memory 45 (shadowed area in Figure 13) is smaller than an amount of data corresponding to one rotation of the optical disk 16, the ring buffer memory 45 would become empty (point H') before reading out is performed again, and accordingly, recovery cannot be performed without having an influence on reading out of data from the ring buffer memory 45. Subsequently, when, for example, the position at which data recorded on the optical disk 16 are being reproduced is an inner circumferential track and accordingly the waiting time for one complete rotation of the optical disk 16 is comparatively short so that there is a sufficient room in the remaining storage capacity of the ring buffer memory 45, or when the storage capacity of the ring buffer memory 45 is set to a sufficiently great value in advance, the recovering faculty by the ring buffer memory 45 when an error takes place in reading out of data from the optical disk 16 is high. In particular, when data are being read out from an inner circumference of the optical disk 16, since the time required for one full rotation is short, even if the amount of data stored in the ring buffer memory 45 is small, error recovery can be performed (Figure 14(a)). Further, in case an error takes place even upon re-reading, re-reading is performed repetitively so far as the remaining amount of data of the ring buffer memory 45 allows this (Figure 14 (b)). Similarly, if the capacity of the ring buffer memory 45 is set to a sufficiently high value in advance, then since surplus data can be stored in the ring buffer memory 45, the error recovering faculty is high. It is to be noted that the optical disk 16 may be replaced by some other disk such as, for example, an opto-magnetic disk or a magnetic disk. While a specific embodiment of the invention has been shown and disclosed, it is to be understood that numeral changes and modifications may be made by those skilled in the art without departing from the scope of the invention as defined by the claims. In at least preferred embodiments this invention provides a digital data reproducing apparatus and method by which a reproduced image free from deterioration in picture quality can be obtained and, more particularly, a reproduced image of uniform picture quality can be obtained from a disk on which a code train at a variable rate is recorded. Further, picture images can be reproduced for a long period of time from a disk of a limited capacity and an image can be reproduced, upon occurrence of an error, without deteriorating the picture quality.

Claims: A digital data reproducing apparatus for reproducing digital data recorded in tracks on a disk (16), comprising: pickup means (41) for picking up said digital data from said disk (16); error detecting means (43,44) for detecting an error in said digital data picked up by said pickup means (41) and outputting an error occurrence signal; first storage means (45) for storing the digital data therein and for reading the stored digital data therefrom at a variable rate, said storage means (45) having a storage capacity greater than twice the amount of data recoverable from an outermost circumferential track of said disk (16); second storage means (21) for storing the digital data read from the first storage means, the second storage means being operative to store said digital data at said variable rate and to read the stored digital data therefrom; decoding means (22-27) for decoding said digital data read out from said second storage means; and jump controlling means (47) for causing said pickup means (41) to perform track jumping in accordance with said error occurrence signal. A digital data reproducing apparatus according to claim 1, comprising sector detecting means (43) for detecting a sector number from said digital data picked up by said pickup means (41), a write address (WP) of said first storage means (45) being set in accordance with said sector number. A digital data reproducing apparatus according to claim 1, wherein said error detecting means (43,44) includes sector detecting means (43) for detecting a sector number from said digital data picked up by said pickup means (41) and outputting, when the sector number is not detected regularly, said output error occurrence signal to said jump controlling means (47). A digital data reproducing apparatus according to claim 1, 2 or 3, wherein said error detecting means (43,44) incudes means (44) for detecting and correcting an error of said digital data picked up by said pickup means (41) and outputting, when data which cannot be corrected are found, said error occurrence signal to said jump controlling means (47). A digital data reproducing apparatus according to any one of claims 1 to 4, wherein said digital data include video data. A digital data reproducing apparatus according to any one of claims 1 to 5, wherein the disk is an optical disk (16), and said pickup means is an optical pickup (41) which makes use of a laser beam. A digital data reproducing apparatus according to any one of claims 1 to 6, wherein said track jumping is performed from a current reading position (A) to a position (B) on an inner circumference side by one track pitch. A digital data reproducing apparatus according to any one of claims 1 to 7, wherein said first storage means is a ring buffer memory (45). A digital data reproducing apparatus according to any preceding claim, wherein the maximum average transfer rate (Rm) to said first storage means (45) is higher than the maximum transfer rate (Rc) from said first storage means (45) to said second storage means (21). A digital data reproducing apparatus according to any one of claims 1 to 9, wherein said decoding means (31) includes inverse variable length code means (22) for inverse variable length code processing a predetermined portion of said digital data. A digital data reproducing apparatus according to any one of claims 1 to 10, wherein said decoding means (31) includes an inverse quantizing circuit (23) for inversely quantizing a predetermined portion of said digital data. A digital data reproducing apparatus according to any one of claims 1 to 11, wherein said decoding means (31) includes inverse discrete cosine transform means (24) for inverse discrete cosine transform processing a predetermined portion of said digital data. A digital data reproducing apparatus according to any one of claims 1 to 12, wherein said decoding means (31) includes movement compensating means (26) for performing movement compensation in accordance with movement vector information included in a predetermined portion of said digital data.

Inventors: AKIYAMA YOSHIYUKI, KOYANAGI HIDEKI, YONEMITSU JUN, AKIYAMA, YOSHIYUKI, KOYANAGI, HIDEKI, YONEMITSU, JUN, Akiyama, Yoshiyuki, c/o Patents Div., Sony Corp., Koyanagi, Hideki, c/o Patents Div., Sony Corp., Yonemitsu, Jun, c/o Patents Div., Sony Corp.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: H04N5
IPC codes: H04N5, G11B20, H04N7, G11B7