Patent Application
20050228940
The invention relates to a device for recording information in a track on a record carrier, which device comprises recording means for recording marks representing the information, control means for controlling recording of digitally encoded real-time information, in particular video, in a stream of packets in the track and of characteristic point information for controlling playback of the packets.
The invention further relates to a method of recording information.
The invention further relates to a computer program product for recording information.
A device and method for recording information on a record carrier are the known from WO 00/28544 (PHN 17161), in which digitally compressed video data is recorded on an optical disc according to a video encoding standard, for example the MPEG2 format. The apparatus has input means for receiving video information, and recording means for recording the video information in a stream of packets in the track, in particular in a video file. The packets may contain video, audio or other information, e.g. subtitles. A number of packets together are required to constitute a reproducible amount of information, e.g. a complete independently coded video frame called I-picture in MPEG. The device further has control means for generating characteristic point information for controlling playback of the packets. The characteristic point information identifies a characteristic point in the video file, i.e. locations of particular data elements such as start of an I-picture or a new scene. The characteristic point information includes pointers to addresses and/or pointers including an offset from a given reference point in the video file, e.g. the start of the file. A problem of the known system is that errors in the recorded data may occur, and that the reproduced video may be affected by the errors. In particular artifacts may be visible because the video decoder receives data not compliant with the applicable video standard.
It is an object of the invention to provide a system for recording and reproducing digitally compressed video information, which is able to hide errors in a more flexible way.
For this purpose, the device as described in the opening paragraph has error detection means for detecting recording errors and for detecting the size of a missing part of the real-time information due to a detected error and for determining an exit point before the detected error and an entry point after the detected error, the real-time information up to the exit point and starting at the entry point constituting decodable streams of packets, and recalculating means for recalculating, in the event of a detected recording error, the characteristic point information by skipping packets between the exit point and the entry point. The method comprises controlling recording of digitally encoded real-time information, in particular video, in a stream of packets in a track and of characteristic point information for controlling playback of the packets, detecting recording errors and detecting the size of a missing part of the real-time information due to a detected error and determining an exit point before the detected error and an entry point after the detected error, the real-time information up to the exit point and starting at the entry point constituting decodable streams of packets, and recalculating, in the event of a detected recording error, the characteristic point information by skipping packets between the exit point and the entry point. The measures have the advantage that recording errors, once detected, can be hidden by adapting the characteristic point information so that the erroneous parts are skipped. In particular the exit point is determined before the erroneous part and on a point in the stream of packets where the decoder has completed a picture. The entry point after the error is determined on a point in the stream from which the decoder can start decoding independently of any preceding information, e.g. an I-picture. Any packets, which are not affected by the detected error, but are part of a video picture that is affected, are skipped. Hence during such subsequent reproduction the error can be completely circumvented, because using the adapted characteristic point information the part of the video stream containing the error is no longer needed. In addition more complicated types of errors, which might result in a decoder reset, undefined audio and/or video data elements leading to visible or audible artifacts, are eliminated by adapting the characteristic point information. The resulting stream may still have a time discontinuity, which is however hardly detectable for the user.
The invention is also based on the following recognition. The inventors have seen that an inconsistency might occur between the characteristic point information and the video data that is actually on disc. There are (at least) the following three different kinds of situations that may lead to that situation. The medium contains no corrupt sectors on the areas considered, but part of the data is not written to disc. This could be a result of mechanical shocks, bad buffer management, a network error in case the disc drive is used as a network drive, etc. Secondly the medium contains corrupt sector(s) on the areas considered. During writing a corrupt sector is encountered and part of the data is lost. Thirdly a disc error is introduced some time after the data has been written correctly, e.g. due to scratches or dirt. Reducing the detrimental effect of the recording error can be achieved by solving the inconsistency. The CPI structure, generated to address the file, is used to check if the file was written correctly. If not, the CPI structure is used to find the part of the file that is missing. The characteristic point information is recalculated to be consistent with the data on disc.
In an embodiment of the device the error detection means comprise means for verifying the real-time information via the characteristic point information. This has the advantage that the characteristic point information constitutes a concise set of accessing information having access points distributed over the real-time information file. Further the characteristic point information indicates the size and location of the real-time information in addition to the file management system data. Using the characteristic point information gives an easy access to relevant packets within the video stream. Any detected error can be repaired by adapting the characteristic point information. In an embodiment said means for verifying are for verifying status information of a file of real-time information, in particular the file size or parts not written correctly as indicated by a file management system, and for determining missing parts of said file by accessing the file via the characteristic point information. This has the advantage that the file management system information may be available indicating that the file contains some errors, but not where and which parts of the file are missing. For real-time information the file may still be usable, and the characteristic point information is used to find which parts are normally accessible and which part are missing. Hence the damaged file can still be used with only a slight degradation of reproduced video quality.
In an embodiment of the device the recalculation means comprise means for removing pointers to packets between the exit point up and the entry point, and/or recalculating offsets of pointers after the entry point. This has the advantage, that the characteristic point information now only points to valid addresses of useful packets within the video file.
It is noted that US 2001/0018727 A1 describes devices for recording video data via a file management system and accompanying characteristic point information called AV addresses. The file management system has the function of defect management, i.e. relocating data which is affected by recording errors during recording. Hence all video data is recorded, and the file management system keeps track of the location of defect areas. The characteristic point information (AV addresses) is not involved in defect management (see [1150-1158]). On the contrary the current invention relates to situations where the data is damaged or missing permanently, and provides a solution using the characteristic point information.
These and other aspects of the invention will be apparent from and elucidated further with reference to the embodiments described by way of example in the following description and with reference to the accompanying drawings, in which
a shows a record carrier (top view),
b shows a record carrier (cross section),
Corresponding elements in different Figures have identical reference numerals.
a shows a disc-shaped record carrier 11 having a track 9 and a central hole 10. The track 9, being the position of the series of (to be) recorded marks representing information, is arranged in accordance with a spiral pattern of turns constituting substantially parallel tracks on an information layer. The record carrier may be optically readable, called an optical disc, and has an information layer of a recordable type. Examples of a recordable disc are the CD-R and CD-RW, and writable versions of DVD, such as DVD+RW, and the high density writable optical disc using blue lasers, called Blue-ray Disc (BD). Further details about the DVD disc can be found in reference: ECMA-267: 120 mm DVD—Read-Only Disc—(1997). The information is represented on the information layer by recording optically detectable marks along the track, e.g. crystalline or amorphous marks in phase change material. The track 9 on the recordable type of record carrier is indicated by a pre-embossed track structure provided during manufacture of the blank record carrier. The track structure is constituted, for example, by a pregroove 14 which enables a read/write head to follow the track during scanning. The track structure comprises position information, e.g. addresses, for indication the location of units of information, usually called information blocks. The position information includes specific synchronizing marks for locating the start of such information blocks. The position information is encoded in frames of modulated wobbles as described below.
b is a cross-section taken along the line b-b of the record carrier 11 of the recordable type, in which a transparent substrate 15 is provided with a recording layer 16 and a protective layer 17. The protective layer 17 may comprise a further substrate layer, for example as in DVD where the recording layer is at a 0.6 mm substrate and a further substrate of 0.6 mm is bonded to the back side thereof. The pregroove 14 may be implemented as an indentation or an elevation of the substrate 15 material, or as a material property deviating from its surroundings.
The record carrier 11 is intended for carrying information representing digitally encoded video according to a standardized format like MPEG2.
MPEG is a video signal compression standard, established by the Moving Picture Experts Group (“MPEG”) of the International Standardization Organization (ISO). MPEG is a multistage algorithm that integrates a number of well known data compression techniques into a single system. These include motion-compensated predictive coding, discrete cosine transform (“DCT”), adaptive quantization, and variable length coding (“VLC”). The main objective of MPEG is to remove redundancy which normally exists in the spatial domain (within a frame of video) as well as in the temporal domain (frame-to-frame), while allowing inter-frame compression and interleaved audio. MPEG-1 is defined in ISO/IEC 11172 and MPEG-2 is defined in ISO/IEC 13818. There are generally three different encoding formats which may be applied to video data. Intra-coding produces an “I” block, designating a block of data where the encoding relies solely on information within a video frame where the macro block 16 of data is located. Inter-coding may produce either a “P” block or a “B” block. A “P” block designates a block of data where the encoding relies on a prediction based upon blocks of information found in a prior video frame (either an I-frame or a P-frame, hereinafter together referred to as “reference frame”). A “B” block is a block of data where the encoding relies on a prediction based upon blocks of data from at most two surrounding video frames, i.e., a prior reference frame and/or a subsequent reference frame of video data. In principle, in between two reference frames (I-frame or P-frame) several frames can be coded as B-frames. However, since the temporal differences with the reference frames tend to increase if there are many frames in between (and consequently the coding size of a B-frame increases), in practice MPEG coding is used in such a way that in between reference frames typically at most two B frames are used, each depending on the same two surrounding reference frames. To eliminate frame-to-frame redundancy, the displacement of moving objects in the video images is estimated for the P-frames and B-frames, and encoded into motion vectors representing such motion from frame to frame. An I-frame is a frame wherein all blocks are inter-coded. A P-frame is a frame wherein the blocks are inter-coded as P-blocks. A B-frame is a frame wherein the blocks are inter-coded as B-blocks. If no effective coding inter-coding is possible for all blocks of a frame, some blocks may be inter-coded as a P-block or even as an I-block. Similarly, some blocks of a P-frame may be coded as I-blocks.
The device comprises error detection means for detecting recording errors. The control unit 20 is arranged for controlling recording of digitally encoded real-time information, in particular video, in a stream of packets in the track and of characteristic point information (CPI) for controlling playback of the packets. CPI is described in detail in WO 00/28544. Further the control unit 20 is arranged for detecting the recording errors in cooperation with the input unit 27, the read processing unit 30 and/or error detection unit 32. It is noted that the error detection means may be arranged for detecting errors during recording, but they are in particular arranged for detecting errors after the recording has taken place at some later time. In both cases we assume that correction of the video data or re-recording is not viable, i.e. the errors are considered to be permanent. The device is equipped to reduce the detrimental effects of such permanent errors. The device further comprises a recalculation unit 31 for recalculating the characteristic point information in the event of a detected recording error by skipping packets between the exit point and the entry point. The recalculation unit 31 has an output 33 coupled to the formatter 28 for supplying the recalculated characteristic point information to be recorded. The error detection unit 32 is arranged for selecting an exit point at the last possible point before the error location based on the type of encoding, e.g. exiting just before a new independently coded video frame (I-picture). Further the recalculation unit 31 is arranged for selecting an entry point at the first possible point after the error location and also based on the type of encoding, e.g. just before an I-picture.
The control unit 20 and error detection unit 32 are arranged for processing as follows. After a recording error has been detected, the size of a missing part of the real-time information is detected using the characteristic point information. For example a mismatch is detected between the length indicated in the CPI table and the file system length information. Then an exit point before the detected error and an entry point after the detected error are determined. The exit and entry point are selected such that the skipped part of the video stream is as small as possible, while the real-time information up to the exit point and starting at the entry point both constitute a decodable stream of packets. The characteristic point information and further references to the video data are recalculated marking the discontinuity as a discontinuity of time and updating pointers and the relevant tables.
In an embodiment of the device the control unit and the functions of error detection and recalculation of characteristic point information are implemented in a software program. The software program running in suitable computer environment is controlling the execution of the recording process in a disc drive unit which is connected to the computer, e.g. an optical disc recorder coupled via an interface bus or network to a PC.
In an embodiment of the device the error detection unit 32 is coupled to the read processing unit 30 via an error output 34 for detecting physical errors, e.g. during writing or reading information on the record carrier 11. The read signals from the record carrier are monitored for detecting a physical defect of the record carrier. Detecting such defects can be based on the reflected laser radiation during writing an optical disc, or on detecting deviations in other writing parameters. For example a read signal can be generated and errors occurring can be detected by the usual error correction codes (ECC) embedded in the data blocks recorded.
In an embodiment the error detection unit 32 is arranged for detecting errors by reading and processing defect information recorded earlier on the record carrier, e.g. defect tables managed by a file management system like UDF. By analyzing the defect information the write error detection unit detects an erroneous location or possibly erroneous locations. For example the file management system creates an Alarm List of locations that show a relatively high number of errors. If the video information is recorded in such a location, the characteristic point information is used to verify the accessibility of the video information in that area. If some video information cannot be accessed, the characteristic point information is recalculated skipping the un-accessible part.
Detecting recording errors can be based on other status information from the file system. The file system usually will be aware of the loss of data when recording and update its file tables to reflect this, e.g. typically a write request will return the number of bytes written so the file system will know how many bytes were lost (but not which ones). In some cases the file system indicates the parts of the file that were lost. In this case the data before and after the last part can be read to determine what data in terms of time was lost. Alternatively the drive may just indicate that the write did not complete. However, in some cases the file system may be consistent with the disc but not with the application layer e.g. in case of networked storage where some data was lost over the network. The characteristic point information can be used to verify such consistency.
In a practical embodiment the file system must be made consistent with the data actually written on disc as follows. In the case where the file system received an ‘action not complete’ event from the drive it may be necessary to read back the data to find the parts that were successfully written. It will be possible to find discontinuities by parsing the stream or looking at invalid ECC blocks. After the file system has been updated to reflect the actual data on disc, there are two further problems to be addressed. First, addressing is done using source packet offsets i.e. the file content is addressed in multiples of the source packet size, in this case 192 bytes. This means that the size of lost data must be a multiple of 192 bytes in order for the addressing to work (otherwise the source packet offsets will not point to the start of source packets). Therefore the file system should be updated to make the lost data a multiple of the source packet size. It is to be verified that the missing data is actually an integral number of full source packets (and not just a multiple of the source packet size). This ensures that the file doesn't include any partial source packets.
In an embodiment of the device the control unit 20 is arranged for performing a repair function for video information recorded earlier. The error detection means comprise a video stream error detection unit 36 coupled to the read processing unit 30. The video stream as recorded is reproduced, and recording errors are detected. The characteristic point information is recalculated and recorded as described above. The video stream error detection unit 36 may be implemented as a function of the MPEG2 video decoder, which decoder necessarily has to process the video data and therefore can easily be adapted to provide error detection of inconsistent video data.
In an embodiment the error detection and characteristic point information recalculation process can be controlled by a user. The user detects errors in the digitally encoded video information, for example glitches or artifacts in the displayed video stream. The device has a user command mode in which the user then indicates which part of the video must be marked as a recording error. The device then recalculates the characteristic point information as described above.
Although the invention has been explained mainly by embodiments using the BD, similar embodiments like DVD are suitable for other optical recording systems. For example in the DVD+RW the playback information is organized according to the DVD standard as in said ECMA document, including Cells containing characteristic point information for reproducible items and VOBUs for physically stored video. Also for the information carrier an optical disc has been described, but other media, such as a hard disc or magnetic tape, can be used. It is noted, that in this document the word ‘comprising’ does not exclude the presence of other elements or steps than those listed and the word ‘a’ or ‘an’ preceding an element does not exclude the presence of a plurality of such elements, that any reference signs do not limit the scope of the claims, that the invention may be implemented by means of both hardware and software, and that several ‘means’ may be represented by the same item of hardware. Further, the scope of the invention is not limited to the embodiments, and the invention lies in each and every novel feature or combination of features described above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 02076947 | May 2002 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB03/01574 | 4/22/2003 | WO | 11/12/2004 |
