WO2005114997A1

WO2005114997A1 - Seamless adding of real-time information

Info

Publication number: WO2005114997A1
Application number: PCT/IB2005/051550
Authority: WO
Inventors: Leonardus M. M. Veugen; Alexander J. J. Bos
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-05-18
Filing date: 2005-05-11
Publication date: 2005-12-01
Also published as: TW200618641A

Abstract

A recorder seamlessly records real-time information, such as video and audio, on a record carrier in a track. The real-time information is encoded and multiplexed according to the required recording format. The recorder has an encoder status unit (50) for, while recording the real-time information, generating encoder status data (54, 55) and storing (56), in a status logbook, the encoder status data at log entry instants. The encoder status data represents the encoder and multiplexer status at respective log entry instant. When a recording of the real-time information is to be appended at an append point to pre-existing recorded real-time information, the encoder status data is retrieved from the status logbook. The log entry instant of the pre-existing recorded real-time information near the append point is selected. Before starting the new recording session, the encoder status at the append point is restored using retrieved real-time information (51, 52, 53) and the retrieved encoder status data (54, 55).

Description

Seamless adding of real-time information

The invention relates to a device for recording real-time information on a record carrier in a track, the device comprising, recording means for recording real-time information, which recording means include a real-time information encoder for encoding the real-time information to encoded real-time information, and a head for recording marks representing the encoded real-time information in the track, and a control unit for controlling recording of the real-time information. The invention further relates to a method of recording real-time information on a record carrier in a track, the method comprising encoding the real-time information via a real-time information encoder process to encoded real-time information, and recording the real-time information by recording marks representing the encoded real-time information in the track. The invention further relates to a computer program product for recording real-time information on the record carrier. In particular the invention relates to seamlessly appending real-time information on an optical disc or hard disk in a recording device. Recording devices for digitally encoding and recording real-time information such as video are well known, e.g. a DVD video recorder (Digital Versatile Disc).

A digital video recorder is known from US Patent US 6,151,443. The device comprises an encoder for compressing audio/video data, in particular according to the MPEG data encoding standard (Moving Picture Experts Group, see ISO/IEC 11172 and ISO/IEC 13818). The video recorder is provided with a "pause and resume" function and a "channel change" function for recording and playing interframe compressed audio video data, particularly MPEG data. The document describes a digital track format and a method for recording one or more sessions from one or more streams of interf ame compressed audio video data which has been synchronized to a reference clock. Special "splice point" indicators are added to recorded video material. The splice point indicators are added either periodically (e.g., by recording a special reference clock) or when a discontinuity occurs in recording audio video data streams. Splice point indicators are used to determine the existence of a point of discontinuity in the recorded audio video data. The system and method enable pausing and later resuming recording of an audio video data stream such that the audio video data can be played back with proper resynchronization at each point of discontinuity (the "pause and resume" function). The format and method also allow for recording an abrupt change from one audio video data stream to a different audio video data stream having a different clock reference, such that the two recorded sequences are spliced together as one continuous recorded sequence that can be played back with resynchronization of the program audio video data at each splice point (the "channel change" function). Although the splice point indicators allow appending new real-time information starting at the splice point, the user may want to append real-time information at a different point. Moreover, problems may occur when data is appended at a splice point. During reproduction the abrupt change of the pre-existing encoded real-time information to the new encoded real-time information at the append point may result in visually or audible detectable discontinuities.

Therefore it is an object of the invention to provide a system for appending real-time information in which the reproduction of real-time information on the append point is seamless. According to a first aspect of the invention the object is achieved with a device for recording real-time information as described in the opening paragraph, in which device the control unit comprises an encoder status unit for, while recording the real-time information, generating encoder status data from the encoder, storing, in a status logbook, the encoder status data at log entry instants, the encoder status data being indicative of an encoder status based on real-time information preceding a respective log entry instant, and, when a recording of the real-time information is to be appended at an append point to preexisting recorded real-time information, determining one of the log entry instants of the preexisting recorded real-time information near the append point, retrieving the encoder status data from the status logbook for said one of the log entry instants, and restoring the encoder status at the append point in dependence of said retrieved encoder status data. According to a second aspect of the invention the object is achieved with a method of recording information on the record carrier as described in the opening paragraph, the method comprising, while recording the real-time information, generating encoder status data from the encoder, storing, in a status logbook, the encoder status data at log entry instants, the encoder status data being indicative of an encoder status based on real-time information preceding a respective log entry instant, and, when a recording of the real-time information is to be appended at an append point to pre-existing recorded real-time information, determining one of the log entry instants of the pre-existing recorded real-time information near the append point, retrieving the encoder status data from the status logbook for said one of the log entry instants, and restoring the encoder status at the append point in dependence of said retrieved encoder status data. According to a third aspect of the invention the object is achieved with a computer program product for performing the method above. The measures have the effect that when real-time information is appended at an arbitrarily selected append point, the encoder is restarted based on a controlled encoder status, which is reconstructed according to the original encoder status at the append point. It is noted that encoding may comprise compression of the real-time information, but at least comprises creating an encoded stream of real-time information to be recorded, e.g. by multiplexing of various types of compressed real-time data units. Due to the reconstructed status the encoder will take into account all requirements of the encoded stream of real-time information, such as requirements in a predefined recording format like DVD. This has the advantage that seamless appending of two recording sessions is achieved, i.e. a virtually continuous reproduction of real-time information is enabled based on the encoded real-time information at the append point. The invention is also based on the following recognition. First the inventors have noted that encoded real-time information does not contain information about the status of the encoder process, but only information related to decoding. Requirements to which the encoded stream of real-time information has to comply are only implicitly present in the recorded encoded real-time information. However such requirements are to be enforced during encoding the real-time information. Secondly, when considering the encoder at an arbitrary point in time, the encoder operates taking into account the past history of real-time information. Hence, when restarting the encoding process on the fly, i.e. seamlessly joining a preexisting stream of real-time information to a new stream of encoded real-time information, the history would have to be retrieved to make sure that no constraints are violated. Although in theory it would be possible to reconstruct the status of the encoder by retrieving, decoding and re-encoding all real-time information from the beginning of the preexisting recording, such would be highly impractical. By storing the encoder status data during recording real-time information, the status of the encoder is easily reconstructable based encoder status data from the last log entry, and re-encoding real-time information (if any) from that moment up to the append point. In an embodiment of the device the encoder status data comprises buffer status data indicating an amount of encoded data in a coding buffer. By monitoring the amount of encoded data the encoder makes sure that no buffer overflow or underflow will occur during decoding, based on a predefined buffer model for the decoder. The buffer status data allows restoring the encoder status relating to the buffer fullness monitoring. This has the advantage that during decoding buffer overflow or underflow is prevented. In a particular embodiment of the device the buffer status data comprises frame descriptors indicating a status of frames present in the coding buffer, in a particular case the frame descriptors indicating an amount of frames and respective sizes of the frames. This has the advantage that the contents of the buffer are reconstructable for the encoder on a frame by frame basis, which allows obviating buffer overflow or underflow on individual frame time instants. In an embodiment of the device the control unit is arranged for storing said encoder status data on the record carrier, in a particular case the logbook being a separate control file or being embedded in the encoded real-time information. It is noted that the logbook may be kept in a temporary memory, such as a local volatile memory of the device, which allows pause or restarting a recording in consecutive recording sessions. However, storing the encoder status data on the record carrier allows reconstructing the encoder status at any time. This has the advantage that seamless appending to an old recording is achieved. In an embodiment of the device the encoder status data comprises a compression rate indicator that is indicative of a compression rate of the preceding real-time information. The compression rate indicator is indicating global compression parameters. For example, in a variable rate video compression such as MPEG2, the total amount of recording capacity used versus the playing time may be monitored, and used to control the compression rate of future encoding to compensate for additional bits used for encoding complex pictures in the past. This has the advantage that the total available recording capacity is presented to the user as a substantially fixed total playing time, and such fixed playing time is maintained in the presence of seamless appending of recording sessions. Further preferred embodiments of the device and method according to the invention are given in the appended claims, disclosure of which is incorporated herein by reference. 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 Figure 1 shows a disc-shaped record carrier, Figure 2 shows a recording device for seamless appending of real-time information, Figure 3 shows a system for seamlessly appending real-time information, Figure 4 shows re-encoding of video frames at an append point, and Figure 5 shows re-multiplexing of video frames at an append point. In the Figures, elements which correspond to elements already described have the same reference numerals.

Figure 1 shows a disc-shaped record carrier 11 having a track 9 and a central hole 10. The track 9 is arranged in accordance with a spiral pattern of turns constituting substantially parallel tracks on an information layer. The record carrier may be an optical disc of a recordable type, and may have multiple recordable information layers. Examples of a recordable disc are the CD-R and CD-RW, and the DVD+RW and DVD+R, and the DVD- RW and DVD-R. The track 9 is indicated by a pre-track structure provided during manufacture of the blank record carrier, for example a pregroove. The pregroove enables a read/write head to follow the track 9 during scanning. The pregroove may be implemented as an indentation or an elevation, or may consist of a material having a different optical property than the material of the pregroove. The pre-track structure may also be formed by regularly spread sub-tracks or pre-pits which periodically cause servo signals to occur. Recorded information is represented on the layer by optically detectable marks written along the track by a beam of radiation, usually a laser beam. The marks are constituted by variations of a physical parameter and thereby have different optical properties than their surroundings, e.g. in the form of areas with a reflection coefficient different from their surroundings, obtained when recording in materials such as dye, alloy or phase change material, or in the form of areas with a direction of polarization different from their surroundings, obtained when recording in magneto-optical material. During reading the marks are detectable by variations in the reflected beam, e.g. variations in reflection. The record carrier may be intended to carry real-time information, for example video or audio information, or other information, such as computer data. Figure 2 shows a recording device for seamless appending of real-time information. The device is provided with scanning means for scanning a track on a record carrier 11 which means include a drive unit 21 for rotating the record carrier 11, a head 22, a servo unit 25 for positioning the head 22 on the track, and a control unit 20. The head 22 comprises an optical system of a known type for generating a radiation beam 24 guided through optical elements focused to a radiation spot 23 on a track of the information layer of the record carrier. The radiation beam 24 is generated by a radiation source, e.g. a laser diode. The head may contain all optical elements, the laser and detectors as an integrated unit, usually called Optical Pickup Unit (OPU), or may contain as a movable unit only some of the optical elements, while the remaining optical elements and laser and detector are located in a unit on a fixed mechanical location, usually called split-optics, the beam being transferred between both units, e.g. via a mirror. The head further comprises (not shown) a focusing actuator for moving the focus of the radiation beam 24 along the optical axis of said beam and a tracking actuator for fine positioning of the spot 23 in a radial direction on the center of the track. The tracking actuator may comprise coils for radially moving an optical element or may alternatively be arranged for changing the angle of a reflecting element. The focusing and tracking actuators are driven by actuator signals from the servo unit 25. For reading the radiation reflected by the information layer is detected by a detector of a usual type, e.g. a four-quadrant diode, in the head 22 for generating detector signals coupled to a front-end unit 31 for generating various scanning signals, including a main scanning signal 33 and error signals 35 for tracking and focusing. The error signals 35 are coupled to the servo unit 25 for controlling said tracking and focusing actuators. The main scanning signal 33 is processed by read processing unit 30 of a usual type including a demodulator and output unit to retrieve the information. The control unit 20 comprises control circuitry, for example a microprocessor, a program memory and control gates. The control unit 20 may also be implemented as a state machine in logic circuits. The control unit 20 is arranged for controlling the recording and retrieval of the real-time information. The device is provided with recording means for recording information on a record carrier of a writable or re- writable type. The recording means include an input unit 27, a modulator 28, a laser unit 29, front-end unit 31 and the head 22 for generating a write beam of radiation. The modulator 28 is for adding control data and encoding the data according to the data recording format, e.g. by adding error correction codes (ECC), synchronizing patterns, interleaving and channel coding. The data units comprise address information and are written to corresponding addressable locations on the record carrier under the control of control unit 20. The data units from the output of the modulator 28 are passed to the laser unit 29 which controls the laser power for writing the marks in a recording layer of the record carrier. Real-time information such as analog audio and/or video, or digital uncompressed audio/video, is presented on the input unit 27 that comprises an encoder 32 for compression of the real-time information. Suitable compression means are for example described for audio in WO 98/16014-A1 (PHN 16452), and for video in the MPEG2 standard. The encoder 32 multiplexes and formats the compressed audio and/or video to an encoded real-time information stream, which is passed to the modulator 28. The read processing unit 30 may comprise suitable audio and/or video decoding units. In an embodiment the input unit may be arranged to receive digital real-time information that already has been compressed and only may need (re-) multiplexing and formatting to the real-time information stream as required by the recording format of the record carrier. In an embodiment the recording device is a storage system only, e.g. an optical disc drive for use in a computer. The control unit 20 is arranged to communicate with a processing unit in the host computer system via a standardized interface, and real-time information encoding and decoding may be performed in the host. In an embodiment the device is arranged as a stand alone unit, for example a video recording apparatus for consumer use. The control unit 20, or an additional host control unit included in the device, is arranged to be controlled directly by the user via a user interface and to perform the functions of the file management system. The control unit 20 is arranged for controlling the recording functions for performing seamless appending as described below with reference to Figures 3 to 5. Thereto the control unit comprises an encoder status unit 34. Alternatively the functions of the encoder status unit for seamless appending may (partly) be implemented in a different processing unit, e.g. in a host computer via a software program. The software program product may be distributed as a software driver, or as a software program on a data carrier or via a network such as the internet. According to the invention the encoder status unit 34 is implemented for seamlessly appending real-time information as follows. The status unit is coupled to the encoder 32 for, during recording the real-time information, generating encoder status data from the encoder 32. It is to be noted that the encoder may also be implemented as a function of the control unit 20. Encoder status data, i.e. encoder parameters that represent relevant aspects of the encoding and multiplexing algorithm status, are transferred from the encoder to the encoder status unit at regular intervals, for example once a second, or at each encoding unit such as an MPEG video GOP (Group Of Pictures). Subsequently, the encoder status data is stored in a logbook, i.e. a data structure having entries for log entry instants, each entry providing said encoder status data at the instant in time corresponding to a time indicator. The encoder status data is indicative of the encoder status as generated by encoding real-time information preceding the respective log entry instant. The logbook data structure may be stored in a system memory, for example a volatile or non- volatile semiconductor memory, which may be used as a limited loop-around buffer (FIFO; first in, first out type) for a most recent period of recording such as the last 10 minutes. However, preferably, the logbook data structure is stored for all of the recorded real-time information, in order to allow seamless appending on any point in a previous recording. In an embodiment the control unit 20 is arranged for storing said encoder status data on the record carrier. Hence the logbook data structure may be stored as a separate file on the record carrier. Alternatively the logbook may be embedded in a distributed way in the encoded real-time information, for example as a private stream in an MPEG multiplexed data stream as defined in ISO/IEC 13818 or the DVD standard. It is to be noted that the data structure and contents of the logbook may be standardized to allow exchange of encoder status data between different recording devices, e.g. from different manufacturers. When a recording of the real-time information is to be appended at an append point to pre-existing recorded real-time information, the encoder status is restored, or at least substantially reconstructed, as follows. First the logbook is retrieved from the memory or record carrier as appropriate, and the relevant section related to the playing time of the last part of the pre-existing data before the append point is retrieved. Subsequently, one of the log entry instants of the pre-existing recorded real-time information near the append point is selected, e.g. usually the last log entry before the append point. The encoder status data from the status logbook for said selected log entry instant is read and applied to the encoder for restoring the encoder status at the append point in dependence of said retrieved encoder status data. Figure 3 shows a system for seamlessly appending real-time information. Real-time information content to be recorded is coupled to input 39 of input unit 40, for example, as an analog or digital TV broadcast signal, an analog camcorder signal, a digital video signal from a DV camcorder, etc. Input unit 40 may filter and digitize an analog signal, or may convert a received digital signal to a uniform digital signal for further processing. An encoder 41 receives the digital content data from input unit 40, and performs encoding operations. The encoder 41 encodes the content data from an uncompressed format to a compressed format, e.g., to MPEG. After MPEG encoding, for example in DVD, data is recorded in 2KB units, referred to as "packets". The following types of packets exist: Video packets, Audio packets, NavPack packets, etc. Video packets carry video data. Audio packets carry audio data. Navpack packets carry navigation data essential to DVD applications. A multiplexer 42 receives the packets from encoder 41, and multiplexes the video data, the audio data, and further data packets into a Program Stream according to a decoder model, e.g. comprising predefined buffers, such as the MPEG P_STD model or the DVD E-STD model. A formatter 43 receives the multiplexed A/V stream from the multiplexer 42 and enters further data, e.g. parameters in the NavPack according to the DVD+RW standard. Finally the packets are passed to a writer 44 for recording marks in a track on the record carrier 11. The writer 44 adds error control data and modulates the completed data stream according to the requirements of the record carrier format, such as DVD+RW. Note that for DVD, when re- multiplexing starts inside a VOBU (i.e. not at the start) the multiplexer has to make sure that the NavPack at the start of the VOBU is updated, e.g. regarding reference pointers to first I picture and first two P pictures, Real time attributes such as Recording date and time, etc. During reading a reader 45 reads the real-time information (title data) from disc, and may temporarily store data packets to a buffer in memory. The packets are passed to a de-multiplexer 46. The de-multiplexer 46 de-multiplexes the title data into Video data packets, Audio data packets, Navigation Packets, and other private data packets. The demultiplexer 46 passes the compressed video data to a video decoder 47, and audio data to an audio decoder (not shown), etc. for reproduction of the real-time information, e.g. as in a DVD player. According to the invention the system is provided with an ADRW unit 50 (Auxiliary Data Reader Writer) that is coupled to the encoder 41 and multiplexer 42 for transferring encoder status data including encoding algorithm status data 54 and multiplexing algorithm status data 55. The ADRW unit 50 is arranged creating the logbook data structure from the encoder status data at given instants in time and for recording and reading the encoder status data 56 as auxiliary data on the disc. The auxiliary data are stored in a specific location (e.g. a file having a predefined name, or in the multiplexed stream), and may include a reference to the recorded real-time information, usually called a title. In an embodiment the encoder status data (including optionally other algorithm dependent parameters) are stored by the ADRW unit 50 for every GOP, or the frequency is reduced to once per several GOPs, which is a tradeoff between storage capacity and processing effort. Note that the nearest preceding log entry may then be a few GOPs in the past, which requires at least re-multiplexing the frames starting at that moment. In an embodiment a compression rate parameter is included in the encoder status data, i.e. a difference between an actual number of bits used till and including the previous GOP on the one hand, and a desired number of bits based on an average target bitrate corresponding to a desired quality level on the other hand. Hence the compression rate indicator is indicating global compression parameters. For example, in a variable rate video compression such as MPEG2, the total amount of recording capacity used versus the playing time may be monitored, and used to control the compression rate of future encoding to compensate for additional bits used for encoding complex pictures in the past. Restoring the compression rate indicator forces the encoder to comply with the global encoder requirements. Reconstructing the encoder status at an append point (as selected by a user for appending a new recording) includes restoring the state of the encoder 41 and multiplexer 42 at the append point by providing the parameters of the encoder algorithm and the multiplexer algorithm respectively as follows. First the encoder status data 56 is retrieved by the ADRW unit 50, and therein the log entry is determined nearest to the append point as explained above. The status of the encoder is reconstructed starting at the log entry instant by setting the encoder 41 and the multiplexer 42 by the encoding algorithm status data 54 and multiplexing algorithm status data 55 respectively. Subsequently data packets are read from the record carrier starting at the log entry instant. Note that the data stream around the append point needs to be re- multiplexed using old and new data packets for creating the seamless link. When re- multiplexing, real-time information may require to be decoded and recompressed, or may be used as compressed data packets 52,53 if encoded without relation to future real-time information. Note that for example in MPEG video encoding video frames may be encoded in so-called B (bi-directional) frames using future frame data also. In addition, the audio data frames may have a different timing than video frames. The de-multiplexer 46 passes compressed audio data 53 to the multiplexer 42, when they are needed for re-multiplexing. Decoder 47 decodes the compressed video data and passes uncompressed video data 51 to the encoder 41 for re-encoding. In an embodiment the de-multiplexer 46 passes the compressed video data 52 to the encoder 41. This increases the speed of re-encoding: when the encoder detects that re- encoding of video data would lead to exactly the same compressed video data, the already compressed video data can be re-used. The de-multiplexer 46 passes the compressed video data to the decoder 47, which transfers uncompressed digital video 51 to the encoder 41 to be used alternatively when the video data must be re-encoded. Note that the encoding algorithm, e.g. for MPEG or DVD, is based on encoding a video frame with reference to previous P-frames (predictive) and an I- frame (Independent) in a GOP (Group Of Pictures) containing the video frame. Therefore, the state of the encoding algorithm at the beginning of the GOP is needed and sufficient. In practical situations the required encoding actions at the append point may depend on the position of the first new video frame in a GOP: a. The new video frame is located at the beginning of a (closed) GOP. The video frame is encoded as an I- frame, and is hence not related to previous video frames. The encoding algorithm may only take into account the restored parameter bitrate-deviation. Note that the first video frame does not always have to be an I-picture, for example in an Open- GOP encoding may start with B-pictures. b. The new video frame is located at a P-frame in a stream that only contains I- frames and P-frames (no B-frames). The encoder is fed with the I-frame and the previous P- frames in the GOP, and also with the decoded video frames. From these data and the restored parameters, the encoder is able to restore the situation at the position of the new video frame. Note that the compressed video data in the I-frame and the previous P-frames can be reused. c. The new video frame is located at a B-frame or a P-frame in a stream containing B-frames. The encoder is fed with all the previous frames in the GOP, and also with the decoded video frames. Note that the B-frames immediately preceding the new video frame must be re-encoded because their succeeding P-frame will be changed as shown in Figure 4. Figure 4 shows re-encoding of video frames at an append point. A number of video frames starting with an I-frame 65 are schematically shown. In a first section 61 a number of video frames are maintained as originally encoded, i.e. re-used and not re- encoded. Note that the first new video frame 64 is to be linked at append point 63 to the preexisting video data. A P-frame 60 is the last frame before the append point 63 that is not influenced, subsequently a series of B-frames 62 up to the new video frame 64 needs to be re- encoded. For audio encoding a similar process and parameters for restoring the status of an audio encoder may be applied. However, without substantial audible effects the audio frames may be maintained unamended, and the linking point of audio frames may be shifted from the append point to accommodate full audio frames. Note that the audio frames have a different frame rate compared to the video frames. Because of the small duration of each audio frame, the audio data may be used in the original encoded frames, i.e. does not need to be re-encoded. The suitable set of audio frames from the original recording can be determined by switching from the previous audio frames to the new audio frames as close as possible to the video append point. It is to be noted that also other data might need to be reworked, for example

NavPacks or Subpicture packets. In case of DVD Digital cam recordings subpicture packets may be used to denote the Recording Date and Time. When the user appends real-time information, the appended Recording date and Time will be included in new Subpicture packets. In particular for the multiplexer it is required to restore the status. Note that restoring the multiplexer algorithm status is different from the video or audio encoding algorithm status, but in this description multiplexing is considered to be part of the encoder status. The main task of the multiplexer is to enter the various types of packets in the recorded data stream taking into account the buffer requirements for each type of real-time information as defined in the applicable buffer model. For MPEG or DVD these are mainly the video buffer and audio buffer. For such applications the multiplexer algorithm is mainly based on constraints of said buffers, one video buffer containing maximally 1 second of video data, and one audio buffer containing maximally 1 second of audio data. The state of the multiplexer algorithm must be restored to a situation of the original recording. The video data and audio data itself is easily retrievable from the existing recorded data. The multiplexer state is determined by buffer status parameters indicating the filling degree and type of data in the buffer at the specified time, i.e. the amount of encoded data in a coding buffer. For example the number of video frames and their sizes in video buffer, and similarly the number of audio frames and their sizes in audio buffer, are to be included in the encoder status data. Figure 5 shows re-multiplexing of video frames at an append point. A series of multiplexed real-time information frames in a multiplexed stream in a video buffer is shown starting at a GOP start 70. In a first section old multiplexed frames 71, which may be part of a previous GOP are located. Note that care is taken to re-multiplex the old multiplexed frames section 71 at the GOP start equal to the original recording so that they need not be rewritten on disc. Note that for DVD rewriting might be necessary because the NAV pack may need to be updated. A fundamental requirement to be maintained during multiplexing is that the total video buffer may contain up to a predefined maximum of video data, e.g. in DVD the video buffer may be filled with maximally 1 second of video data. Hence after the old multiplexed frames section 71, the subsequent video data that has to be linked to the new video frame 74 must be re-multiplexed. A first part 72 may have video frames that are not re- encoded, whereas a second part 73 may have re-encoded frames. Note that all sections 72,73,74 of the encoded video stream in the buffer after the old multiplexed section 71 have to be re-multiplexed according to the normal requirements build in the multiplexer for the respective recording format. In an embodiment, the parameters of the multiplexer algorithm are not be stored for each video frame, but with a certain frequency, e.g. once per GOP or in DVD once per VOBU (Video Object Unit). For reconstructing the multiplexer algorithm status, several extra video frames have to be multiplexed starting at the nearest stored log entry. The interval between log entries is a tradeoff between storage of multiplexer parameters and multiplexer processing effort. In an embodiment timing parameters are included for easily restoring the multiplexer algorithm status. It is noted that in some recording formats, such as DVD, such additional parameters can be retrieved from the recorded data, e.g. from NavPacks, and that some additional parameters may also be restored from other elements of an existing recorded Program Stream. However, such retrieval requires additional time, and some recorded data may not include such navigation packets. Hence, advantageously, the timing parameters may be included in the encoder status data. The most relevant state parameters are related to timing and synchronization of packets and frames: system clock reference SCR, decoder time stamp DTS, presentation time stamp PTS. Note that DTS, PTS of video packs and DTS, PTS of audio packs may be stored separately in the logbook. However DTS and PTS of the audio are always the same, and need not be stored both, but PTS, DTS of video may differ due to reordering of video frames (B-pictures). Finally the PTS timestamp is not really required to be stored because it may be easily reconstructed. The parameters for the audio buffer may be stored with the same frequency as the parameters for the video buffer. Due to the non-alignment (in time stamps) between video frames and audio frames, one extra audio frame must be taken into account for restarting the algorithm. In a practical embodiment the invention deals with a recording feature in a DVD-recorder, HDD-recorder (magnetic Hard Disk) or similar equipment. The problems that may occur in prior art recorders will be described by 3 typical example situations based on a DVD-recorder:

1. The user has recorded a Title on disc, and wants to append another piece of recording to this existing Title. At the moment, this introduces a discontinuity in the recording characteristics at the position where the new piece of recording starts. Consequently, the new combined recording can only be played back non-seamlessly at the mentioned position, causing an annoying, visible hiccup.

2. The user plays back an existing recorded Title, and pauses at a certain position. From this position onward he wants to overwrite the remaining part of the Title with a new piece of recording. Overwriting can not be done video frame accurate, but it has to start at the beginning of a VOBU (or GOP). Furthermore, a discontinuity as described above with 1. may occur.

3. The user is making a recording (e.g. from camcorder connected to the input), and watches the video being recorded. If he wants to remove a piece at the latest part of this recording, e.g. because of unwanted recorded video, the recording must be stopped. Automatically the disc will be updated with a new file management data like a new table of contents. The user must then start playback and navigate to the position where the part to be overwritten starts as described in 2. Hence stopping and navigating will take a lot of time. In general the above problems are caused by the MPEG encoding and multiplexing algorithms that depend on the characteristics of the already processed data. The invention teaches to collect, store, and use encoder status data at periodic moments, e.g. at the start of each VOBU or GOP. The encoder status data is used to restore the state of the algorithms at the position where the new piece of recording will start. A limited amount of re-processing is needed to fully restore the state of the algorithms at the time of the original recording. The advantages of the invention are that the playback of the resulting recording will be seamless without visible hiccups. In a DVD recorder, e.g. for DVD+RW recording, there is no need to introduce a new cell at the place where the recording restarts. Since cells are a limited resource, the titles to be recorded might run out of cells. For example, a disc with a 4 hour recording mode may be "full" after only 2 hours of recording if all the available cells were consumed due to extensive restarting. When interrupting recording real-time information (as described above with problem situation 3), there is no need to generate a TOC, because next real-time information is joined seamlessly. It is noted that quick switching between (or simultaneous) playback and recording facilitates such interrupted recording. Finally, due to the reconstructing of the encoder algorithm status, starting a new piece of recording can be done frame accurate, in stead of being forced to move to a VOBU/GOP boundary. Although the invention has been mainly explained by embodiments using optical discs having spiral shaped tracks, the invention is also suitable for other record carriers such as rectangular optical cards, magneto-optical discs, magnetic discs, semiconductor memory like FLASH cards or any other type of information storage system that may require appending real-time information in different recording sessions. 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' or 'units' may be represented by the same item of hardware or software. 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.

Claims

CLAIMS:

1. Device for recording real-time information on a record carrier in a track, the device comprising,

- recording means for recording real-time information, which recording means include a realtime information encoder (32) for encoding the real-time information to encoded real-time information, and a head (22) for recording marks representing the encoded real-time information in the track, and

- a control unit (20) for controlling recording of the real-time information, the control unit comprising an encoder status unit (34) for,

- while recording the real-time information, generating encoder status data from the encoder, - storing, in a status logbook, the encoder status data at log entry instants, the encoder status data being indicative of an encoder status based on real-time information preceding a respective log entry instant,

- and, when a recording of the real-time information is to be appended at an append point to pre-existing recorded real-time information, - determining one of the log entry instants of the pre-existing recorded real-time information near the append point,

- retrieving the encoder status data from the status logbook for said one of the log entry instants, and

- restoring the encoder status at the append point in dependence of said retrieved encoder status data.

2. Device as claimed in claim 1, wherein the encoder status data comprises buffer status data indicating an amount of encoded data in a coding buffer.

3. Device as claimed in claim 2, wherein the buffer status data comprises frame descriptors indicating a status of frames present in the coding buffer, in a particular case the frame descriptors indicating an amount of frames and respective sizes of the frames.

4. Device as claimed in claim 1, wherein the control unit (20) is arranged for storing said encoder status data on the record carrier (11), in a particular case the logbook being a separate control file or being embedded in the encoded real-time information.

5. Device as claimed in claim 1, wherein the encoder status data comprises a compression rate indicator that is indicative of a compression rate of the preceding real-time information.

6. Device as claimed in claim 1, wherein the encoder status data comprises timing information of encoded real-time information, in a particular case the timing information being indicative of a system clock reference (SCR), a decoder time stamp (DTS) or a presentation time stamp (PTS).

7. Device as claimed in claim 1, wherein the real-time information comprises video and audio, and the encoder status data comprises video status information indicative of a video encoding process and audio status information indicative of an audio encoding process.

8. Method of recording real-time information on a record carrier in a track, the method comprising

- encoding the real-time information via a real-time information encoder process to encoded real-time information,

- recording the real-time information by recording marks representing the encoded real-time information in the track, and - while recording the real-time information, generating encoder status data from the encoder,

- storing, in a status logbook, the encoder status data at log entry instants, the encoder status data being indicative of an encoder status based on real-time information preceding a respective log entry instant,

- and, when a recording of the real-time information is to be appended at an append point to pre-existing recorded real-time information,

- determining one of the log entry instants of the pre-existing recorded real-time information near the append point,

- retrieving the encoder status data from the status logbook for said one of the log entry instants, and - restoring the encoder status at the append point in dependence of said retrieved encoder status data.

9. Method as claimed in claim 8, wherein the encoder status data comprises buffer status data indicating an amount of encoded data in a coding buffer.

10. Computer program product for recording real-time information, which program is operative to cause a processor to perform the method as claimed in claim 8 or 9.