WO2001063936A1 - Compressed video bitstreams processing - Google Patents

Compressed video bitstreams processing Download PDF

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Publication number
WO2001063936A1
WO2001063936A1 PCT/GB2001/000724 GB0100724W WO0163936A1 WO 2001063936 A1 WO2001063936 A1 WO 2001063936A1 GB 0100724 W GB0100724 W GB 0100724W WO 0163936 A1 WO0163936 A1 WO 0163936A1
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WO
WIPO (PCT)
Prior art keywords
bitstream
bit rate
mode
storage
transcoded
Prior art date
Application number
PCT/GB2001/000724
Other languages
French (fr)
Inventor
Bruce Fairbairn Devlin
Michael James Knee
Original Assignee
Snell & Wilcox Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Snell & Wilcox Limited filed Critical Snell & Wilcox Limited
Priority to AU37538/01A priority Critical patent/AU3753801A/en
Publication of WO2001063936A1 publication Critical patent/WO2001063936A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
    • H04N19/103—Selection of coding mode or of prediction mode
    • H04N19/107—Selection of coding mode or of prediction mode between spatial and temporal predictive coding, e.g. picture refresh
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/146—Data rate or code amount at the encoder output
    • H04N19/147—Data rate or code amount at the encoder output according to rate distortion criteria
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/146—Data rate or code amount at the encoder output
    • H04N19/152—Data rate or code amount at the encoder output by measuring the fullness of the transmission buffer
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/162—User input
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/17—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object
    • H04N19/172—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object the region being a picture, frame or field
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/17—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object
    • H04N19/176—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object the region being a block, e.g. a macroblock
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/40—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using video transcoding, i.e. partial or full decoding of a coded input stream followed by re-encoding of the decoded output stream
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/60—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding
    • H04N19/61—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding in combination with predictive coding
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
    • H04N21/40—Client devices specifically adapted for the reception of or interaction with content, e.g. set-top-box [STB]; Operations thereof
    • H04N21/43—Processing of content or additional data, e.g. demultiplexing additional data from a digital video stream; Elementary client operations, e.g. monitoring of home network or synchronising decoder's clock; Client middleware
    • H04N21/44—Processing of video elementary streams, e.g. splicing a video clip retrieved from local storage with an incoming video stream or rendering scenes according to encoded video stream scene graphs
    • H04N21/4402—Processing of video elementary streams, e.g. splicing a video clip retrieved from local storage with an incoming video stream or rendering scenes according to encoded video stream scene graphs involving reformatting operations of video signals for household redistribution, storage or real-time display
    • H04N21/440254—Processing of video elementary streams, e.g. splicing a video clip retrieved from local storage with an incoming video stream or rendering scenes according to encoded video stream scene graphs involving reformatting operations of video signals for household redistribution, storage or real-time display by altering signal-to-noise parameters, e.g. requantization
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00—Details of colour television systems
    • H04N9/79—Processing of colour television signals in connection with recording
    • H04N9/7921—Processing of colour television signals in connection with recording for more than one processing mode
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/76—Television signal recording
    • H04N5/78—Television signal recording using magnetic recording
    • H04N5/782—Television signal recording using magnetic recording on tape
    • H04N5/783—Adaptations for reproducing at a rate different from the recording rate

Definitions

  • This invention relates to compressed bitstreams and in the most important example to the processing of compressed bitstreams prior to storage.
  • the television viewer has, through the video cassette recorder (VCR) become accustomed to time-shifting broadcast television programmes that is to say recording a television programme off air for viewing at a more convenient time.
  • the VCR has also created an alternative distribution channel for video content in the form of the sold or hired video tape.
  • the VCR has been developed to offer a range of additional features. Typically, a video tape can be played at an increased speed with a recognisable picture still being displayed, to enable particular material to be located. Advanced VCRs will often provide the capability of freeze-framing or of stepping frame-by-frame through selected scenes. Another feature that is frequently available is that of "long play", where the user elects to compromise the recording quality in return for a longer recording interval on a given length of tape.
  • the present invention consists in one aspect in a method of processing a compressed digital video bitstream, comprising the steps of: receiving a bitstream; establishing a plurality of user-defined storage modes, comprising at least two of a long play mode, having a lower bit rate than the received bitstream, a compress-to-fit mode, having a bit rate related to available storage, and a rapid access mode, having an increased number of non-predicted frames; on user selection of a given one of said storage modes, transcoding the received bitstream to a transcoded bitstream having a different bit rate, reflecting the selected storage mode; and storing the transcoded bitstream.
  • the present invention consists in another aspect in a method of processing a compressed digital video bitstream to facilitate rapid access, comprising the steps of: receiving a bitstream; transcoding the received bitstream to a transcoded bitstream having a higher bit rate and less prediction; and storing the transcoded bitstream.
  • the present invention consists in yet another aspect in a method of processing a compressed digital video bitstream, comprising the steps of: receiving a bitstream having a constant bit rate; establishing a plurality of user- defined storage modes; on user selection of a given storage mode, determining a fixed quality level reflecting that storage mode; transcoding the received bitstream, employing the fixed quality level, to a transcoded bitstream having a variable bit rate; and storing the transcoded bitstream.
  • Figure 1 is a block diagram illustrating an embodiment of the present invention
  • FIG. 2 is a block diagram of one form of the transcoder shown in Figure 1;
  • FIG. 3 is a block diagram of another form of the transcoder shown in Figure 1.
  • a compressed bitstream is received at terminal 100 and, instead of passing directly to the store 102 is received by a transcoder 104.
  • the user is provided the option of selecting storage modes through mode selection block 106.
  • One such storage mode is "long play" and upon selection of this mode, the transcoder receives from the mode selection block a control signal.
  • the transcoder operates to transcode the received bitstream to a transcoded bitstream having a selected lower bit rate. In this way, the programme material represented by the received bitstream can be accommodated by using a lesser portion of the store 102.
  • Another storage mode is "compress-to-fit" mode, for use where only a certain amount of storage remains.
  • a user interface is provided, enabling the user to instruct recording of a given television programme.
  • the arrangement according to this example of the invention can operate to determine first the amount of available storage capability in the store 102. If this is less than that required to record the entire programme at the broadcast bit rate, the mode selection block 106 can recalculate a bit rate at which the entire programme can be recorded. This recalculated bit rate is provided to the transcoder which performs the transcoding operation as before.
  • the bit rate calculation may be performed by estimating the total amount of information to be recorded. This estimate may be made by calculating an approximate average bit rate of the material being broadcast, typically over a short period of the broadcast, and using programme length information transmitted with the broadcast.
  • the user can select to store the received bitstream at the lowest bit rate consistent with a defined quality of video image.
  • the bitstream can be analysed and a quality measure determined. This measure can be produced without reference to the source picture, in contrast to conventional methods of quality measurement, such as PSNR.
  • the transcoder will in this storage mode reduce the bit rate of the transcoded bitstream until the quality measure reaches a defined threshold.
  • this measure is derived using the quantisation levels either detected in the bitstream, or passed from a previous encoding. The quality level may therefore also be defined by the use of a fixed quantisation level.
  • the input bitstream has a constant bit rate
  • this will produce a variable bit rate output stream.
  • variance of the output bit rate is not forbidden.
  • This permits several advantageous features. For example, in a long play mode, a user may select a certain quality level with which they are satisfied, in order to increase recording capacity. This quality level will equate to a particular quantisation level, rather than a simple "halving" of the quality, as is common with VCR long play modes.
  • several quality levels may be selected for different types of material.
  • a finer quantisation may be selected for recording a film than for recording a documentary, which may be finer than that selected for recording a game show.
  • a maximum quality may also be specified, by selecting a quantisation no finer than a certain level. This may save storage capacity, as the recording will not be of a higher quality (and hence, bit rate) than that perceivable by the viewer.
  • bit rate either calculated or selected may be varied during recording, should the viewer recognise that a lower quality recording may be sufficient for the particular programme.
  • the transcoder 104 operates in accordance with the disclosure of EP 0 765 576 and EP 0 913 058.
  • an incoming bitstream can be decoded to an uncompressed or less compressed bitstream, with the coding decisions being extracted.
  • the uncompressed or less compressed signal can then be re-encoded at the required bit rate utilising the same coding decisions, so as to minimise or remove losses in re-encoding.
  • These coding decisions may accompany the bitstream in an information bus signal, or the information may be embedded in the bitstream itself.
  • the transcoding operation can also usefully take advantage of the disclosure of EP 0 963 657.
  • the mode selector 106 permits selection of a mode in which the transcoder operates to increase bit rate.
  • the received bitstream will typically take the form of a long GOP MPEG bitstream, that is to say a bitstream in which there are relatively few intra- coded or I frames and a relatively large number of predicted frames which may take the form of forward predicted frames (P frames) or both forward and backward predicted frames (B frames). If such a long GOP bitstream is written directly to store, rapid access operations, such as a high-speed search for picture material, freeze frame and so on will be complex and time consuming. Unless the available functionality is to be confined to the relatively infrequent I frames, some form of decoding operation will be required within each trick mode function.
  • this difficulty is overcome, where storage capacity permits, by modifying the received bitstream before it is written into the store.
  • the received bitstream is modified by reducing the amount of prediction coding.
  • identified P frames in the received bitstream will be decoded and re-encoded as I frames.
  • the incoming bitstream is received by a controller 200. This is under the control of the mode selection block 106. In dependence upon user preference, aspects of the bitstream and the amount of available storage, it is elected to convert all or a given proportion of P frames into I frames.
  • the controller passes some or all P frames to a decoder 202 where, with information of course from the associated I frame, a decompressed picture signal is derived.
  • I frame encoder 204 This is re-encoded in I frame encoder 204 to provide a sequence of I frames which are multiplexed in multiplexer 206 with the remaining, undisturbed frames, to form a transcoded bitstream output.
  • coding decisions are forwarded from the decoder 202 to the I frame encoder 204 (which may select or use only those required), and to the multiplexer 206.
  • the coding decisions may also be stored, in order to enable the lossless reversal of the re-encoding if required at a later stage.
  • the controller passes to the decoder those P frames which are in defined positions in the GOP structure. For example, in a typical structure, between each I frame, there are interposed three P frames, with intervening B frames. The controller might therefore pass every P frame falling in the middle of such a structure.
  • the controller passes all P frames to the decoder, in order to produce an "I frame only" bitstream.
  • the storage space available may not accommodate such a large amount of information, and thus the transcoder may be instructed to re-encode the l-frames at a lower resolution, in order to save bits, but nevertheless produce a fully intra-coded bitstream.
  • the P frames which are passed to the decoder are those falling in areas of the original material where a substantial amount of motion is present.
  • the bitstream is passed as usual to the controller 200, and is also passed to a monitor of the motion vector bit rate 300. Where there is a high motion vector bit rate, the material will contain significant motion.
  • the output of block 300 is passed to the controller 200, and the related P frames having the high motion vector bit rate are passed to the decoder 202.
  • the "cost" in bit rate terms of converting such P frames to I frames will be less than that of converting P frames where there is little or no motion.
  • the number of intra-coded macroblocks within a picture is increased by a defined proportion.
  • a given number of P type macroblocks will be converted to I type macroblocks.
  • This feature has particular advantage in those trick modes which utilise a portion only of any one picture.
  • a recognisable picture is provided in a search mode by combining respective different picture regions from different incoming frames.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Television Signal Processing For Recording (AREA)
  • Compression Or Coding Systems Of Tv Signals (AREA)

Abstract

In compression of bitstreams prior to storage, typically on a PDR, a user selects one of a number of storage modes, including long play (having lower bit rate than the received bitstream), compress-to-fit (having a bit rate related to available storage space), and rapid access (having an increased number of non-predicted frames) modes. The received bitstream is transcoded to a bitstream having a bit rate reflecting the selected storage mode, and the transcoded bitstream is then stored.

Description

COMPRESSED VIDEO BITSTREAMS PROCESSING
This invention relates to compressed bitstreams and in the most important example to the processing of compressed bitstreams prior to storage. The television viewer has, through the video cassette recorder (VCR) become accustomed to time-shifting broadcast television programmes that is to say recording a television programme off air for viewing at a more convenient time. The VCR has also created an alternative distribution channel for video content in the form of the sold or hired video tape. The VCR has been developed to offer a range of additional features. Typically, a video tape can be played at an increased speed with a recognisable picture still being displayed, to enable particular material to be located. Advanced VCRs will often provide the capability of freeze-framing or of stepping frame-by-frame through selected scenes. Another feature that is frequently available is that of "long play", where the user elects to compromise the recording quality in return for a longer recording interval on a given length of tape.
In the move to digital television, proposals are already being made to imitate the functionality of a VCR through other storage media such as hard disks or re-writable CDS or DVDs. It has already been recognised that new storage media, and new storage facilities, such as personal digital recorders (PDRs) offer capabilities considerably beyond those of the VCR, such as the ability to record and play back simultaneously to give a "pause" effect in essentially real time viewing.
The compressed nature of broadcast digital video does, however, present challenges in providing functionality analogous to that of an advanced VCR. Generally, a bitstream delivered to the viewer (whatever the method of distribution) will be a relatively low bit rate and in the MPEG case will contain relatively few intra-coded frames.
It is an object of one aspect of the present invention to address such difficulties.
Accordingly, the present invention consists in one aspect in a method of processing a compressed digital video bitstream, comprising the steps of: receiving a bitstream; establishing a plurality of user-defined storage modes, comprising at least two of a long play mode, having a lower bit rate than the received bitstream, a compress-to-fit mode, having a bit rate related to available storage, and a rapid access mode, having an increased number of non-predicted frames; on user selection of a given one of said storage modes, transcoding the received bitstream to a transcoded bitstream having a different bit rate, reflecting the selected storage mode; and storing the transcoded bitstream.
It is an object of another aspect of the invention to address in particular the difficulties arising in providing the rapid access functionality analogous to that of an advanced VCR.
Accordingly, the present invention consists in another aspect in a method of processing a compressed digital video bitstream to facilitate rapid access, comprising the steps of: receiving a bitstream; transcoding the received bitstream to a transcoded bitstream having a higher bit rate and less prediction; and storing the transcoded bitstream.
The present invention consists in yet another aspect in a method of processing a compressed digital video bitstream, comprising the steps of: receiving a bitstream having a constant bit rate; establishing a plurality of user- defined storage modes; on user selection of a given storage mode, determining a fixed quality level reflecting that storage mode; transcoding the received bitstream, employing the fixed quality level, to a transcoded bitstream having a variable bit rate; and storing the transcoded bitstream.
The invention will now be described by way of example with reference to the accompanying drawings in which:
Figure 1 is a block diagram illustrating an embodiment of the present invention;
Figure 2 is a block diagram of one form of the transcoder shown in Figure 1; and
Figure 3 is a block diagram of another form of the transcoder shown in Figure 1.
Referring initially to Figure 1 , a compressed bitstream is received at terminal 100 and, instead of passing directly to the store 102 is received by a transcoder 104. In accordance with the invention, the user is provided the option of selecting storage modes through mode selection block 106. One such storage mode is "long play" and upon selection of this mode, the transcoder receives from the mode selection block a control signal. In accordance with this control signal, the transcoder operates to transcode the received bitstream to a transcoded bitstream having a selected lower bit rate. In this way, the programme material represented by the received bitstream can be accommodated by using a lesser portion of the store 102.
Another storage mode is "compress-to-fit" mode, for use where only a certain amount of storage remains. For example, in certain known arrangements, a user interface is provided, enabling the user to instruct recording of a given television programme. In such a circumstance, the arrangement according to this example of the invention can operate to determine first the amount of available storage capability in the store 102. If this is less than that required to record the entire programme at the broadcast bit rate, the mode selection block 106 can recalculate a bit rate at which the entire programme can be recorded. This recalculated bit rate is provided to the transcoder which performs the transcoding operation as before.
If, as may be in the above example, the transcoding is being performed "live" as a programme is broadcast, the bit rate calculation may be performed by estimating the total amount of information to be recorded. This estimate may be made by calculating an approximate average bit rate of the material being broadcast, typically over a short period of the broadcast, and using programme length information transmitted with the broadcast.
In another storage mode, the user can select to store the received bitstream at the lowest bit rate consistent with a defined quality of video image. Using, for example, the quality measure disclosed in PCT/GB99/03356, the bitstream can be analysed and a quality measure determined. This measure can be produced without reference to the source picture, in contrast to conventional methods of quality measurement, such as PSNR. Using this measure in feedback, the transcoder will in this storage mode reduce the bit rate of the transcoded bitstream until the quality measure reaches a defined threshold. In one embodiment, this measure is derived using the quantisation levels either detected in the bitstream, or passed from a previous encoding. The quality level may therefore also be defined by the use of a fixed quantisation level. Where the input bitstream has a constant bit rate, this will produce a variable bit rate output stream. However, as the bitstream is to be written to a store, rather than passed along a transmission chain requiring a constant rate bitstream, variance of the output bit rate is not forbidden. This permits several advantageous features. For example, in a long play mode, a user may select a certain quality level with which they are satisfied, in order to increase recording capacity. This quality level will equate to a particular quantisation level, rather than a simple "halving" of the quality, as is common with VCR long play modes. Moreover, several quality levels may be selected for different types of material. For example, a finer quantisation may be selected for recording a film than for recording a documentary, which may be finer than that selected for recording a game show. A maximum quality may also be specified, by selecting a quantisation no finer than a certain level. This may save storage capacity, as the recording will not be of a higher quality (and hence, bit rate) than that perceivable by the viewer.
In another embodiment, where compress-to-fit or long play modes are employed, the bit rate either calculated or selected may be varied during recording, should the viewer recognise that a lower quality recording may be sufficient for the particular programme.
In one form of this invention, the transcoder 104 operates in accordance with the disclosure of EP 0 765 576 and EP 0 913 058. As disclosed in these references, an incoming bitstream can be decoded to an uncompressed or less compressed bitstream, with the coding decisions being extracted. The uncompressed or less compressed signal can then be re-encoded at the required bit rate utilising the same coding decisions, so as to minimise or remove losses in re-encoding. These coding decisions may accompany the bitstream in an information bus signal, or the information may be embedded in the bitstream itself. The transcoding operation can also usefully take advantage of the disclosure of EP 0 963 657. Here, the play-off in the quantisation step between decreasing the error and decreasing the bit rate of the coding can be controlled advantageously by dynamic variation of λ, a parameter defining the bounds of the quantisation levels selected. For example, if λ=0.75, a signal between two quantisation levels will be "rounded down" to the lower level if it is less than 75% of the interval between the two levels. This uses fewer bits, as the higher quantisation level is only used for signals occupying the highest 25% of the interval.
In another form of the present invention, the mode selector 106 permits selection of a mode in which the transcoder operates to increase bit rate. As has been explained, the received bitstream will typically take the form of a long GOP MPEG bitstream, that is to say a bitstream in which there are relatively few intra- coded or I frames and a relatively large number of predicted frames which may take the form of forward predicted frames (P frames) or both forward and backward predicted frames (B frames). If such a long GOP bitstream is written directly to store, rapid access operations, such as a high-speed search for picture material, freeze frame and so on will be complex and time consuming. Unless the available functionality is to be confined to the relatively infrequent I frames, some form of decoding operation will be required within each trick mode function. In accordance with the present invention, this difficulty is overcome, where storage capacity permits, by modifying the received bitstream before it is written into the store. Thus the received bitstream is modified by reducing the amount of prediction coding. In one example, identified P frames in the received bitstream will be decoded and re-encoded as I frames. Thus as shown in Figure 2, the incoming bitstream is received by a controller 200. This is under the control of the mode selection block 106. In dependence upon user preference, aspects of the bitstream and the amount of available storage, it is elected to convert all or a given proportion of P frames into I frames. Thus the controller passes some or all P frames to a decoder 202 where, with information of course from the associated I frame, a decompressed picture signal is derived. This is re-encoded in I frame encoder 204 to provide a sequence of I frames which are multiplexed in multiplexer 206 with the remaining, undisturbed frames, to form a transcoded bitstream output. Advantageously, coding decisions are forwarded from the decoder 202 to the I frame encoder 204 (which may select or use only those required), and to the multiplexer 206. The coding decisions may also be stored, in order to enable the lossless reversal of the re-encoding if required at a later stage.
In one embodiment, the controller passes to the decoder those P frames which are in defined positions in the GOP structure. For example, in a typical structure, between each I frame, there are interposed three P frames, with intervening B frames. The controller might therefore pass every P frame falling in the middle of such a structure.
In another embodiment, the controller passes all P frames to the decoder, in order to produce an "I frame only" bitstream. In certain cases, the storage space available may not accommodate such a large amount of information, and thus the transcoder may be instructed to re-encode the l-frames at a lower resolution, in order to save bits, but nevertheless produce a fully intra-coded bitstream.
Shuttle, still, or slow motion modes are likely to be particularly useful in scenes involving large amounts of movement. Thus, in a further embodiment, illustrated in Figure 3, the P frames which are passed to the decoder are those falling in areas of the original material where a substantial amount of motion is present. The bitstream is passed as usual to the controller 200, and is also passed to a monitor of the motion vector bit rate 300. Where there is a high motion vector bit rate, the material will contain significant motion. Thus the output of block 300 is passed to the controller 200, and the related P frames having the high motion vector bit rate are passed to the decoder 202. The "cost" in bit rate terms of converting such P frames to I frames will be less than that of converting P frames where there is little or no motion. In another form of the invention, the number of intra-coded macroblocks within a picture is increased by a defined proportion. Typically, a given number of P type macroblocks will be converted to I type macroblocks. This feature has particular advantage in those trick modes which utilise a portion only of any one picture. Thus, by analogy with a VCR shuttle mode, it can be arranged that a recognisable picture is provided in a search mode by combining respective different picture regions from different incoming frames. By generally increasing the number of intra-coded macroblocks and by specifically ensuring that those picture regions which are to be used to produce a combined picture are coded as I type macroblocks, the present invention can considerably facilitate such an arrangement.
It should be understood that this invention has been described by way of example only and that a wide variety of further modifications are possible without departing from the scope of the invention.

Claims

1. A method of processing a compressed digital video bitstream, comprising the steps of: receiving a bitstream; establishing a plurality of user-defined storage modes, comprising at least two of a long play mode, having a lower bit rate than the received bitstream, a compress-to-fit mode, having a bit rate related to available storage, and a rapid access mode, having an increased number of non-predicted frames; on user selection of a given one of said storage modes, transcoding the received bitstream to a transcoded bitstream having a different bit rate, reflecting the selected storage mode; and storing the transcoded bitstream.
2. A method according to Claim 1 , wherein the transcoding step comprises the steps of decoding to an uncompressed or less compressed signal, extracting coding decisions and re-encoding the uncompressed or less compressed signal utilising some or all of said extracted coding decisions.
3. A method according to Claim 2, wherein the step of re-encoding utilises the same arrangement of predicted and non-predicted frames, the same coding modes and the same motion vectors as the received bitstream, and selects different quantisation levels in accordance with the user selected mode.
4. A method according to any one of the preceding claims, comprising the steps, on user selection of said compress-to-fit storage mode, of determining an amount of available storage; estimating an aggregate bit count of the received bitstream; and transcoding the received bitstream to a bit rate calculated with reference to said available storage and said bit count.
5. A method according to Claim 4, wherein the step of estimating the aggregate bit count comprises measuring bit rate over an interval of the received bitstream, and receiving information regarding the temporal extent of the bitstream being received.
6. A method according to Claim 1 , wherein the step of transcoding in the rapid access mode comprises converting at least a portion of the prediction coded matter into intra-coded matter.
7. A method according to Claim 6. wherein said portion is a macroblock.
8. A method according to Claim 6, wherein said portion is a frame.
9. A method according to any of claims 6 to 8, further comprising producing a fully intra-coded transcoded bitstream, wherein the resolution of the frames may be varied in dependence upon the available storage capacity.
10. A method according to any of claims 6 to 8, further comprising selecting for conversion those prediction coded frames furthest from the nearest intra-coded frame.
11. A method according to any of claims 6 to 8, further comprising selecting for conversion those prediction coded frames having the highest motion vector bit rate.
12. A method of processing a compressed digital video bitstream to facilitate rapid access, comprising the steps of: receiving a bitstream; transcoding the received bitstream to a transcoded bitstream having a higher bit rate and less prediction; and storing the transcoded bitstream.
13. A method according to Claim 12, wherein the step of transcoding in the rapid access mode comprises converting at least a portion of the prediction coded matter into intra-coded matter.
14. A method according to Claim 13. wherein said portion is a macroblock.
15. A method according to Claim 13, wherein said portion is a frame.
16. A method according to any of claims 13 to 15, further comprising producing a fully intra-coded transcoded bitstream, wherein the resolution of the frames may be varied in dependence upon the available storage capacity.
17. A method according to any of claims 13 to 15, further comprising selecting for conversion those prediction coded frames furthest from the nearest intra-coded frame.
18. A method according to any of claims 13 to 15, further comprising selecting for conversion those prediction coded frames having the highest motion vector bit rate.
19. A method according to any of the claims 6 to 18, further comprising storing sufficient coding decisions to enable lossless recreation of the received bitstream.
20. A method according to any of claims 1 to 4, wherein the step of transcoding in the long play mode employs a fixed quality level.
21. A method according to Claim 20 where the received bitstream has a constant bit rate, wherein the step of transcoding employs a fixed quantisation level, to produce a stored bitstream having a variable bit rate.
22. A method according to Claim 21, wherein the fixed quantisation level may be selected according to user-defined quality requirements.
23. A method according to Claim 21, wherein the fixed quantisation level is selected to reflect available storage.
24. A method according to Claim 21 , wherein the fixed quantisation level is selected to produce the maximum user-perceivable picture quality.
25. A method according to Claim 21, wherein the fixed quantisation level is constrained to never be finer than that reflecting a maximum user-perceivable picture quality.
26. A method of processing a compressed digital video bitstream, comprising the steps of: receiving a bitstream having a constant bit rate; establishing a plurality of user-defined storage modes; on user selection of a given storage mode, determining a fixed quality level reflecting that storage mode; transcoding the received bitstream, employing the fixed quality level, to a transcoded bitstream having a variable bit rate; and storing the transcoded bitstream.
27. A method according to any of the above claims, wherein the received bitstream is compliant with a given compression standard and the transcoded bitstream is compliant with the same standard.
28. A digital video recorder, comprising a bitstream receiver, a bitstream store and a transcoder, the transcoder comprising a controller and a user interface, wherein the controller is adapted to establish, in response to user interface, a plurality of user-defined storage modes, comprising at least two of a long play mode, having a lower bit rate than the received bitstream, a compress- to-fit mode, having a bit rate related to available storage, and a rapid access mode, having an increased number of non-predicted frames, and wherein the transcoder is adapted to, on user selection of a given storage mode, transcode the received bitstream to a transcoded bitstream having a different bit rate, reflecting the selected storage mode.
29. A digital video recorder, comprising a bitstream receiver, a bitstream store and a transcoder, the transcoder comprising a controller and a user interface, wherein the transcoder is adapted to transcode a received bitstream to a transcoded bitstream having a higher bit rate and less prediction.
30. A digital video recorder, comprising a bitstream receiver, a bitstream store and a transcoder, the transcoder comprising a controller and a user interface, wherein the controller is adapted to establish, in response to user interface, a plurality of user-defined storage modes, wherein the controller is adapted, on user selection of a given storage mode, to determine a fixed quality level, and wherein the transcoder is adapted to employ the fixed quality level in transcoding the received bitstream to a transcoded bitstream having a variable bit rate.
PCT/GB2001/000724 2000-02-21 2001-02-21 Compressed video bitstreams processing WO2001063936A1 (en)

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