Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-06-16
2003-09-09
Britton, Howard (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C348S705000, C375S240260, C375S240280, C386S349000, C386S349000
Reexamination Certificate
active
06618438
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for switching MPEG streams, for example video sequences or program segments coded according to the MPEG standard, the acronym standing for the expression Motion Picture Expert Group.
2. Description of the Related Art
Films or video sequences are nowadays commonly stored in compressed form, so as to reduce the memory capacities required. Switchings of sequences are then performed not in baseband but directly on the basis of the MPEG coded data streams. Thus, successive video sequences are broadcast or one sequence is inserted into another sequence through the use of digital data stored in compressed form. A first stream of compressed video data corresponding to a first sequence of images, and referred to as the old stream, is followed, starting from a given point referred to as the switching point, by a second stream of compressed video data which is referred to as the new stream and corresponds to a second sequence of images.
The switching from one data stream coded according to the MPEG standard to another data stream coded according to this standard is beset with a number of problems in the area of the switching point.
Management of the buffer memory of the decoder is performed, in the MPEG standard, on the basis of modeling referred to as a virtual memory. Indeed, the coding of each image constituting a sequence is not carried out at constant cost and a buffer memory at the output of the coder is used to deliver a data stream at constant mean bit rate. On the decoder side, a buffer memory is also required so as to deliver, from the constant bit rate data stream, variable-cost images. Management of the buffer memory of the decoder is performed on the basis, among other things, of the clocks transmitted by the coder in the data stream and of the tags allocated to the images transmitted and defining the instant of exit from the memory of the decoder, the time gap between the instant of storage of an image in the memory of the coder and the instant of reading of the memory of the decoder having to be the same for each image.
This involves the PCR clock, standing for Program Clock Reference in the MPEG standard, which is a time cue transmitted in the data stream and on the basis of which the synchronization of the decoder is performed.
This also involves the DTS tag, standing for Decoding Time Stamp in the MPEG standard, which is a field transmitted in the stream and which indicates the instant at which an image is to be decoded in the decoder.
A first problem arises concerning the management of the memory of the decoder, owing to the fact that the number of bits per image (or frame) is not fixed. The state of the buffer memory varies over time and upon switching, the elements taken into account by the new stream for the management of this memory correspond to a state of fill of the memory which is different from the actual state corresponding to the old stream. This may result in saturation of the memory entailing image loss or a voiding of the memory creating a freeze-frame for example.
A second problem concerns the synchronization as regards the decoding and display of the images. The tags contained in the data streams relate to clocks specific to each stream (namely to a specific PCR) and the instants of decoding and display of the images need to be resynchronized.
Another problem is related to the very coding of the images. The various types of codings called upon by the MPEG standard are:
intra coding (I image) making reference to no preceding or following image,
inter coding of predictive type (P image) which may call upon a preceding reference image,
inter coding of bi-directional type (B image) which may call upon a preceding reference image and/or upon a following reference image.
The images are grouped together into a GOP or Group Of Pictures as it is called. The first of a GOP, when considering the order of transmission, is an image of intra type and the first few succeeding images of type B may call upon images of the same GOP, which is then a closed GOP, or else upon images of the same GOP and of the preceding GOP, which is then an open GOP.
The problem arises when the stream of data following the switching point begins with an open GOP. This is because the decoding of the compressed images of the first GOP of this second stream may require, if this is an open GOP, the utilization of images of the preceding GOP. The preceding GOP corresponding to a stream of data of another sequence, the decoded images corresponding to this open GOP will exhibit defects. These defects are generally highly visible and are due to the utilization, for the decoding of these first few images of type B of the second sequence, of a different reference image from that which was used for the coding. The image blocks constituting the image or the images of the open GOP which are coded in bi-directional mode are image blocks coded in inter mode on the basis of a P- or I-type reference image of the preceding GOP for the same image sequence. The decoding of the blocks coded in inter mode, blocks composed of residuals or of prediction errors, for the first few images of bi-directional type after the switching point, will utilize different blocks from those which served in calculating these inter-coded residuals blocks. The reference image taken into account at decoding level will in fact be an image of the old stream, and hence different from the image taken into account at coding level. Very annoying block effects consisting of the displaying of erroneous image blocks will therefore appear in the first few images of the second sequence.
A partial solution to these problems exists and is known to be described in the MPEG II standard (part 1: Systems, paragraph 2.4 and appendix K). This involves incorporating, within the system layer, information relating to possible switching points (referred to as “splicing points” in the standard) for which the transport stream has favorable characteristics, allowing decoding of the images without the defects cited above (for example by using closed GOPs). It will then be possible for switching to be performed at these points only. Of course, this information must be incorporated into the data stream right from the coding of the images and it is no longer possible to incorporate it thereafter other than by carrying out a new decompression and compression of the data.
A permitted switching point, at the start of a GOP, adds a constraint on the coding of the first few images of this GOP, which are coded in bi-directional mode, this GOP having to be closed. These images can no longer use, as reference images, preceding images but only succeeding images and they are therefore forced into monodirectional mode.
The use of closed GOPs, hence the coding of the first two B images following an I image on the basis of the following P image (order of coding) and of the one I image (instead of the preceding P image of the preceding GOP), it then being possible to regard these B images as P images, severs the regulating of the coder on the basis of the states of the buffer memory and hence degrades the quality of the images, on account of a higher coding cost. To give an order of magnitude, the images of P type have a coding cost which is about twice as high as that of the images of B type.
This MPEG II coding compatible with stream switchings is not ordinarily used for coding films or video sequences. There are at present very few products on the market which force closed GOPs for the requirements of switching, the constraints induced in respect of bit rate regulation at coder level being an annoyance. Moreover, a priori determination of whereabouts a closed GOP must exist in the data stream, that is to say prediction of whereabouts in the sequence the cuts will be, presupposes that one knows the future uses of the sequences. Otherwise, it would be necessary to produce closed GOPs exclusively, although one wishes to utilize only a very small number of them.
The drawbacks generated by
Chapel Claude
Guillemot Jean-Charles
Le Roux Jean
Britton Howard
Eriksen Guy M.
Kurdyla Ronald M.
Thomson Licensing S.A.
Tripoli Joseph S.
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