Image analysis – Image compression or coding
Reexamination Certificate
1998-07-22
2001-03-27
Couso, Jose L. (Department: 2694)
Image analysis
Image compression or coding
C382S235000, C382S236000
Reexamination Certificate
active
06208759
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the switching of bit-rate reduced signals and in the most important example to the switching of bit-rate reduced video.
To achieve reasonable levels of bit-rate reduction—or compression—in video, it is generally not sufficient merely to reduce the number of bits required to transmit each picture or frame. Additional compression is achieved by transmitting a reduced number of complete frames, accompanied by prediction information (utilizing motion compensation) which enables a downstream decoder to generate “missing” frames from the prediction information, using a transmitted frame as a reference. The compressed signal accordingly lacks the regular frame sequence of conventional video and typically has in its place an arrangement of relatively infrequent reference frames with interspersed prediction frames. Depending on the compression scheme employed, these prediction frames may be all of the same kind, utilizing forward prediction only, or may include both forward and backward prediction frames.
It will be understood that conventional techniques of switching between video signal sources at a convenient frame boundary, if employed with a compressed signal of this nature, would most times separate prediction frames from the reference frames to which they relate. This would lead to nonsensical results at the downstream decoder, lasting for a significant time period.
A prior suggestion for dealing with this difficulty relies on forcing the switching to take place at a reference frame. This is not a satisfactory solution, however, since constraining the switching points will not generally be acceptable unless it is separately ensured that the reference frames are close together. This may not always be possible and will, of course limit the degree of bit-rate reduction that can be achieved.
A further substantial obstacle to the switching of bit-rate reduced video arises from the use in a typical compression coder/decoder pair of coder and decoder data buffers. A typical coder, from the nature of the transform employed and from the use of variable length coding, will produce data at variable rates and will require a data buffer in order to feed a constant bit-rate transmission channel. Similarly, the downstream decoder will require a data buffer to enable it to use data from the constant bit-rate channel at the variable rates required to de-compress the video signal. The upstream coder is able to regulate the rate at which data is output, in such a manner as to prevent overflow or underflow of data in the coder buffer. Typically this is done by controlling quantization levels.
In the conventional coder/decoder pair, this control exercised over buffer occupancy in the coder buffer serves automatically to prevent overflow or underflow of data in the downstream decoder buffer. However, it can readily be seen that if the input of a downstream decoder is switched from one upstream coder to another, this control over buffer occupancy is lost and overflow or underflow of data would be expected in the downstream decoder.
The lack of a workable method of switching bit-rate reduced video has had a number of unfortunate consequences. One example lies in broadcast networks where at a number of local centers, a facility is required for switching between a compressed network feed and a regional opt-out signal. The option of compressing the locally produced signal and merely switching between compressed network and regional signals, has not been available because of the problems that have been described above. At such centers therefore, the signal distributed on the national network would need to be decoded to analog or to uncompressed digital form, passed into a conventional mixer for switching to and from the regional program and the output of the mixer then recoded in compressed form before conveying it to the local transmitter.
In such an arrangement the digital decoder and encoder are constantly in circuit; this leads to a number of problems. There is for example a reduction in distributed picture quality because of the cascaded coding and decoding processes. Moreover, reliability is reduced because the signal path constantly includes additional and complicated decoder and re-coder.
On its path up the distribution chain, the signal may have to pass through two or three such regional opt-out centers and the loss in picture quality and loss of reliability are two reasons why such a solution would be unattractive.
It should be understood that the same problems will arise in switching between two bit-rate reduced video data streams.
What has been said in relation to bit-rate reduced video applies to bitstreams such as those generated by ITU/R 721 standard codecs but more importantly to bitstreams generated according to the MPEG1 and MPEG2 standards. The invention will also apply to other bit-rate reduced signals having reference frames separated by frames defined with respect to a reference frame and to other variable length coding schemes.
A method for being able to switch between bitstreams at frame boundaries—not restricted to reference frames—would also provide the basis for editing functions carried out on the compressed bitstream. Hitherto it has not been considered practicable to edit compressed sequences to frame accuracy when the compression system uses motion-compensated interframe prediction because—as has been explained—the frames are not independent. To decode one particular frame, many previous frames have to be decoded in order to generate a “prediction” using motion compensation from previous frames. Current, nonlinear editing systems based around the storage on disc of video in compressed form use compression schemes in which each frame is coded independently of the previous frames (eg. JPEG).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method of switching a bit-rate reduced signal which permits switching of the bitstream at frame boundaries and which minimizes the impairments introduced by cascaded decoding and recoding.
Accordingly, the present invention includes in one aspect in a method of switching a bit-rate reduced signal, comprising the steps of developing a re-coded signal through cascaded decoding and re-coding processes; switching between the bit-rate reduced signal and the re-coded signal at an appropriate signal point and subsequently switching to an alternative bit-rate reduced signal.
Preferably, the re-coded signal is developed by passing the bit-rate reduced signal through a decoder and re-coder pair and wherein the alternative bit-rate reduced signal is derived by switching the input of the re-coder to an alternative signal source.
In another aspect, the present invention includes an apparatus for switching a bit-rate reduced signal at an output terminal, comprising an input terminal for receiving a bit-rate reduced signal; an output switch; a direct path for passing the bit-rate reduced signal to the output switch without decoding; a decoder for receiving the bit-rate reduced signal; a coder for receiving decoded signal from the decoder and supplying re-coded signal to the output switch, the output switch being adapted to switch at the output terminal between the bit-rate reduced signal and the re-coded signal; and re-coder switch means for receiving an alternative signal and applying it to the coder in substitution for said decoded signal.
In a further aspect, the present invention includes an arrangement of first and second signals where each signal is capable of representation in coded and decoded forms, the coded form comprising a sequence of frames including both reference frames and frames defined with respect to reference frames, the method of switching between the first signal in coded form and the second signal in coded form, comprising the steps of receiving the first signal in coded form; decoding and recoding the first signal to provide a recoded first signal in frame synchronisation with the coded first signal as received; switching in the coded doma
British Broadcasting Corporation
Couso Jose L.
Do Anh Hong
Kinberg Robert
Venable
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