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`européischen Patentanmel-
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`Patentappiicatlon No. Demande de brevet :1“
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`00402939. 3
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`Der Présicient des £uropéiscnen Patentamts;
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`lm Auflrag
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`For the President of the European Patent Office
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`Le President de E'Oflioe européen des brevets
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`DEN HAAG,DEN
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`Bezeicknung der Emnéung:
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`Method of transcoding and transcod‘lng device H1 th embedded f13ters
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`In Anspruch genommene Prioriéuen) I Priorityiies) claimed I Prioritfls} ravendiquée(s)
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`Am Anmaatfetag benannta Vertragsiaaten:
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`Contracting states dasignaied at date of fifing: ATIBE/CWCYIDEIDKIESIFVFR/GB’GRIIEIITIUILUIMCINL/P‘DSEl?
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`Method of transcoding and transcoding device with embedded filters
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`FIELD OF THE INVEKIION
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`The present invention relates to a method of transcoding a primary encoded signal
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`comprising a sequence of pictures, into a secondary encoded signal, said method of
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`oanscoding comprising at ieast a step of decoding a current picture of the primary encoded
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`signal for providing a first transformed signal, an encoding step, following the decoding step,
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`for obtaining the secondary encoded signal, and a step of prediaing a transformed motion
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`compensated signal from a transformed encoding error derived from the encoding step, said
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`prediction step being located between the encoding and decoding steps. The invention also
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`relates to a corresponding device for carrying out such a method of transcoding.
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`signals.
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`This invention is particularly relevant for the transooding of MPEG encoded video
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`BACKGROUND OF THE INVENTION
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`Bit'rate transcoding is a technique which allows a primary video stream encoded at
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`a bit-rate am to be converted into a secondary video stream encoded at a bit-rate 8R2
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`lower than Bill, the bit-rate reduction being performed in order to meet requirements
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`imposed by the means of transport during broadcasting. A transcoding device as described
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`in the opening paragraph is disclosed in the European Patent Application n° EP 0690 392
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`(PilF 94001) and is depicted in Fig. 1. Said device (100) for transcoding enwded digital
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`signals (51) which are representative of a sequence of images, comprises a decoding
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`channel (11,12) followed by an encoding channel (13,14,15). A prediction channel is
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`connected in cascade between these two diannels, and said prediction channel comprises,
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`in series, between two subtractors (101,102), an inverse discrete cosine transform sub-
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`assembly IDCT (16), a picture memory MEM (17), a circuit MC (18) for motion compensation
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`and a discrete cosine transform sub-assembly OCT (19).
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`SUMMARY OF THE INVENTION
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`It is an object of the invention to provide a method of transcoding and a
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`corresponding device that allows a better quality of pictures for low bit—rate applications. The
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`present invention takes the following aspect into consideration.
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`With the advent of home digital video recording of MPEG broadcasts, transcoders
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`can be used in consumer devices to implement long play modes or to guarantee the
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`recording time. However, the input signal to be transcoded has often been encoded at a
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`variable bit-rate with a low average bit-rate. This is due to the generalisation of statisti-l
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`multiplexing that allows broadcasters to put a lot of video programs in a multiplex in order to
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`save the bandwidth. It is likely that a coarser re—quantisation of the input signal, using a
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`method of bansooding according to the prior art, will lead to conspicuous quantisation
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`artefacts. As a consequence, such a transcoding method is not adapted for low bit-rate
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`To overcome this drawback, the method of transcoding in amordance with the
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`invention is characterised in that it comprises a filtering step, between the decoding and
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`encoding steps, for providing a filtered transformed signal, and the prediction step further
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`an adding substep for determining a sum of the transformed motion compensated
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`signal and a transformed signal; and
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`a subtracting sub—step for determining the transformed encoding error from a difference
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`between said sum and a second bansformed signal provided by the encoding step.
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`The transcoding method in accordance with the invention allows to implement filters
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`in the transcoder of the prior art at a negligible cost. Those filters can be tuned to control
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`the static and dynamic resolution and also to perform noise reduction. For the same number
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`of bits, the filtered transformed signal is encoded with a smaller quantlsation scale thus
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`reducing visual artefacts such as blocking, ringing and mosquito noise.
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`In a first embodiment of the invention, the adding sub-step is intended to provide
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`the sum of the transformed motion mmpensated signal and the first transformed signal, and
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`the filtering step is a temporal filtering step for receiving said sum and for providing the
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`filtered transformed signal to the encoding step. Such a temporal filtering step allows to
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`perform noise reduction using, for example, a recursive filter. As a consequence, bits are
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`only spent on the useful inforn'iation and the picture quality is thus increased.
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`In another embodiment of the invention, the filtering step is a spatial filtering step
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`for receiving the first transformed signal, and the adding sub-step is intended to provide the
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`sum of the transformed motion compensated signal and the filtered transformed signal to
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`the encoding step. Such a spatial filtering allows a reduction of the sharpness of the picture
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`and decreases the possible source of ringing and mosquito noise.
`The present invention also relata to a corresponding device for carrying out such a
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`method of transcoding.
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`The present invention finally relates to a computer program product for a receiver,
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`such as a digital video recorder or a set-top-box, that comprises a set of instructions, which,
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`when loaded into the receiver causes the receiver to carry out the method of canscoding.
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`These and other aspecs of the invention will be apparent from and will be
`elucidated with reference to the embodiments described hereinafter.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`The present invention will now be described in more detail, by way of example, with
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`reference to the accompanying drawings, wherein :
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`Fig. 1 is a bbck diagram corresponding to a transcoding device according to the prior
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`Fig, 2 is a block diagram corresponding to a first embodiment of a transcoding device
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`according to the invention, said device comprising a temporal filter,
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`Fig. 3 is a block diagram corresponding to a second embodiment of a transcoding device
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`according to the invention, said device comprising a spatial filter, and
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`Fig. 4 a block diagram corresponding to a third embodiment of a transcoding device
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`according to the invention, said device also comprising a spatial filter.
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`DETAILED DESCRIPHON OF THE INVENTION
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`The present invention reiats to an improved method of and a corresponding device
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`for transcoding video encoded signals. It relates, more especially, to MPEG-2 encoded
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`signals but it will be apparent to a person skilied in the art that said method of transcoding
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`stays also applicable to any type of video signals encoded using a block—based technique
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`such as, for example, those provided by MPEG-1, MPEG-4, H-261 or H-263 standards.
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`A transcoding device allows a primary encoded signal (51) previously encoded with a
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`first quantisation scale and comprising a sequence of pickers, to be converted into a
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`secondary encoded signal (52) encoded with a second quantisation scale.
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`Such a hanscoding device comprise at least the following eiemenls :
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`a decoding sub-assembiy comprising a variable iength decoder VLD and a first
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`dequantiser IQ for decoding a current picture of the primary encoded signal and for
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`obtaining the secondary encoded signal, and a second ciequantlser IQ,
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`o ‘ an inverse discrete transform sub-assembly 1061' (an Inverse Discrete Cosine
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`compensated signal (Rmc) from a transformed encoding error (Re) derived from the
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`an adder for determining a sum of the transformed motion compensated signal and
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`sub-assembly,
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`a filter, between the decoding sub—assembly and the encoding sub-assembly, for
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`providing a filtered transformed signal (Rf).
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`Said filter can be a temporal or a spatiai filter intended to control the static and
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`dynamic resolun‘on and-to perform noise reduction on a picture. The different
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`implementations of such filters are described in the following Figs. 2 to 4.
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`In a first embodiment of the invention, the transcoder implements a motion
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`compensated temporal filter. Temporal filtering allows to reduce signals which are not
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`correlated from frame to frame. it can very effectively reduce noise when combined with
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`motion compensation, as motion compensation tries to correlate the image content from
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`frame to frame. In this embodiment, a recursive filter is implemented since it provides a
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`A naive transcoding chain with a motion compensated recursive tempera! filter
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`usually comprises in uscade :
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`a decoder for providing motion compensated blocks [)1 of decoded pictuaes from an
`input stream,
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`a recursive temporal filter for providing filtered blocks Df of decoded pictures, and
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`an encoder for providing an output stream and motion compensawd blocks DZ of locally
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`decoded pictures after encoding.
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`To reduce costs, the motion compensation in the encoder is re-used in the recursive
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`temporal filter. Thus, the signal DZ is fed back to said filter instead of BF. The fiitering
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`equation of a motion compensated block Dfinnn) is then :
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`Df(n, m) = (1 — a). 01(n, m) + a -Mc(oz(p(n)), V(n,m)),
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`(1)
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`where :
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`m is the index of a block of said current picture,
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`V(n,m) is the motion associamd with biock EB, of picture n,
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`p(n) is the index of the anchor picture assodated with image n,
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`MC is the motion compensation operator, and
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`.t-s«-v 'hMm—m—u-m-r-t
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`gflifi 0-2980
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`mooaioases G-ZGOQ)
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`DEJSC
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`5
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`a is a positive scalar smaller than one that tunes the filter response.
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`An expression similar to equation (1) can be drawn for bi—direcdonai motion
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`compensation. However, without loss of generality, we shall restrict the demonstration to
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`the unidirectional case. Note that intra encoded blocks cannot be filtered since no prediction
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`is formed for them. Yet, intra encoded blocks in non intra pictures correspond most often to
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`newly exposed regions that could not possibly be temporally filtered.
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`WM'
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`The naive transcoding drain can be simplified using the hypothesis that the motion
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`compensation information is unchanged. To this end, the motion compensated block
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`01(n,m) is expressed as follows :
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`r
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`D1(n, m) = Mt «R1(n,m)- M+MC(Dl{p{n)),V(n,m)),
`where :
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`(2)
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`M is the 8 x 8 discrete cosine transform matrix,
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`M‘ is the corresponding transposed matrix, and
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`R1(n,m) is the residue retrieved from the input bit-stream alter variable length
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`15
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`decoding VLC and dequantisation IQ.
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`M is defined by equation (3) and is such that MM‘ = I :
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`M _ {45/4
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`i" .- 80an (21' +1)/16)/2 otherwise.
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`ifi = o,
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`Then, the filtered block is encoded using the same motion compensation
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`information. Let Rf(n,m) be the corresponding residue :
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`Rf(n, m) = M . Df(n,m)- Ml — M - MC(D2(p(n)), v(n,m))- Mt .
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`The residue is then quantised and deqoantised‘ again to compute the iocaliy decoded
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`pictures DZ. Let R2(n,m) be the quantised and dequantised residue :
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`R2(n,m) = M . DZ(n,m)- Mt — M . nc(ozb(n)), v(n, m))- Mt .
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`(5)
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`The equations (1) and (4) are oombined so that Rf is derived directly from DI and
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`Rf(n, m) = (1 - a.) {M‘ o1(n,m). Mt — M .MC(oz(p(n)),v(n,m))- Mt ].
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`Combining the equation {2) with equation (6) gives :
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`Rr(n,m) = (1 — u)[ R1(n,m)+ M - MC(Dl(p(n}), V(n,m))- Mi
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`— M . MCiDzipiniiflian- M‘].
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`(7)
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`Since motion compensation is performed identically from DI and from DZ, the
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`motion compensation operator MC can operate on the picture difference, i.e., on the error
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`signal due to the transcoding operation. Defining SD = Di - DZ, equation (7) is rewritten as
`follows :
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`Him-2801
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`(5) and (6) :
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`(a)
`er(n,m) = {1 _ ur) [R1(n,rn) + M . MCGSDfpm», V(n,m))- w].
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`The error signai 50 an be derived from the prediction errors, combining equations
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`ao(n,m)= 141%?)-R2(n,m)] .M.
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`(9)
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`Equations (8) and (9) define the transcoder structure depicted in Fig. 2. Said
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`transcoder (200) comprises :
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`a decoding channel comprising a variable length decoder VLD (11) and a first
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`dequantiser IQ (12) for decoding a current picture of a primary encoded signal (51) and
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`for providing a first transformed signal (R1),
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`an encoding channei comprising a quantiser Q (13), a variable iength encoder VLC (14)
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`for olitaining the secondary encoded signal (52), and a second deduantiser IQ (15) for
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`providing a second cansforrned signai (R2),
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`a prediction channel comprising, in series ;
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`a subtractor (201) for determining a transformed encoding error (Re) and whose
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`negative input receives the second transformed signal,
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`an inverse discrete cosine transform IDCF (16),
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`a picture memory HEM (17),
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`a circuit MC (18) for motion compensation,
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`a discrete cosine transform DCI’ (19) for predicting a transformed motion
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`compensated signal (Rmc),
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`In the second and third embodiments of the invention, the transcoder implements a
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`spatial filter. Spatial fiitering is not so efficient to reduce the noise as motion compensated
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`temporal filtering is. Yet, it can prevent block artefacis at tow bit-rate, smoothing down
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`sharp edges that wouid otherwise create ringing effects. It can also simplify complex
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`an adder (202) for providing a sum of the transformed motion compensamd signai
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`and the first transformed signal (R1) to the positive input of the subtractor,
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`a temporal filter W (21) for receiving said sum and for providing the fiitered transformed
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`signal (Rf) to the quantiser Q (13).
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`The strength of the motion compensated recursive temporal filter can be adjusted
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`separately for each transformed coefficient Rffii}, i.e., for each DCT sub-band. The
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`transformed coefficient of rank r is muitiplied by wri] = 1 - am such as :
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`thii = Wfi](R1[i] + RmCUD
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`Thus, the noise reduction can be tuned to the spectzal shape of the noise. It can
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`also be decided not to filter low frequencies in order to avoid visible artefact in case of a bad
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`motion compensation and to reduce the noise.
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`6304-92939 3(2-53612308)
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`neso,
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`Weh~—--—-—~
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`7
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`patterns that would be otherwise randomly distorted from one picture to the other, resulting
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`in the so-called mosquito noise.
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`Let us consider again the naive banscoding chain. The pixel domain filter shall have
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`the same granularity that the granularity of the decoder. Thus we consider a 'biock-w'
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`filter. Let Dl(n,m) be block rn of picture n. The filtered block 01(n,m) is computed as
`foiEows :
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`Df(n, m) = Fv(n)- 01(n, m)- Fh‘(n)
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`where Fv(n) and Fh{n) are matrices that define respectively the vertical and
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`horizontal filtering within the block.
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`Combining the equation (11) with the equation (2), we find :
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`Df(n,m) = l=\r(n)-P“lt ~ R1(n,m) - M -Fh‘(n)
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`(11)
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`+Mn).nc(pzoz(n)),v(n,m))-Fh'(n)
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`(12)
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`If the filter is the same for a group of pictures, then Fv{n} = Fv{p(n)) and Fh(n) =
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`Fh(p(n)). Thus, the following approximation can be given for equation (12) based on the
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`assumption that block—wise filtering commuts with motion compensation :
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`Df(n, m) = Fv(n)- Mt - R1(n,m)- M - Fl'fI (n)+ MC(Df(p(n)). V(n,m))
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`(13)
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`It follows that the mock-wise filter can be appiied to residue R1(n,m) after an
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`inverse discrete cosine transform IDCT. To implement the spatial filter in the transcoder, the
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`residue R1(n,rn) needs to be substituted by :
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`Rf(n,m) = M - Mn). Mt -R£(n,m)-M - th(n)-M‘
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`Even lithe matrices M ~ Fv(n)- Mt and M- Fh‘(n)-Mt can be precomputed, their
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`computing seems to involve many operations. Said computing an be simplified for a class of
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`block-wise filters for which the two matrices are diagonal. Such filters are symmetric filters
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`with an even number of taps. In our embodiment, we consider normalised 3-tap symmetric
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`filters since they are more suitable for small blocks. Such filters have a single parameter,
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`denoted a. The corresponding pixel domain filtering matrix, (enema, is defined by :
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`Thus. to implement filtering with horizontal parameter a. and vertical parameter a,"
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`the residue R1(n,rn) needs to be weighted (component-wise) by (Wuhgfia defined as
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`w z
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`Zoos(i nf8)+ av ' 2005(1‘ n/8)+ 3;,
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`Fig. 3 shows a transcoder with spatial pro-filtering according to the second
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`embodiment of the invention. Said transcoder (300) comprises :
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`a decoding channel comprising a variable length decoder VLD (11} and a first
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`dequantiser IQ (12) for providing a first transformed signal (R1),
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`a spatial filter W (31) for receiving said first transformed signal and for providing the
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`filtered transfomied signal (Rf),
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`an encoding channel comprising a quantiserQ (13), a variable length encoder VLC (14)
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`and a second dequantiser IQ (15) for providing a second transformed signal (R2),
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`a prediction channel comprising, in Series :
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`a subtractor (261) for determining a transformed encoding error (Re) and whose
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`negative input receives the second transformed signal,
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`an inverse discrete cosine transform IDCT (16),
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`compensated signal (Rmc), and
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`an adder (302) for providing a sum of said transformed motion compensated signal
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`and the filtered transformed signal (Rf) to the positive input of the subtractor.
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`Fig. 4 is a transooder according to the third embodiment of the invention, with
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`spatial post-filtering whose weighting factors are Wu. Said transcoder (460) compriss :
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`a decoding channel {1 1,12) for providing a first transformed signal (R1),
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`an encoding channei (13,14,15) further comprising an inverse filter (42) for providing a
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`a subtractor (201) for determining a transformed encoding error (Re) and whose
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`negative input receives the second transformed signal,
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`an adder (202) for providing a sum of said transformed motion compensated signai
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`and the first transformed signal (R1) to the positive input of the subtractor, and
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`a Spatial filter W (41) for receiving said sum and for providing a filtered bansformed
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`signal (Rf) to the encoding channel.
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`Compared to pre-flitering, the spau‘al filter is performed in the encoding part of the
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`In such transcoders (200,300,400), the fitter is performed on the dequantised data
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`which results In a better accuracy. Sest results, so far, were obtained for the combination of
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`spatial filtering of pictum and temporal filtering of predicted pictures. Successive fiitering
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`biocks W can aiso be merged into a single block whose weighting is the product of the
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`individual weightings without departing from the scope of the invention.
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`The drawings and their description hereinbefore refer both to a transcoding device
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`and a method of transcoding, a functional biock ofa diagram corresponding to a sub-
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`assembly of said device or a step of said method, rspectively. They illustrate rather than
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`limit the invention. It wiil be evident that there are numerous alternatives, which fall within
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`the some of the appended claims. In this respect. the following closing remarks are made.
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`There are numerous ways of implementing functions by means of items of hardware
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`or sofiware, or both. In this respect, the drawings of Fig. 2 to 4 are very diagrammatic, each
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`representing only one possible anbodirnent of the invention. Thus, almough a drawing
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`shows different functions as different blade, this by no means excludes that a singie item of
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`hardware or software denies out several functions. Nor does it exclude that an assembly of
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`items of hardware or software or both carry out a function. For example, the filtering step
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`an be combined with the quantisation step, thus fanning a single step without modifying
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`the method of transcoding in accordance with the invention.
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`Said method of transcoding can be implemented in several manners, such as by
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`means of wired electronic circuits or, alternatively, by means of a set of instructions stored
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`in a computer-readable medium, said instructions replacing at least a part of said circuits
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`and being executable under the control of a computer or a digital processor in order to carry
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`Any reference sign in the foliowing claims should not be construed as limiting the
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`claim. It wilt be obvious that the use of the verb “to comprise“ and its conjugations does not
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`exclude the presence of any eater steps or elements besides those defined in any claim. The
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`word “a” or “an" preceding an element or step does not exclude the presence of a plurality
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`of such elements or steps.
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`A method of transcoding a primary