DIGITAL VIDEO SIGNALS CODING METHOD AND CORRESPONDING CODING OR TRANSCODING SYSTEM

Abstract: The disclosure relates to a method for transmitting a video bitstream corresponding to n multiplexed variable bitrate coded video bitstreams, when coded audio bitstreams are associated to said parallel video bitstreams. According to the proposed method, one or several audio parameters are defined from said audio bitstreams, and the bitrate allocations respectively associated to the parallel video bitstreams are each one weighted by a weighting parameter corresponding respectively to each of said audio parameter(s).

Description: Digital video signals coding method and corresponding coding or transcoding system. The present invention relates to a method for transmitting a constant bitrate digital video bitstream corresponding to n multiplexed variable bitrate coded video bitstreams, comprising the steps of : - coding said video bitstreams in n parallel video coding branches ; - multiplexing the coded video bitstreams into said constant bitrate video bitstream, and to a corresponding video coding or transcoding system. This invention is useful in the field of digital television transmission, for implementing a statistical multiplexer that allows to have an improved dynamic allocation of the bitrate in each channel. In digital television transmission systems, sometimes also called statistically multiplexed systems, n digital video signals have to be transmitted through an identical number of channels having a predetermined global allocated bitrate. A coding apparatus of this type is described for instance in the document EP 0685971. This apparatus comprises a plurality of encoders, for coding the input video signals at variable rates, and a bit allocating stage, supplied with estimated information coming from the encoders and provided for sending back to the encoders, on the basis of said information, an indication of an allowed allocation of bits. Such a dynamic allocation allows to grant more bitrate on more complex sequences, which not only reduces the probability of getting strong artifacts and improves the bandwidth efficiency, but also makes sure that the quality perception will be the same in each channel (the video quality is evaluated for each parallel video signal and the bitrate is requested in correspondence with said evaluation). However, the quality perception does not take into account the audio information when digital audio signals accompany the video ones, which is often the case. It is therefore an object of the invention to propose a new kind of bitrate allocation in statistical multiplexing systems. To this end, the invention relates to a method such as described in the opening paragraph of the description and which is moreover characterized in that it also comprises the steps of : - defining at least one audio parameter from a coded audio bitstream respectively associated to each video bitstream ; - transmitting to a common controller said audio parameter created in association with each video bitstream ; - weighting the bitrate allocation respectively associated to each video bitstream according to a weighting parameter sent back by said controller in correspondence with each audio parameter respectively. The applicant has indeed discovered that, at the end of the receiving system, when pictures are finally displayed, the users do not have the same perception of these displayed pictures and the same ability to detect artifacts depending on the type of the audio signals accompanying the pictures, whereas it was up to now recognized that a conventional video bitrate allocation depended on parameters linked only to the video signals. The document US 5606362 describes a coding apparatus in which the global bit allocation granted to the apparatus depends on a parameter evaluation related to the audio signal that is associated to a given picture. However, if the global allocation is influenced by the audio signal, it is clear that the video bitrate allocation itself is not modified. The quality perception is therefore not modified by the audio signal, whereas the principle carried out according to the invention is to influence the video bitrate allocation by weight Ji with one or several relevant audio parameters created from these audio signals. It is also an object of the invention to propose a system for carrying out the method according to the invention. To this end, the invention relates to a constant bitrate digital video transmission system comprising n parallel video coding branches, the variable bitrate output signals of said branches being directed towards a constant bitrate transmission channel via a multiplexer and a bitrate allocation being defined for each encoding branch by means of a common controller that receives from each coding branch at least one respective video parameter created in association with each video bitstream, characterized in that it also comprises : (1) means for generating at least one audio parameter from a coded audio bitstream respectively associated to each video bitstream ; (2) means for processing in said common controller each of said audio parameters in order to define in association with each coding branch a so-called weighting parameter ; (3) means for weighting the bitrate allocation respectively associated to each video bitstream according to each of said corresponding weighting parameters. Such a system may also be adapted to another type of environment, for controlling transcoders, when for instance a transport stream is processed and bitrate is re-allocated by having bitrate transcoding. The particularities and advantages of the invention will now be explained in a more detailed manner, with reference to the accompanying drawings in which : - FigJ shows a television transmission system incorporating an embodiment of the invention ; - Fig.2 shows a similar system in another type of environment. A digital video signals coding system carrying out the method according to the invention is illustrated in FigJ. It comprises n parallel encoding channels, including variable bitrate video encoders 110a to 1 lOn. Each of these encoders receives a respective video signal to be encoded, said signal being in fact a bitstream of data that have been previously subdivided into blocks of pixels, DCT transformed and quantized. These n input video signals are referenced EN(1) to EN(n), and the n output signals of the encoders are referenced N(l) to N(n). These n output signals are coded bitstreams sent with a variable bitrate towards a multiplexer 20, the output of which is a single coded bitstream S with a fixed bitrate. Said bitrate is the limited bitrate allowed by the transmission channel, referenced 100. It is often desirable to have a transmission system in which the quality perception is not modified, whatever the channel. A rate controller 30 is then provided in order to receive from each encoder 110a to 1 lOn one or several specific parameters associated to each video signal processed by each respective encoder and to send back to said encoders specific signals for controlling the bitrate allocation of these encoders on the basis of these specific parameters and with the object of being in accordance with the bitrate allowed in the transmission channel. The requested bitrate corresponding to that evaluation is obtained by controlling the quantizer step size of each encoding channel. According to the invention, the audio signals that accompany the video signals and have also to be transmitted are also sent towards the rate controller 30 and participate in the allocation of the bitrate in each channel. To that end, the base band audio signal - referenced EA(1) to EA(n) - associated to each video signal is sent towards an associated analysis circuit 40a to 40n. The circuits 40a to 40n yield to the controller 30 a specific parameter which is intended to influence the bitrate allocation of the corresponding channel. In the present case, the parameter is the acoustic level of the signals EA(1) to EA(n). When the acoustic level of one or several signals EA(i) is higher, the corresponding bit allocations may be decreased, since an user brings less attention to a picture accompanied by a higher acoustic level and which moreover is often a picture changing quickly (on the contrary, a low acoustic level is often associated to pictures that are rather still). The bitrate finally obtained at the output of the multiplexer 20 is the same as when the analysis circuits are not provided, but the distribution of the respective bit allocations given by the signals N(l) to N(n) is now different, each of said allocations being now weighted by one (or several) audio parameters(s). The invention is obviously not limited to the implementation that has been hereabove described, from which variants may be proposed without departing from the scope of the invention. For instance, the law that links the audio parameters and the modification of the bitrate allocation in each channel may be easily changed according to the type of user. It is also worth to note that the audio parameter may be different (contrast, presence of low or high frequencies, and so on). The system according to the invention has been described in the case the encoding channels comprise video encoders, but the invention is similarly implemented even with another type of environment, when the input signals EN(1) to EN(n) are, as shown in Fig.2, signals already coded sent to transcoders 210a to 210n (in place of encoders). When such a transport stream is processed, the bitrate of each channel is re-allocated by having bitrate transcoding. This re-allocation is then, as in the first described implementation, influenced by the audio parameters measured in the analysis circuits 40a to 40n. It is also clear that the present coding method can be implemented either by means of wired electronic circuits or, alternatively, by means of a set of instructions stored in a computer-readable medium, said instructions replacing at least a part of the circuits of the system and being executable under the control of a computer or a digital processor in order to carry out the same functions as fulfilled in said replaced circuits. The invention then also relates to a computer-readable medium comprising a software module that includes computer-executable instructions for performing the steps, or some steps, of the hereinabove described method.

Claims: , 6 CLAIMS: 1. A method for transmitting a constant bitrate digital video bitstream corresponding to n multiplexed variable bitrate coded video bitstreams, comprising the steps of : - coding said video bitstreams in n parallel video coding branches ; - multiplexing the coded video bitstreams into said constant bitrate video bitstream ; characterized in that it also comprises the steps of : - defining at least one audio parameter from a coded audio bitstream respectively associated to each video bitstream ; - transmitting to a common controller said audio parameter created in association with each video bitstream ; - weighting the bitrate allocation respectively associated to each video bitstream according to a weighting parameter sent back by said controller in correspondence with each audio parameter respectively. 2. A constant bitrate digital video transmission system comprising n parallel video coding branches, the variable bitrate output signals of said branches being directed towards a constant bitrate transmission channel (100) via a multiplexer (20) and a bitrate allocation being defined for each encoding branch by means of a common controller (30) that receives from each coding branch at least one respective video parameter created in association with each video bitstream, characterized in that it also comprises : (1) means for generating at least one audio parameter from a coded audio bitstream respectively associated to each video bitstream ; (2) means for processing in said common controller (30) each of said audio parameters in order to define in association with each coding branch a so-called weighting parameter ; (3) means for weighting the bitrate allocation respectively associated to each video bitstream according to each of said corresponding weighting parameters. 3. A constant bitrate digital video transmission system comprising n parallel video transcoding branches, the variable bitrate output signals of said branches being directed towards a constant bitrate transmission channel (100) via a multiplexer (20) and a bitrate allocation being defined for each transcoding branch by means of a common controller (30) that receives from each transcoding branch at least one respective video parameter created in association with each video bitstream, characterized in that it also comprises : (1) means for generating at least one audio parameter from a coded audio bitstream respectively associated to each video bitstream ; (2) means for processing in said common controller (30) each of said audio parameters in order to define in association with each transcoding branch a so-called weighting parameter ; (3) means for weighting the bitrate allocation respectively associated to each video bitstream according to each of said corresponding weighting parameters. 4. A storing medium comprising a software module for stoπng a set of instructions executable under the control of a computer or a processor and provided for performing at least some of the steps of the transmitting method according to claim 1.

Inventors: FERT ETIENNE, FERT, ETIENNE
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H04N7
IPC codes: H04N7