Mobile Video Communications Using a Wyner Ziv Transcoder Eduardo Peixotoa Ricardo L de Queiroza and Debargha Mukherjeeb a Departamento de Engenharia Eletrica Universidade de Brasilia Brasilia Brazil Packard Laboratories Palo Alto CA USA E mail eduardo image unb br queiroz ieee org debargha mukherjee hp com b Hewlett ABSTRACT In mobile to mobile video communications both the transmitting and receiving ends may not have the necessary computing power to perform complex video compression and decompression tasks Traditional video codecs typically have highly complex encoders and less complex decoders However Wyner Ziv WZ coding allows for a low complexity encoder at the price of a more complex decoder We propose a video communication system where the transmitter uses a WZ reversed complexity coder while the receiver uses a traditional decoder hence minimizing complexity at both ends For that to work we propose to insert a transcoder in the network to convert the video stream We present an efficient transcoder from a simple WZ approach to H 263 Our approach saves a large amount of the computation by reusing the motion estimation performed at the WZ decoder stage among other things Results are presented to demonstrate the transcoder performance Keywords Wyner Ziv video transcoding mobile video H 263 1 INTRODUCTION Mobile to mobile video communications is a highly desirable goal in personal communications Nevertheless the requirements to have low complexity at both encoder and decoder sides have been prohibitive These requirements do not allow traditional codecs to work in their best modes so that neither the rate nor the quality are at their peak The traditional codecs are more complex at the encoder side where most of the computation is done to decide the best way to encode each macroblock The decoders however are less complex just performing entropy decoding motion compensation and inverse transforms Reverse complexity video codecs are a new paradigm where the encoder has low complexity and the decoder exploits the video statistics thus being more complex Neither traditional codecs nor reverse complexity codecs are suitable to video communications where low complexity is required at both ends To solve this problem we propose a transcoder from a reverse complexity codec to a traditional codec thus achieving the best of each codec Such a transcoder has already been mentioned in the literature 1 however to our knowledge it has never been implemented The traditional video compression solutions are based on the hybrid architecture of block based motion compensated prediction and on a spatial transform followed by entropy coding of the residue and related information 2 3 The goal of these tools is the reduction of spatial and temporal redundancy In this architecture the encoder has a higher complexity than the decoder Typically the encoder is many times more complex than the decoder mainly due to motion estimation 2 4 These codecs are suited to applications where the video content is encoded once and decoded multiple times e g home DVDs or decoded by many devices e g broadcasting Distributed video coding DVC is based on reverse complexity codecs where the encoder is less complex than the decoder 1 DVC allows us to explore the video statistics at the decoder side theoretically achieving the same rate distortion of a traditional coder which exploits the video statistics at the encoder These codecs are based on the Slepian Wolf 5 and Wyner Ziv6 theorems In a Wyner Ziv WZ codec the encoder is simpler at a price of a more complex decoder They are suitable to applications where the encoding has to be performed within a low resources enviroment but the decoding can be made at a more powerful machine or offline While DVC fits well an encoder for a mobile camera it just shifts the need of computational resources to the decoder The goal of low encoding complexity and low decoding complexity within a single scheme and without giving up quality seems unreacheable In this paper we propose a transcoder from a WZ scheme to a traditional Figure 1 The transcoder within the proposed framework hybrid DPCM DCT scheme In this scenario a mobile camera phone performs WZ encoding transmitting the data to a server The server acts as a transcoder changing the video sequence to another format which requires a less complex decoder The transcoded data is sent to another mobile device the real addressee of the video content This scheme is shown in Fig 1 2 THE CODECS USED To achieve our goal of low complexity at both ends we need two codecs one with a low complexity encoder and the other one with a low complexity decoder Traditional codecs usually have low complexity decoders since they explore the video statistics at the encoder The codec chosen to implement our framework is the H 263 7 It was chosen because it is an efficient ITU T standard and it is significantly less complex than the newest state of the art video codec H 264 AVC 8 therefore reducing the total complexity of the transcoder and of the decoder WZ coding has been the focus of many studies in recent years 9 10 11 12 The WZ codec used in this paper was recently proposed 9 It is a simple pixel domain WZ codec whose architecture is shown in Fig 2 Figure 2 Wyner Ziv encoder and decoder framework At the encoder some frames called key frames are encoded with a regular intra frame encoder in this case H 263 INTRA The other frames called WZ frames are encoded as follows the difference between the current frame X and the encoder reference frame Xer is fed to the WZ encoder Xer is taken as the previous reconstructed key frame which is available both to the encoder and decoder in order to avoid drifting errors The pixels of the residual frame R X Xer are quantized using an uniform scalar quantizer generating RQ which is then reordered and encoded by a low density parity check accumulated LDPCA code 13 The WZ encoder may have a group of pictures GOP 2 length of 2 1 key frame followed by 1 WZ frame 3 1 key frame followed by 2 WZ frames or even more More WZ frames may be encoded between each key frame decreasing the encoder complexity However the correlation of the side information decreases as the GOP length increases thus reducing the quality of the encoded sequence The decoder generates the side information SI Y using motion compensated interpolation The SI generation process is crucial to any DVC framework and will be of greater relevance to the transcoder The
View Full Document
Unlocking...