Dirac Video Codec: Introduction

Dirac Video Codec: Introduction

Kok Keong (Middlesex University, UK), Myo Tun (Dialogic Corporation, Canada) and Yoong Choon Chang (Multimedia University, Malaysia)
DOI: 10.4018/978-1-61692-831-5.ch004
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Dirac was started off by British Broadcasting Corp. (BBC) in 2003 as an experimental video coding system based on wavelet technology, which is different from that used in the main proprietary/standard video compression systems. Over the years, Dirac has grown out of its initial development and it is now on offer as an advanced royalty-free video coding system designed for a wide range of users, from delivering low-resolution web content to broadcasting high-definition (HD) and beyond, to near-lossless studio editing. The Dirac’s video coding architecture and algorithms are designed with the “keep it simple” mindset. In spite of that the Dirac seems to give a two-fold reduction in bitrate over MPEG-2 for HD video and broadly competitive with state-of-the-art video codecs. This chapter introduces the architecture of Dirac video encoder. The overall encoding structure is discussed followed by the detail description of motion estimation, Overlapped Block-based Motion Compensation (OBMC), Discrete Wavelet Transform (DWT), Rate Distortion Optimization (RDO) quantization and entropy coding. The Dirac’s bitstream syntax for compressed video data storage and streaming is described. Besides that, the coding performance of Dirac in terms of compression ratio, PSNR, SSIM and VQM in comparison with H.264 as a reference are discussed. Related issues such as transcoding and streaming over packat erasure channel are also discussed.
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Nowadays, analogue video recording is a mature technology and has almost reached its limits. Investments in enhancing the technology provide increasingly small returns. On the other hand, digital video technology has the potential to achieve much higher levels of quality and the technology is being improved at an increasing rate. It has a number of unique properties that make possible applications that could not be realized using analogue video. Firstly, digital video can be manipulated more easily than analogue video. In addition to this, digital video can be stored on random access media, whereas analogue video is generally stored sequentially on magnetic tape. This random access allows for interactivity, since individual video frames are addressable and can be accessed quickly. Digital video can be duplicated without loss of quality which is important for editing applications.

The ability to easily store and transmit is by far its most important property. Video in digital form can be transmitted across channels where transmission of analogue video was almost impossible. Because compressed digital video can be transmitted using less bandwidth than analogue television, it is possible to provide many channels where before there were only a few or none. By exploiting the digital technology, cable TV systems can have enough capacity to provide hundreds of channels of digital video. Video-on-demand is currently available on trial basis. The video was delivered to the consumers homes via their copper telephone wire and normal telephone service was not disrupted. In future, video-on-demand services might eventually replace the trip to the video store.

In addition to the applications mentioned above, modern digital video applications also include storage on different media such as Video-CD, DVD, Blu-Ray Disc, broadcasting over wireless mobile channel, streaming over the internet, satellite and terrestrial digital TV, video-conferencing and video-telephony and many more. Wide development of the digital video applications has led the generation of the international standards for different types of applications under the auspices of the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and the International Organization for Standardization / International Electrotechnical Commission (ISO/IEC). The ITU-T’s H.261 video coding standard is originally designed for transmission over ISDN lines on which data rates are multiples of 64 Kbit/s and H.263 is designed as a low-bitrate compressed format for video conferencing. On the other hand, the Moving Pictures Expert Group (MPEG), established under the ISO/IEC, standardized MPEG-1 to compress Video Home System (VHS) quality raw digital video. Under the same group, another standard called MPEG-2 is widely used as the format of digital television signals that are broadcast by terrestrial (over-the-air), cable, and direct broadcast satellite TV systems. It also specifies the format of movies and other programs that are distributed on DVD. MPEG-4 is for compression of Audio Video data for web (streaming media) and CD distribution, voice (telephone, videophone) and broadcast television applications. It provides improved coding efficiency, ability to encode mixed media data (video, audio, speech) and error-resilience to enable robust transmission.

The latest standard, H.264 which is also called MPEG-4 Part 10 was developed by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG) as the product of a partnership effort known as the Joint Video Team (JVT). It is aimed to elaborate an open standard that is not application-specific and that perform significantly better than the existing standards in terms of compression, network adaptation and error robustness.

In the near future, H.264 will gain wide acceptance on many applications especially on Internet broadcasting. However, the usage of H.264 [License (2010)] incurs royalty fees which may not be cost effective for non-profit and public content owners such as public service broadcasters, archive institutes, etc., for deployment of Internet-based services. Whilst these costs are manageable initially, these could become prohibitive if the services scaling up to millions of users, or if new services are deployed which were not envisaged in the original license agreements.

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