Sandro Moiron

Video transcoding from H.264/AVC to MPEG-2 with reduced computational complexity

This paper addresses video transcoding from H.264/AVC into MPEG-2 with reduced complexity and high rate-distortion efficiency. While the overall concept is based on a cascaded decoder-encoder, the novel adaptation methods developed in this work have the advantage of providing very good performance in H.264/AVC to MPEG-2 transcoding. The proposed approach exploits the similarities between the coding tools used in both standards, with the objective of obtaining a computationally efficient transcoder without penalising the signal quality. Fast and efficient methods are devised for conversion of macroblock coding modes and translation of motion information in order to compute the MPEG-2 coding format with a reduced number of operations, by reusing the corresponding data embedded in the incoming H.264/AVC coded stream. In comparison with a cascaded decoder-encoder, the fast transcoder achieves computational complexity savings up to 60% with slightly better peak signal-to-noise ratio (PSNR) at the same bitrate.

A prioritization scheme for H.264/AVC video congestion resilience with IEEE 802.11e

Intra-refresh is an efficient technique to mitigate temporal error propagation resulting from network packet losses. However, while including intra-refresh macroblocks (MBs) can prevent error propagation, conversely when network congestion occurs, packets carrying this type of MBs should be dropped at wireless network access time, in preference to dropping packets containing data of other types of MBs. Exploiting this observation, the proposed network access prioritization scheme combines intra-refresh MB lines with the H.264/AVC codecs Flexible Macroblock Ordering (FMO) to enhance congestion resilience. The proposed scheme is shown to improve the resulting video quality by up to 4 dB during periods of congestion. As an application of the proposed algorithm, the paper applies a crosslayer mapping of video traffic to IEEE802.11e access categories. The potential gains are demonstrated both for random packet drops at input buffers and for a wireless sensor network scenario.

Reduced complexity intra mode decision for resolution reduction on h.264/AVC transcoders

Intra frame prediction is one of the most compression efficient tools of the H.264/AVC codec over its predecessor. In video transcoding for picture size reduction, pattern directionality of the output frames may differ from the input, affecting the H.264/AVC intra prediction modes. This paper proposes two simple algorithms for efficient mode decision by exploiting the input modes. To further improve the conversion performance two refinement methods are also proposed. One is to look at the neighboring of the selected modes and the other to look at horizontal, vertical and DC modes, as the most frequently used modes. Compared with the full rate-distortion mode decision, the proposed algorithms achieve more than 50% complexity reduction while showing a negligible PSNR penalty of only 0.1 dB and an excess bitrate of 1-6%.

Automatic Language Translation for Mobile SMS

In any form of communication it is vital that both parties can understand the same language, if they cannot a translator is required. Currently mobile users engage the service of a third party provider to translate an SMS text into a different language. The existing services have a number of drawbacks e.g. high cost to the user, not user friendly, they reduce the message space, and are inefficient. To communicate with a foreign person the sender must know the recipients preferred language and device display capability. What is needed is a service where a sender can send message in their native language without regard for the target tongue. We show that a mobile operator can provide a transparent service where the text message is automatically converted to the recipients preferred language. In comparison to the existing system, our implementation is efficient and cost effective and has large implications for commerce, language learning and person-to-person communication. A large number of services such as health care management, education, emergency notification, news, weather, and traffic reports and commerce applications can be delivered to vast mobile populations who are not able to enjoy the benefit of these services due to language barriers.

Video Transcoding Techniques

This chapter addresses the most recent advances in video transcoding and processing architectures for multimedia content adaptation. The first section provides an overview of the state of the art, where video transcoding is described as a key solution to enable seamless interoperability across diverse multimedia communication systems and services. Then, a detailed analysis of relevant transcoding functions is presented, addressing their implementation, coding performance and computational complexity. The different processing architectures that can be used for video transcoding are also described, according to their respective functionalities and application scenarios. Other solutions capable of providing adaptation of coded video to heterogeneous environments, such as scalable video coding and multiple description video coding are also discussed and their main advantages and disadvantages are highlighted. A case study is also presented to illustrate the need and effectiveness of video transcoding in modern applications.

Video streaming over an Ad Hoc network using data partitioning and path diversity

Multiple description video coding can exploit path diversity to improve error resiliency by generating several independently decodable video descriptions and sending them over different paths. Such paths are readily available in ad hoc wireless networks. To approach the original quality, all descriptions need to be successfully received. However, in a wireless channel, error bursts or link disruption may still affect one or more descriptions, resulting in significant drops in video quality. This article describes a lower complexity yet effective scheme for video streaming over such wireless networks. In order to improve error resiliency, the scheme combines unequal packet importance with the advantages of path diversity. The scheme can be implemented with a minimal increase in computational complexity, as spatial decomposition of video frames is employed to generate the descriptions. Experimental results for an ad hoc wireless network show that a significant objective quality gain can be achieved (up to 3 dB) in the presence of signal shadow fading.

Innovations in Video Error Resilience and Concealment

Router buffer overflow and random bit errors have always been a problem in the Internet. The growth in multimedia applications in mobile extensions of the Internet has increased the threat from error bursts, random errors, and packet erasures that are a manifestation of wireless channels. Source-coded error resilience has come to prominence in the H.264/AVC codec standard, and this paper reviews such innovations surrounding this codec. Some innovations are new variants of an existing technique such as datapartitioning and some are entirely new such as the powerful dispersed-mode flexible macroblock ordering. Other techniques, such as reversible variable length coding, were not be transferred to H.264 from earlier codecs and, hence, innovations are proposed to still provide robust entropy coding. Other variants such as adaptive intra-refresh are still being experimented with. When error resilience or other protection methods fail to present an adequate video frame then error concealment can be brought to bear. This paper reviews error concealment which is important to commercial companies as they are able to distinguish their codec from others. The paper considers spatial, temporal and hybrid error concealment methods. Though this paper contains over one hundred references it can only provide a slice through this rich field.

No-Reference H.264/AVC Statistical Multiplexing for DVB-RCS

Replacement of MPEG-2 by the H.264/AVC codec for satellite video services, including aggregated video for the DVB-RCS uplink, presents an opportunity to develop efficient statistical multiplexing. In this paper, a scheme is developed that effectively models the relationship between number of non-zero coefficients and video quality. The result is the ability to equalize video quality, reduce quality fades, and smooth the overall bitrate presented. The number of channels in the multiplex can be increased through the scheme if there is a need to achieve an average of 40 dB PSNR across all inputs.

Enhanced prioritization for video streaming over wireless home networks with IEEE 802.11e

Cyclic intra refresh is able to mitigate temporal error propagation. This paper exploits the unequal error sensitivity introduced within individual video frames when using a cyclic intra refresh line of macroblocks. Slice-level priority is proposed based on the relative position of a slice with respect to the intra-refresh line. Two in-frame prioritization schemes are investigated: a region-based priority assignment and a packet-based priority assignment. The region-based scheme assigns packet priorities in the order of bitstream arrival to the packetizer, whereas the packet-based scheme assigns priorities within regions and does not follow bitstream arrival order to the packetizer. The proposed schemes do not add any bitrate or computational overheads and no decoder modification is required. Simulation results for streaming high-definition video over a quality-of-service enabled home wireless network show that the packet-based scheme can achieve quality gains up to 3 dB over sending video without applying priorities.

Prioritized packetization for video with intra-refresh macroblock line

When transmitting video over low bandwidth networks a cyclic intra-refresh (IR) line is preferable to periodic intra frames as an error mitigation technique. This paper shows that IR lines introduce non-uniform error sensitivity among regions within a picture that are identifiable by the position of the IR line. To exploit this characteristic, a packet prioritization scheme is proposed in which priorities are assigned on a packet-level basis. Thus, higher priority packets can be given better protection. The proposed scheme is demonstrated for prioritized access to a congested IEEE 802.11e network. Experimental results indicate that, when compared with the classic non-prioritized packet access scheme, the proposed scheme can achieve quality gains of up to 3.6 dB for a 10% data loss rate.