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Video Transcoding - Reduced Complexity Transcoding, Receiver Aware Transcoding, Quality Vs Complexity

transcoder bitrate capabilities resources

Definition: Video transcoding is the process of converting a compressed video in a given format into another compressed video bitstream.

Transcoding is necessary when a given compressed bitstream is not suitable for a video player. For example, the high bitrate video used for a digital TV broadcast cannot be used for streaming video to a mobile device. For delivery to mobile devices, we need video content that is encoded at lower bitrate and lower resolution suitable for low-resource mobile terminals. Pre-encoding video at a few discrete bitrates leads to inefficiencies as the device capabilities vary and pre-encoding video bitstreams for all possible receiver capabilities is impossible. Furthermore, the receiver capabilities such as available CPU, available battery, and available bandwidth vary during a session and a pre-encoded video stream cannot meet such dynamic needs. To make full use of the receiver capabilities and deliver video suitable for a receiver, video transcoding is necessary. A transcoder for such applications takes a high bitrate video as input and transcodes it to a lower bitrate, lower resolution, and lower complexity video suitable for a mobile terminal. The main challenges of video transcoding are: 1) complexity reduction at the transcoder, 2) complexity management at the receiver and 3) quality Vs complexity tradeoff.

Video transcoding has received significant attention from the industry and academia. An overview of the issues in video transcoding was presented in. Adaptation of the compressed video through bitrate reduction, sometime referred to as trans-rating, has been proposed using techniques such as dynamic rate shaping .

Reduced resolution transcoding is another approach to meeting receiver requirements that not only reduces the output bitrate but also the resources required at the receiver.

Reduced Complexity Transcoding

Figure 1 shows the general architecture of a video transcoder. A video transcoder is used to convert a compressed video bitstream in format A to a format B, where the video bitstream in format B is usually intended for a device with limited resources and capabilities. A full-complexity transcoder encompasses the full complexity of decoding the format “A bitstream” and the full complexity of encoding into a format “B bitstream.” The dotted box in the figure shows a full complexity transcoder: a video decoder A followed by a video encoder B. If video formats A and B are similar or based on the same principles, the complexity of the transcoding operation can be reduced. Reduced complexity transcoders try to gather useful information during the decoding stage and reuse it in the encoding stage. Developing algorithms for reduced complexity is a challenging problem and the complexity of the transcoding algorithms depends on the video formats involved. The MPEG-2 to MPEG-4 transcoding algorithms can reuse significant amount of information because of the similarity between MPEG-2 and MPEG-4. Since the MPEG-2 and H.264 video coding algorithms are substantially different, innovative approaches have to be developed to reduce transcoding complexity. Complexity reduction allows transcoding servers to server larger number of users simultaneously.

Receiver Aware Transcoding

Considering the video transcoding at a sender, a transcoder converts a given video to a format that is more suitable for playback on the receiver. A transcoder can modify the coding algorithm, resolution, frame rate, or bitrate. For a given receiver, there are a number of possible transcoding options that result in a video suitable for the receiver. Figure 2 depicts a case with five different options for the algorithm, resolution, frame rate, and bitrate. The options for the coding algorithm can be entirely different coding algorithms (MPEG-2 Vs H.264) or the same algorithm with different tools (e.g., MPEG-4 with and without error resilience). Not all of the possible combinations of options are meaningful for a given receiver. Transcoders have to determine which of the meaningful combinations meet the available receiver resources while maximizing the quality of experience for the end user. The knowledge of receiver capabilities allows a transcoder to select transcoding options that maximize the quality of the video received.

Quality Vs Complexity

The quality of the transcoded video is directly related to the transcoding complexity and the decoding complexity. Producing high quality transcoded video requires significant computing resources. Similarly decoding bitstreams encoded using sophisticated algorithms requires more receiver resources. The upper bound on the required computing resources is the amount of resources required for full decoding and re-encoding and this also represents the upper bound on the achievable quality. For a given computing budget (receiver resources and capabilities), a transcoder can output bitstreams with a number of different qualities, where a bitstream quality is a function of its bitrate, frame rate, resolution, and SNR. The key challenge is making the optimal tradeoff given the playback environment and receiver capabilities. The traditional metric of peak signal to noise ratio (PSNR) is not sufficient to represent the quality of transcoded video as the quality is complex function of the playback environment, usage context, and user perception. The quality of user experience (QoE) has to be considered as a whole. The QoE of a transcoded bitstream is a function of the output bitrate, frame rate, resolution, PSNR, and the usage environment and all these factors have to be considered in quantifying the QoE.

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