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final_F05_solution - Polytechnic University Dept Electrical...

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Polytechnic University, Dept. Electrical and Computer Engineering EE4414 Multimedia Communication System II Fall 2005, Yao Wang ___________________________________________________________________________________ Second Exam (12/8, 11:00-12:50) Closed-book, 1 sheet of notes (single or double sided) allowed, no peeking into neighbors! SOLUTION 1. Video coding standards (20 pt) a) Describe two features incorporated in the H.263 video coding standard that helped to improve the coding efficiency over the earlier H.261 video coding standard. (2.5pt for each feature) i) Using half-pel accuracy motion estimation instead of integer-pel. ii) Variable block size for motion compensation. (allow a 16x16 block to be divided into 4 8x8 blocks, and estimating motion vector for each 8x8 block separately. This is helpful when the 16x16 block includes two objects moving differently). If a student list another legitimate difference, it is acceptable too. b) Why do MPEG-1 and MPEG-2 use the GOP structure with periodic I-frames? (2 pt) For video conferencing or video phone applications, can the encoder insert I-frames periodically? What may be the problem? (3 pt) The GOP structure enables random access, which is important for video broadcasting, video streaming, and DVD playback applications, which are the targeted applications of MPEG1/2. Inserting I-frames periodically generally cause the bitstream to have spikes at I-frames. When the bit stream is sent through a constant rate channel, the I-frame data will take longer time to send, this will cause variable delay at the receiver. In order to display the video at constant frame rate, a large smoothing buffer is needed at the receiver. This will significantly increase the delay time between when a frame is sent at the sender and when it is decoded and displayed. The delay may exceed several seconds. For video distribution applications targeted by MPEG1/2, this delay is typically acceptable. However, for video conferencing/telephony applications, the acceptable delay is between 150 ms and 400 ms. Therefore, inserting I-frames periodically is not advisable for video conferencing/telephony applications. c) What is scalable coding? (2 pt) Why is it beneficial for video streaming applications? (3pt) Scalable coding generates, for each group of video frames, a bit stream that can be truncated either at any point or at several defined points. When a user receives a truncated bit stream, he/she will see a correspondingly lower video quality (either in spatial resolution, temporal frame rate, or color accuracy, or a combination of these). In video streaming applications, the same video is often requested by users with different access bandwidth or decoding/display capability. Without scalable coding, multiple versions of this video has to be encoded at different bit rate, with different
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spatial/temporal resolutions. With scalable coding, only a single scalable bit stream needs to be generated. Based on the users available bandwidth and decoding/display capability, only a partial
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final_F05_solution - Polytechnic University Dept Electrical...

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