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Video Compression Techniques

Video Compression Techniques. By David Ridgway. Telecine or 3.2 Pulldown is the process of converting film which is 24 frames/sec. progressive to NTSC video (29.97 frames/sec interlaced). NTSC video is interlaced, a frame consists of 2 sets of lines which may not be the same.

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Video Compression Techniques

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  1. Video CompressionTechniques By David Ridgway

  2. Telecine or 3.2 Pulldown is the process of converting film which is 24 frames/sec. progressive to NTSC video (29.97 frames/sec interlaced). • NTSC video is interlaced, a frame consists of 2 sets of lines which may not be the same. •   A standard method for conversion was developed that repeated 12 video fields distributed throughout each second rather than repeating 6 entire frames.

  3. In MPEG-1, the decoder could only infer the intended frame rate, or derive it based on the Systems layer time stamps. MPEG-2 provides specific picture header variables called repeat_first_field and top_field_first which explicitly signal which frames or fields are to be repeated, and how many times.

  4. An I-frame is encoded as a single image, with no reference to any past or future frames. • A P-frame (Predictive) is encoded relative to the past reference frame. • A B-frame (Bi-directional) is encoded relative to the past reference frame, the future reference frame, or both frames.

  5. I B B P B B P B B I frame #: •  1 2 3 4 5 6 7 8 9 10 Frames 1, 4, and 7 must be decoded before frame 9 can be decoded.

  6. Macroblock predictions are formed out of arbitrary 16x16 pixel (or 16x8 in MPEG-2) areas from previously reconstructed pictures

  7. In P-Pictures, each 16x16 sized macroblock is predicted from a macroblock of a previously encoded I picture. • The highlighted block in the target image( image to be coded) is similar to the reference image except it shifted to upper right.

  8. Macroblocks within a P picture may either be coded as Intra or Non-intra (temporally predicted from a previously reconstructed picture). • Macroblocks within the B picture can be independently selected as either Intra, Forward predicted, Backward predicted, or both forward and backward (Interpolated) predicted.

  9. Limited motion activity in P pictures=skipped macroblocks • Co-planar motion within B picture=skipped macroblocks

  10. Discrete Cosine Transform Mapping • Zigzag ordering converts a two-dimensional array to a one-dimensional sequence run-length coding.

  11. For MPEG-1 standard, Zig-zag progressive scan is an appropriate choice. • The MPEG-2 standard introduced a new run-length entropy scanning pattern (on the right hand side), it is more efficient for the interlaced video signal.

  12. Discrete Cosine Transform data stream shall be organized in field or frame order. • Quantization reduces by frame compression the amount of information to represent an image. • Run-length entropy scanning pattern is generally more efficient for the statistics of interlaced video signals.

  13. Sources: • bmrc.berkeley.edu/frame/research/storage/index.html#vod • fh.friedberg.de/fachbereich/e2/telekom-labor/zinkel/mk/mpeg2beg/whatisit.htm • aluminumstudios.com • ict.ewi.tudelft.nl

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