Video CompressionVideo CompressionTechniquesTechniques

By David RidgwayBy David Ridgway

Telecine or 3.2 Pulldown is the process of Telecine or 3.2 Pulldown is the process of converting film which is 24 frames/sec. progressive converting film which is 24 frames/sec. progressive to NTSC video (29.97 frames/sec interlaced). to NTSC video (29.97 frames/sec interlaced).

NTSC video is interlaced, a frame consists of 2 sets NTSC video is interlaced, a frame consists of 2 sets of lines which may not be the same.of lines which may not be the same.

    A standard method for conversion was developed A standard method for conversion was developed that repeated 12 video fields distributed that repeated 12 video fields distributed throughout each second rather than repeating 6 throughout each second rather than repeating 6 entire frames. entire frames.

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.

An An I-frameI-frame is encoded as a single is encoded as a single image, with no reference to any past or image, with no reference to any past or future frames. future frames.

A A P-frameP-frame (Predictive) is encoded (Predictive) is encoded relative to the past reference frame. relative to the past reference frame.

A A B-frameB-frame (Bi-directional) is encoded (Bi-directional) is encoded relative to the past reference frame, the relative to the past reference frame, the future reference frame, or both frames. future reference frame, or both frames.

I B B P B B P B B I I B B P B B P B B I frame #:frame #:   1 2 3 4 5 6 7 8 9 10 Frames 1, 4, and 7 must 1 2 3 4 5 6 7 8 9 10 Frames 1, 4, and 7 must

be decoded before frame 9 can be decoded.be decoded before frame 9 can be decoded.

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

In P-Pictures, each 16x16 sized macroblock is In P-Pictures, each 16x16 sized macroblock is predicted from a macroblock of a previously predicted from a macroblock of a previously encoded I picture.encoded I picture.

The highlighted block in the target The highlighted block in the target image( image to be coded) is similar to the image( image to be coded) is similar to the reference image except it shifted to upper right. reference image except it shifted to upper right.

Macroblocks within a P picture may either be Macroblocks within a P picture may either be coded as Intra or Non-intra (temporally predicted coded as Intra or Non-intra (temporally predicted from a previously reconstructed picture).from a previously reconstructed picture).

Macroblocks within the B picture can be Macroblocks within the B picture can be independently selected as either Intra, Forward independently selected as either Intra, Forward predicted, Backward predicted, or both forward predicted, Backward predicted, or both forward and backward (Interpolated) predicted. and backward (Interpolated) predicted.

Limited motion activity in P Limited motion activity in P pictures=skipped macroblocks pictures=skipped macroblocks

Co-planar motion within B picture=skipped Co-planar motion within B picture=skipped macroblocks macroblocks

Discrete Cosine Transform Discrete Cosine Transform MappingMapping

Zigzag ordering converts a two-Zigzag ordering converts a two-dimensional array to a one-dimensional array to a one-dimensional sequence run-length dimensional sequence run-length coding. coding.

For MPEG-1 standard, Zig-zag progressive For MPEG-1 standard, Zig-zag progressive scan is an appropriate choice.scan is an appropriate choice.

The MPEG-2 standard introduced a new The MPEG-2 standard introduced a new run-length entropy scanning pattern (on run-length entropy scanning pattern (on the right hand side), it is more efficient the right hand side), it is more efficient for the interlaced video signal.for the interlaced video signal.

Discrete Cosine Transform data stream Discrete Cosine Transform data stream shall be organized in field or frame shall be organized in field or frame order.order.

Quantization reduces by frame Quantization reduces by frame compression the amount of information compression the amount of information to represent an image.to represent an image.

Run-length entropy scanning pattern is Run-length entropy scanning pattern is generally more efficient for the statistics generally more efficient for the statistics of interlaced video signals. of interlaced video signals.

Sources:Sources:

bmrc.berkeley.edu/frame/research/bmrc.berkeley.edu/frame/research/storage/index.html#vodstorage/index.html#vod

fh.friedberg.de/fachbereich/e2/fh.friedberg.de/fachbereich/e2/telekom-labor/zinkel/mk/telekom-labor/zinkel/mk/mpeg2beg/whatisit.htmmpeg2beg/whatisit.htm

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