CHAPTER-VIDEO COMPRESSION STANDARDS

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Contents What is Video Compression? Why Video compression? Video Compression Standards Evaluation of Video standards H.26X (X=1,3,4)Video Standards Comparison of H.261 with MPEG Comparison of H.26X Question –Answer References

Storing video in less bit is known video compression.

Video Compression follow lossy method .Video compression use modern coding

techniques to reduce redundancy in video data.Video compression focus on reduction of

Spatial and Temporal redundancy.Major role played by Temporal redundancy in

video compression.

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What is Video Compression ?

„640 x 480 resolution, 8 bit color, 24 Fps un compressed video contains-

„307.2 Kbytes per image (frame) „7.37 Mbytes per second „442 Mbytes per minute „26.5 G-Bytes per hours

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Why Video Compression ?

640 x 480 resolution, 24 bit colors, 30 fps uncompressed video contains-

„921.6 Kbytes per image (frame) „27.6 Mbytes per second „1.66 G-bytes per minute „99.5 G-bytes per hour „ 100 Gigabyte disk (100 x 109 bytes), can

store about 1- 4 hours of high quality video „5

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Raw video contains vast amount of data. Sending these video through communication

channel required very high bandwidth and storage capacity.

Without compression communication of video take huge time and very high cost.

20 Mb/s HDTV channel bandwidth require compression by factor 70.

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Data compression Methods

Loss of information is acceptable in a picture of video.

The reason is that our eyes and ears cannot distinguish subtle changes.

Loss of information is not acceptable in a text file or a program file.

Examples of lossy method- Joint photographic experts group (JPEG) Motion picture experts group (MPEG)

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Lossy Compression Methods

„H.261/H.262/H.263/H.263+/H.263++/H.264/H.265

Video „MPEG-1, MPEG-2, MPEG-4, etc. AVI„Cinepak „Sorensen „Indeo „Real

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Video Compression Standards

Evaluation of Video Standards

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Evaluation of Video Standards

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Symmetric compression „Requires same time for encoding and decoding

„Used for dialog mode applications (teleconference).

„Asymmetric compression„Performed once when enough time is available

(„Two Pass Encoding „used for retrieval mode applications) Asymmetric compression does not acquire same time for encoding and decoding.

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Types of Compression

H.261 is a video coding standard adopted by by the ITU(CCITT) in 1990.

It is the most widely used international video compression standard for video coding.

H.261 is usually used in conjunction with other control and framing standards.

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H.261 Video Compression Standard

CIF(Common Intermediate format)-This is mainly used for high bit rate application.

QCIF(Quarter CIF)- This is one quarter of CIF and it is mainly used for low-bit rate application.

(Aspect ratio, Temporal rates, Interlacing are same in CIF and QCIF but they differ in Chroma(U,V) and number of active pels/line Lum(Y) )

H.261 Input Image Formats

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H.261 have mainly two compression mode-

1.Intra Mode- The intra Mode is similar to JPEG still image compression which is based on block by block DCT Coding technique.

2.Inter Mode- In this mode Temporal prediction is employed with or without motion compensation. Then the interframe prediction error is DCT encoded.

H.261 Compression mode

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The H.261 standard describes the video coding/decoding methods for the video portion of an audio visual service.

Designed for data rates of p*64 kbps, where p is in the range 1-30.

Targeted for circuit-switched networks (ISDN was the communication channel considered within the framework of the standard)

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H.261 Features-

Defines two picture formats: i) CIF (352x288)

ii)QCIF(176x144)

The H.621 encoding algorithm is a combination of:inter-picture prediction (to remove temporal redundancy)transform coding (to remove spatial redundancy)motion vectors (for motion compensation)

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It provide maximum coding delay of 150 msec (because it is mainly intended for bidirectional video communication)

It provide low cost VLSI implementation which is well suited for commercialization of videophone and teleconferencing equipment.

MPEG 1 part 2 design is heavily influenced by H.261

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H.261 Coding Process

H.261 Encoder-E

DCT-1

Q-1

ME

MC

Codingcontrol

VLCQDCT

Framebuffer

-

+

Video in

Intra/inter Mode

Qp

Video out

MV

H.261 Decoder

D

MC FrameMemory

+

IDCT

Q-1VLCDecoderBufferError

Correction

Input data

Decoded data

Intra/inter

Motion Vector

Step size

H.261 Summary

Used in video/teleconferencingTwo dimensional (2-D) 8 X 8 DCT to remove intra-frame

correlationZig-zag order to scan the transform coefficientsRun Length coding for zero-valued coefficients after

quantizationMotion estimation is applied to video sequence to improve

the prediction between successive frames

Continue……….Transmission rates control in the range of p X 64

KbpsError resilience including synchronization and

concealment technique required in transmission code, to cover up channel errors

Common Intermediate Format (CIF) and Quarter CIF (QCIF) for a single solution to different video formats (NTSC / PAL)

H.261 Based on JPEG Encodes video only Lossy algorithm with compression in space and time Uses I and P-frames Uses DCT on 8x8 blocks Best for video with little motion (eg. video

conferencing) Optimized for bandwidth efficiency and low delay

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Comparison of H.261 with MPEG

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MPEG Based on H.261 and JPEG Encodes audio & video Lossy algorithm with compression

in space and time Uses I, P, and B-frames Uses DCT on 8x8 blocks Designed to handle moving picture

components Less bandwidth efficient

Video conferencing productsMS Windows NetMeeting Audio VisionIntel Video Phone Internet PhoneH.261 is currently used in teleconferencing standards

H.320, H.323 and H.324

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Application of H.261

Internet Phone

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Intel Video Phone

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Intel video Phone S/w

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Intel Video Phone Upgrade Kit

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ITU- International Telecommunication UnionCIF - Common Intermediate Format

DPCM – Differential Pulse Code Modulation

DCT – Discrete Cosine Transform

ISDN – Integrated Services Digital Network

Key Word Used in H.261

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Spatial Redundancy Temporal Redundancy Motion Compensation

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Initially created for videoconferencing and video on internet, this codec was a major step towards the standardization of video compression.

In this capability for progressive scan is better than H.261

It was primarily used as a starting point for the development of MPEG ( that is optimized for higher data rates).

MPEG 4 part 2 similar to H.263

H. 263

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It is now used to compress video in Flash format.

Its disadvantages is that makes a fairly intensive CPU use and may not be successful in lower-end machines.

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This codec provides high quality encoding and decoding for streaming video applications in real time, at rates ranging from one quarter to half the file size of previous video formats.

The file size achieved is 3 times smaller than those achieved with MPEG-2 codec’s

Good quality images achieved both high and low ratios as better picture quality than MPEG-2, MPEG-4 or H.263.

MPEG 4 part 10 and H.264 are technically identical

H.264

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H.264

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H.264 Picture Quality

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It is twice more efficient than MPEG-4. It is easy to integrate and covers a wide range of image

Its disadvantages are that requires more time to coding and licensing agreements are complicated

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Comparison of H.26X Standards

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Refrences:-1.Murat Tekalp “ Digital video processing’’ Prentice Hall Signal Processing series publication, ISBN: 0-13-190075-7 (alk paper) published Year 1995.2.AL Bovik, “The Essential Guide to Video Processing” Academic Press is an imprint of Elsevier publication, ISBN: 978-0-12-374456-2 published Year 2009.3.Video codec for audiovisual services at p x 64 kbit/s ITU-T (International Telecommunication Union – Telecommunication Standardization Sector) Recommendation H.261, 19934.Stephen J. Solari, Digital Video and Audio Compression, McGraw-Hill Professional; (March 1, 1997) 5.K. Rijkse, “H.261 : Video Coding for Low-Bit-Rate Communication”, IEEE Communications Magazine, pp. 42 – 456.H.261 Video Coding , http://www-mobile.ecs.soton.ac.uk/peter/h261/h261.html7. www.itglitz.in/ICT/unit2/unit%203/H261.PPT