MPEG: (Moving Pictures Expert Group) A Video Compression Standard for Multimedia Applications Seo Yeong Geon Dept. of Computer Science in GNU.

MPEG: (Moving Pictures Expert Group)A Video Compression Standard for

Multimedia Applications

Seo Yeong Geon

Dept. of Computer Science in GNU

MPEG Overview

• MPEG-1 : a standard for storage and retrieval of

moving pictures and audio on storage media

• MPEG-2 : a standard for digital television

• MPEG-3 : Originally developed for HDTV, but abandoned when MPEG-2 was determined to be sufficient

• MPEG-4 : a standard for multimedia applications

• MPEG-7 : a content representation standard for

information search

• MPEG-21: offers metadata information for audio and

video files

Relevant Standards

• Joint picture Experts Group (JPEG)– Compress still images only

• Expert Group on Visual Telephony (H.261)– Compress sequence of images– Over ISDN (64 kbits/sec)– Low-delay

• Other high-bandwidth “H” standards:• H21 (34 Mbits/sec)• H22 (45 Mbits/sec)

5 Parts of MPEG 1

• Part 1: Combining video and audio inputs into a single/multiple

data stream

• Part 2: Video Compression

• Part 3: Audio Compression

• Part 4: Requirements Verification

• Part 5: Technical report on the software implementation of the

Parts 1 - 3

10 Parts of MPEG-2

• Part 1Part 1:: Combine video and audio data into single/multiple streams

• Part 2:Part 2: Offers more advanced video compression tools

• Part 3:Part 3: Is a multi-channel extension of the MPEG-1 Audio standard

• Part 4/5:Part 4/5: Correspond to and build on part 4/5 of MPEG-1

• Part 6:Part 6: Specifies protocols of managing MPEG-1 & MPEG-2 bitstreams

• Part 7:Part 7: Specifies a multi-channel audio coding algorithm

• Part 8:Part 8: (was discontinued because of obsolescence)

• Part 9: Part 9: specifies the Real-time Interface (RTI) to Transport Stream

decoders

• Part 10:Part 10: the conformance part of Digital Storage Media Command and

Control (currently under development)

MPEG video spatial domain processing

• Spatial domain handled very similarly to JPEG– Convert RGB values to YUV colorspace– Split frame into 8x8 blocks– 2-D DCT on each block– Quantization of DCT coefficients– Run length and entropy coding

Spatial Redundancy Reduction

Zig-Zag Scan,Run-length

coding

Quantization• major reduction• controls ‘quality’

“Intra-FrameEncoded”

Types of frames

• I frame (intra-coded)– Coded without reference to other frames

• P frame (predictive-coded)– Coded with reference to a previous reference frame

(either I or P)– Size is usually about 1/3rd of an I frame

• B frame (bi-directional predictive-coded)– Coded with reference to both previous and future

reference frames (either I or P)– Size is usually about 1/6th of an I frame

Group of Pictures (GOP)

• Starts with an I-frame

• Ends with frame right before next I-frame

• “Open” ends in B-frame, “Closed” in P-frame– (What is the difference?)

• MPEG Encoding parameter, but ‘typical’:– I B B P B B P B B I– I B B P B B P B B P B B I

• Why not have all P and

B frames?

Spatial Redundancy

• Take advantage of similarity among most neighboring pixels

Temporal Redundancy

• Take advantage of similarity between successive frames

950 951 952

“Talking Head”

Temporal Activity

Temporal Redundancy Reduction

Temporal Redundancy Reduction

• I frames are independently encoded

• P frames are based on previous I, P frames

• B frames are based on previous and following I and P frames– In case something is uncovered

• Sequence Layer

• Group of Pictures Layer

MPEG Layers

MPEG audio• MPEG-1 – 3 layers of increasing quality, layer 3 being the most

common (MP3)– 16 bits– Samping rate - 32, 44.1, or 48 kHz– Bitrate – 32 to 320 kbps– De facto - 44.1 kHz sample rate, 192 kbps bitrate

• MPEG-2 – Supports > 2 channels, lower sampling frequencies, low bitrate improvement

• AAC (Advanced Audio Coding) – More sample frequencies (8 kHz to 96 kHz)– Higher coding efficiency and simpler filterbank– 96 kbps AAC sounds better than 128 kbps MP3

• Usually CBR, but can do VBR

Typical Compression Performance

Type Size Compression

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I 18 KB 7:1

P 6 KB 20:1

B 2.5 KB 50:1

Avg 4.8 KB 27:1

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• Note, results in Variable Bit Rate, even if frame rate is constant