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 1: Part 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-Frame Encoded”

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 1/3 rd 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 1/6 th 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

“Talking Head” Temporal Activity

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 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 I 18 KB 7:1 P 6 KB 20:1 B 2.5 KB 50:1 Avg 4.8 KB 27: Note, results in Variable Bit Rate, even if frame rate is constant