MPEG & MP3 -supplement - from “ Graham McAllister - Nortel Networks ”

TechniqueCompression Ratio When? Huffman1.5-2:11952 RLE4-10:11966 LZW2-10:11977&84 Quadtree2:11980 VQ10:11984 Directional Filtering 10-40:11985 Fractals :11988 MPEG :11993 Surface Methods 10-50:11995 MPEG :11995 MPEG :11999 time

MPEG Today MPEG-2 –Super-set of MPEG-1 –Rates up to 10 Mbps (720x486) –Can do HDTV (no MPEG-3) MPEG-4 –Around Objects, not Frames –Lower bandwidth MPEG-7 –Not (yet) a standard –Allows content-description (ease of searching) MPEG-21 MP3 –For audio –MPEG-1 Layer-3

Digital video formats MPEG / MJPEG Files with a.mpg extension Apple QuickTime Files with a.mov or.qt extension Microsoft AVI Files with a.avi extension Real Player Files with a.rm extension

1. Image Sequence Compression Uses Inter-frame encoding Also known an image sequence or temporal coding

Temporal Redundancy Reduction

MPEG Process How does it work? 1. Sub-sample Reduces data by around 50% RGBYUV luminance

MPEG Process II 2. Motion Detection – on luminance block only three types of frame … I Frames – intra-coded P Frames – prediction from previous frame B frames – use bi-directional prediction 1 I 2 B 3 B 4 P 5 B 6 P 7 B 8 I

Example sequence transmitted as I P B B I B B Frame 1Frame 2Frame 3Frame 4Frame 5Frame 6Frame 7 I frameB frame P frameB frame I frame MPEG compression Forward prediction Bidirectional prediction Compressed stream Input stream

Motion Compensation I-Frame –Independently reconstructed P-Frame –Forward predicted from the last I- Frame or P-Frame B-Frame –forward predicted and backward predicted from the last/next I- frame or P-frame Transmitted as - I P B B B P B B B

Motion Prediction

Motion Estimation

Motion Estimation (cont ’ d)

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

MPEG Process III - Schematic Image Sequences RGB To YUV Motion Estimation DCT QuantizeRLEVLC Output Buffer No compression achieved

MPEG-1 & MPEG-2 MPEG-1 –Designed for video playback at 150KB/s - single speed CD-ROM –Used in VCD technology MPEG-2 –Much higher bandwidth – 3MB/s –DVD technology

MPEG-4 Very different from previous generations Aimed at low-bandwidth applications – at upper end, good enough for digital TV Digital Camcorders

MPEG-4 II - Meshes 2-D animated meshes Textures mapped onto meshes Store vertices of mesh and movement parameters

MPEG-4 III - Sprites MPEG-4 is object based – state of the art Panoramic images – massive compression ratios – 1000:1

2. Audio Compression Techniques from image compression can be used –Huffman encodes output –DCT MP3 – huge!!!!

MP3 – MPEG-1 Layer 3 1. Minimal Audition Threshold –Don ’ t store anything under 5Khz 2. Masking Effect –Uses psychoacoustic model of the ear –Don ’ t store quiet and loud noises simultaneously

MP3 II 3. Joint Stereo (JS) coding –1. Intensity Stereo (IS) Ear unable to locate some frequencies – bass Store signal in mono + minimum for spatialization –2. Mid/Side (MS) Stereo Used if left and right speakers are similar Store middle (L+R) plus a side speaker (L or R) e.g. LR Store: Raw: Fewer bits Decompress:105

MP3 III - schematic