mpeg & mp3 -supplement - from “ graham mcallister - nortel networks ”
TRANSCRIPT
MPEG & MP3MPEG & MP3MPEG & MP3MPEG & MP3-supplement supplement
- from “from “Graham McAllister - Nortel NetworksGraham McAllister - Nortel Networks””
Technique Compression Ratio
When?
Huffman 1.5-2:1 1952
RLE 4-10:1 1966
LZW 2-10:1 1977&84
Quadtree 2:1 1980
VQ 10:1 1984
Directional Filtering
10-40:1 1985
Fractals 10-1000:1 1988
MPEG-1 10-100:1 1993
Surface Methods
10-50:1 1995
MPEG-2 10-200:1 1995
MPEG-4 10-500:1 1999
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 / MJPEGFiles with a .mpg extension
Apple QuickTimeFiles with a .mov or .qt extension
Microsoft AVIFiles with a .avi extension
Real PlayerFiles 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%
R G B Y U V
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 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7
I frame B frame B frame P frame B frame B frame I frame
MPEGcompression
Forwardprediction
Bidirectionalprediction
Compressedstream
Inputstream
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
RGBTo
YUV
Motion Estimation DCT
QuantizeRLEVLCOutput 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. L R
10 57 5Store:
Raw:Fewer bits
Decompress: 10 5
MP3 III - schematic