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Paul Cockshott |
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Faraday Partnership |
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To develop a video compressor suitable for use
on GSM (8Kbit) data channels |
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To support colour images at 12.5 frames per
second. |
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The compressor and decompressor to run
simultaneously in software on a 200mhz processor. |
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Compressor developed for Orange and Acorn,
intially tailored to the StrongARM processor. |
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Target device, hand held mobile video phones. |
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No use of custom video compressor chips. |
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Efficient use of bandwidth ( high compression
ratios) |
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Low computational complexity compressor. |
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Minimal use of floating point arithmetic. |
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Simple decompression. |
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Image first converted to YUV then |
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Vector quantized using high quality codebooks. |
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Blocks of image replaced by best matching
codebook entries. |
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Motion compensation also used |
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Use of codebooks from 32x32 down to 4x4 using a
modified quad tree with block occlusion |
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This contrasts with fixed sized blocks in MPEG
(8x8). |
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Larger blocks give higer compression. |
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Quad tree involves total replacement |
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Block occlusion allows selective replacement |
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Half as many bits required. |
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Vector quantization by table lookup |
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16 memory accesses to find best matching vector. |
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Code tables constructed by recursive subdivision
of probability density functions |
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Subdivision ensures that codes for like patterns
have similar numerical values |
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Brighter patches have higher codes |
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This allowes recursive application of algorithm |
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Determine axis of maximun variance |
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Split PDF through centroid perpendicular to this
axis. |
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Assign leading 0 bit of code to lower half, 1 to
upper half |
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Recurse on each sub portion of the PDF |
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Videophones over GSM |
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Suitable for streaming video over the web
decompressor in Java |
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Low bandwidth Digital TV (200 K baud), allows
low powered satellite transmiters to send TV to wider areas. |
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