SCT Video Compression

Paul Cockshott

Faraday Partnership

Objective

To develop a video compressor suitable for use on GSM (8Kbit) data channels

To support colour images at 12.5 frames per second.

The compressor and decompressor to run simultaneously in software on a 200mhz processor.

Customer Requirements

Compressor developed for Orange and Acorn, intially tailored to the StrongARM processor.

Target device, hand held mobile video phones.

No use of custom video compressor chips.

Technical features

Efficient use of bandwidth ( high compression ratios)

Low computational complexity compressor.

Minimal use of floating point arithmetic.

Simple decompression.

High compression ratio

Image first converted to YUV then

Vector quantized using high quality codebooks.

Blocks of image replaced by best matching codebook entries.

Motion compensation also used

Example codebooks

Multiple Scales of Blocks

Use of codebooks from 32x32 down to 4x4 using a modified quad tree with block occlusion

This contrasts with fixed sized blocks in MPEG (8x8).

Larger blocks give higer compression.

Block Occlusion

Quad tree involves total replacement

Block occlusion allows selective replacement

Half as many bits required.

CPU efficient

Vector quantization by table lookup

16 memory accesses to find best matching vector.

Code table design

Code tables constructed by recursive subdivision of probability density functions

Subdivision of Probabilty Map

Subdivision ensures that codes for like patterns have similar numerical values

Brighter patches have higher codes

This allowes recursive application of algorithm

Algorithm for PDF splitting

Determine axis of maximun variance

Split PDF through centroid perpendicular to this axis.

Assign leading 0 bit of code to lower half, 1 to upper half

Recurse on each sub portion of the PDF

Strength of SCT

Key Benefits

Videophones over GSM

Suitable for streaming video over the web decompressor in Java

Low bandwidth Digital TV (200 K baud), allows low powered satellite transmiters to send TV to wider areas.


	To develop a video compressor suitable for use on GSM (8Kbit) data channels
	To support colour images at 12.5 frames per second.
	The compressor and decompressor to run simultaneously in software on a 200mhz processor.


	Compressor developed for Orange and Acorn, intially tailored to the StrongARM processor.
	Target device, hand held mobile video phones.
	No use of custom video compressor chips.


	Efficient use of bandwidth ( high compression ratios)
	Low computational complexity compressor.
	Minimal use of floating point arithmetic.
	Simple decompression.


	Image first converted to YUV then
	Vector quantized using high quality codebooks.
	Blocks of image replaced by best matching codebook entries.
	Motion compensation also used


	Use of codebooks from 32x32 down to 4x4 using a modified quad tree with block occlusion
	This contrasts with fixed sized blocks in MPEG (8x8).
	Larger blocks give higer compression.


	Quad tree involves total replacement
	Block occlusion allows selective replacement
	Half as many bits required.


	Vector quantization by table lookup
	16 memory accesses to find best matching vector.


	Code tables constructed by recursive subdivision of probability density functions


	Subdivision ensures that codes for like patterns have similar numerical values
	Brighter patches have higher codes
	This allowes recursive application of algorithm


	Determine axis of maximun variance
	Split PDF through centroid perpendicular to this axis.
	Assign leading 0 bit of code to lower half, 1 to upper half
	Recurse on each sub portion of the PDF


	Videophones over GSM
	Suitable for streaming video over the web decompressor in Java
	Low bandwidth Digital TV (200 K baud), allows low powered satellite transmiters to send TV to wider areas.