Vector quantization

Vector quantization is a classical quantization technique from signal processing which allows the modeling of probability density functions by the distribution of prototype vectors. It was originally used for data compression. It works by dividing a large set of points (vectors) into groups having approximately the same number of points closest to them. Each group is represented by its centroid point, as in k-means and some other clustering algorithms.

The density matching property of vector quantization is powerful, especially for identifying the density of large and high-dimensioned data. Since data points are represented by the index of their closest centroid, commonly occurring data have low error, and rare data high error. This is why VQ is suitable for lossy data compression. It can also be used for lossy data correction and density estimation.

Vector quantization is based on the "competitive learning" paradigm, so it is closely related to the self-organizing map model.

Training

A simple training algorithm for vector quantization is:
# Pick a sample point at random
# Move the nearest quantization vector centroid towards this sample point, by a small fraction of the distance
# Repeat

A more sophisticated algorithm reduces the bias in the density matching estimation, and ensures that all points are used, by including an extra sensitivity parameter:
# Increase each centroid's sensitivity by a small amount
# Pick a sample point at random
# Find the quantization vector centroid with the smallest
## Move the chosen centroid toward the sample point by a small fraction of the distance
## Set the chosen centroid's sensitivity to zero
# Repeat

It is desirable to use a cooling schedule to produce convergence: see Simulated annealing.

The algorithm can be iteratively updated with 'live' data, rather than by picking random points from a data set, but this will introduce some bias if the data is temporally correlated over many samples.

Applications

Vector quantization is used for lossy data compression, lossy data correction and density estimation.

Lossy data correction, or prediction, is used to recover data missing from some dimensions. It is done by finding the nearest group with the data dimensions available, then predicting the result based on the values for the missing dimensions, assuming that they will have the same value as the group's centroid.

For density estimation, the area/volume that is closer to a particular centroid than to any other is inversely proportional to the density (due to the density matching property of the algorithm).

Use in data compression

Vector quantization, also called "block quantization" or "pattern matching quantization" is often used in lossy data compression. It works by encoding values from a multidimensional vector space into a finite set of values from a discrete subspace of lower dimension. A lower-space vector requires less storage space, so the data is compressed. Thanks to the density matching property of vector quantization, the compressed data have errors that are inversely proportional to their density.

The transformation is usually done by projection or by using a codebook. In some cases, a codebook can be also used to entropy code the discrete value in the same step, by generating a prefix coded variable-length encoded value as its output.

The set of discrete amplitude levels is quantized jointly rather than each sample being quantized separately. Consider a "K"-dimensional vector $[x\_1,x\_2,...,x\_k]$ of amplitude levels. It is compressed by choosing the nearest matching vector from a set of "N"-dimensional vectors $[y\_1,y\_2,...,y\_n]$.

All possible combinations of the "N"-dimensional vector $[y\_1,y\_2,...,y\_n]$ form the codebook.

Block Diagram:A simple vector quantizer is shown below

Only the index of the codeword in the codebook is sent instead of the quantized values. This conserves space and achieves more compression.

Twin vector quantization (VQF) is part of the MPEG-4 standard dealing with time domain weighted interleaved vector quantization.

Video codecs based on vector quantization

* Cinepakand old versions of its spiritual successors:
* Sorenson codec
* Indeo
* Westwood's VQA format, used in many gamesAll of which are superseded by the MPEG family.

Audio codecs based on vector quantization

* CELP
* G.729
* TwinVQ
* Ogg Vorbis [cite web
title = Vorbis I Specification
publisher = Xiph.org
date = 2007-03-09
url = http://xiph.org/vorbis/doc/Vorbis_I_spec.html
accessdate = 2007-03-09 ]
* AMR-WB+
* DTS

See also

* speech coding
* Ogg Vorbis
* Voronoi diagram
* rate-distortion function
* data clustering
* Learning Vector Quantization
* Centroidal Voronoi tessellation

"Part of this article was originally based on material from the Free On-line Dictionary of Computing and is used with under the GFDL."

References

External links

* http://www.data-compression.com/vq.html
* [http://qccpack.sourceforge.net QccPack — Quantization, Compression, and Coding Library (open source)]
* http://www.geocities.com/mohamedqasem/vectorquantization/vq.html