Sub Band Coding

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SUB BAND CODING

S.I.Rosaline

II sem/ COS


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Introduction

Compression schemes depend on datacharacteristics.

Vector Quantizationsource output has high

degree of clustering. Differential Encodingsource output sample

to sample difference is small.

Scalar Quantizationsource output is random

But, most source outputs exhibit acombination of characteristics.

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Introduction(contd..)

Decomposing source blocks into different

frequency bandrequires TRANSFORM

CODING.

DisadvantageBlocking (generation of coding

artifacts at the block edges).

Overcome by LOT( Lapped Orthogonal

Transform).

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Example

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Consider a sequence {xn} as shown above.

Inference - fast sample to sample variation

- slow varying trend (dotted lines)

To extract trendavg. samples in a moving

windowsmooths out rapid variations, makes

evident the slow trend.

Avg {xn} , window size -2generate new

sequence {yn} .

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Now {yn} easily encoded compared with {xn}.

However, we want to encode {xn} . Hence

done by, encoding {yn} by difference equation{zn}.

Decomposinganalysis

Recomposingsynthesisimplemented using

discrete time filters.

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FILTERS

Filter - A system that isolates different

frequency components.

Low pass filter, high pass filter, Band pass filter

cut off frequency.

Magnitude Transfer function - ratio of

magnitude of input & output of the filter as a

function of frequency.

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Ideal & Practical Magnitude Response

Low Pass Filter

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Realistic filtersripple occurs in pass band &

stop band.

Filtersdigital in nature. Hence analog s/gundergoes sampling first.

SamplingNyquist theorem2f0 .

If Nyquist ruleviolatedaliasing occursoriginal signal reconstruction is impossible

anti aliasing filter used.

Input output relation of the filter is:

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Filters used in Sub Band Coding

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Filter in sub band codingQuadrature Mirror

Filter.

High pass impulsive response is

Filters are symmetric :

Filters with fewer taps are less efficient than

that with more taps. ExampleJohnston eight tap filter & Smith

Barnwell eight tap filter.

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Basic Sub Band Coding Algorithm

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Analysis

Source output passed thro analysis filter bank

Passband of the filters can be overlapping ornon overlapping.

Reducing the number of samples-

decimation/downsampling. Amount of decimation depends on the ratio of

the bandwidth of the filter output to the filter

input. After decimation, the output is encoded using

ADPCM, PCM, or vector quantization.

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Non Overlapping & Overlapping filter

banks

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Synthesis

Quantization and codingQuantized & codedcoefficients used to reconstruct the originalsignal. (Bit Allocation)

Encoded samplesDecodedUpsampledpassed thro a bank of reconstruction filters output added to get the final output.

Three major componentsanalysis &synthesis filters, Bit allocation, EncodingScheme.

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Sub Band Coding

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