Speech CodingUsing LPCR.KRISHNAKANTHME SSP 1st YEARROLL NO :100514744203OU ENGINEERING COLLEGE*

CONTENTSAimSpeech CodingSpeech analsiys techniquesLinear predictive codingBlock diagramParametersConclusion*

AIMThe main aim of this LPC Speech Coding is to develop a system for encoding good quality speech at low bitrate*

What is Speech CodingSpeech coding is the procedure of transforming speech signal into more compact form for

transmitting in smaller memory to access in available bandwidth Message encryption

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Uncompressed Speech signalAnalog speech is a bandpassed signal between 200 and 3400 Hz.Uncompressed digital speech is a bit stream at 64kB/s.

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Speech codingBy coding we mean an efficient representation of the signal COMPRESSION

The main approaches:waveform codingtransform codingParametric / hybrid coding}smart quantizers*

Comparison of speech coders*

Speech Production in HumansThe speech signal is created by:A pressure source (lungs), exciting ...A Filter (Vocal tract: pharynx - mouth [soft palate, tongue] - nasal cavity)*

For DSP Engineer An excitation sourceA time varying filterH(t,)filter:Excitationspeech*

Block Diagram LPC Vocoder*

Why the name Linear Predictive CodingIt is assumed that the new sample is the weighted linear combination of previous samples

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Z-Plane RepresentationIn the z-plane we can write the model as a transfer function:Clearly this transfer function has only poles - which is why it represents an all pole filter. *

What are these ParametersThe Coefficients of the All Pole FilterGain

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How do we find the Coefficients: least squaresFormulation:Given a signal s(n);Defining an error as:

Find the set of that will minize the mean square error: *

Solution:Simply equate the derivative of E to zero:Which gives us the Normal Equations:These are no more than p linear equations in p unknowns...*

Or in matricial form:*

A correlation; in other words: take the signal, multiply it by a shifted version, and sum.Since our signal is long and time varying- we did it on short windows

What is each element of the form- *

Solving the MatrixFound the Coefficients a(i) by Using the

Levinson-Durbin recursion method *

Second ParameterThe model gain G is determined by matching the signal energy with energy of the linearly predicted samples. *

Bit rate for LPC vocoder

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Predictor coefficients18 * 8 = 144Gain5Pitch period40*4DCT coefficients309Total156Overall bit rate50 * 309 = 15450 bits / second

Case studyIn digital cellular technology it makes it possible to share available systems.In digital voice storage ,it makes possible to store longer messages in available memory*

ConclusionSound produced through LPC method is not exactly the real sound but it sounds intelligibly understandableLPC can be used in Speech recognition systemsLPC was widely used in Military because of low bit rate in transmissionThere are many variants over the basic scheme: LPC-10, CELP, MELP, RELP, VSELP, ASELP, LD-CELP...

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