lpc and horn noise detection

LINEAR PREDICTIVE CODING METHODS AND HORN NOISE DETECTION

By P GOPIKRISHNA (12)EASHWAR JEEVAN(15)V V N PRANATHI(30)

• Introduction to speech coders.• Speech production and

modeling(coding).• Basic Principles of Linear

Predictive Analysis.• The Autocorrelation Method.• The Covariance Method.• Solution of the LPC Equations.• Horn noise detection using LPC

Contents

Speech Coders

Waveform Codec• Waveform codec’s attempt, without using any

knowledge of how the signal to be coded was generated, to produce a reconstructed signal whose waveform is as close as possible to the original.

• This means that in theory they should be signal independent and work well with non-speech signals.

• Generally they are low complexity codec’s which produce high quality speech at rates above about 16 kbits/s.

• When the data rate is lowered below this level the reconstructed speech quality that can be obtained degrades rapidly

Source Codec• Source coders operate using a model of how the

source was generated, and attempt to extract, from the signal being coded, the parameters of the model.

• It is these model parameters which are transmitted to the decoder.

• Source coders for speech are called vocoders, and work as follows.

• The vocal tract is represented as a time-varying filter and is excited with either a white noise source, for unvoiced speech segments, or a train of pulses separated by the pitch period for voiced speech.

• Therefore the information which must be sent to the decoder is the filter specification, a voiced/unvoiced flag, the necessary variance of the excitation signal, and the pitch period for voiced speech.

Desirable Properties of a Speech Coder• Low Bit-Rate• High Speech Quality• Robustness Across Different Speakers /

Languages• Robustness in the Presence of Channel Errors• Good Performance on Nonspeech Signals• Low Memory Size and Low Computational

Complexity• Low Coding Delay

Applications of Speech CodingDigital Transmissions

◦On wired telephone: Multiplexing Integration of services

◦On wireless channels: Spectral efficiency For better protection against errors

Voice mail/messagingStorage: telephone answering

machineSecure phone

• study human hearing, especially masking

• speech production mechanism• speaker characteristics• linguistic code (recognition-

synthesis)• thought-to-speech

Future in speech coding

Speech production and modeling

Speech Signal

Speech Spectrum for a Voiced Sound

current sample

time

short-term prediction

short-term - resonance of vocal tractlong-term - periodicity of voiced speech (vocal cord vibration)

long-term prediction

source filter speech

log spectrum

LPC is short term linear prediction that predicts the coefficients based on previous signal samples which can be transmitted instead of speech at a low bit rate than speech itself

Linear model of speech production

A(Z)-Analysis Filter 1/A(Z)-Synthesis Filter

At transmission end-coding

At reception end-decoding

i/p-speecho/p-coded speech parameters

i/p-coded speech parameters o/p- speech

LPC vocoder

The same principle as in H.Dudley’s VocoderUsed by US Government (LPC-10) - 2.4 kbs

LPC vocoder

Short Term Linear Prediction

• The coefficients of H(z)=1/A(z) can be obtained by linear prediction.

• Short term analysis on x(n) speech signal– Frames of 10 to 30 ms.

• Least square error criterion

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LPC Analysis Equations

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Autocorrelation Method

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The Covariance Method

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The resulting covariance matrix is symmetric, but not Toeplitz, and can be solved efficiently by a set of techniques called Cholesky decomposition


Autocorrelation/Covariance Summary

1.Autocorrelation Method => signal is windowed by a tapering window in order to minimize discontinuities at beginning (predicting speech from zero-valued samples) and end (predicting zero-valued samples from speech samples) of the interval

2. Covariance method => the signal is extended by p samples outside the normal range of to include p samples occurring prior to m=0 (they are available) and eliminates the need for a tapering window; resulting matrix of correlations is symmetric

COMPARISON

Results

•we have added a speech signal and known horn noise .• Threshold value observed practically for given horn noise.•LPC residue for horn and silence is low whereas for speech it is high. •The above property is used to develop algorithm to detect horn noise.

Horn noise detection

Results

Softwares:

REFERENCES:•SPEECH CODING ALGORITHMS Foundation and Evolution of Standardized Coders -WAI C. CHU

•Lecture Notes in Speech Production, Speech Coding, and Speech Recognition Mark Hasegawa-Johnson of University of Illinois

•Introduction to Speech Processing | Ricardo Gutierrez-Osuna | CSE TAMU(Texas A & M U niversity)Linear Predictive Coding, Jeremy Bradbury, December 5, 2000

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lpc and horn noise detection

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