Vocoders

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Vocoders

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The Channel Vocoder (analyzer): The channel vocoder employs a bank of

bandpass filters, Each having a bandwidth between 100 HZ and 300

HZ. Typically, 16-20 linear phase FIR filter are used.

The output of each filter is rectified and lowpass filtered. The bandwidth of the lowpass filter is selected to

match the time variations in the characteristics of the vocal tract.

For measurement of the spectral magnitudes, a voicing detector and a pitch estimator are included in the speech analysis.

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The Channel Vocoder (analyzer block diagram):

BandpassFilter

A/DConverter

LowpassFilter

A/DConverter

LowpassFilter

Rectifier

Rectifier

BandpassFilter

Voicingdetector

Pitchdetector

Encoder

S(n)To

Channel

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The Channel Vocoder (synthesizer): At the receiver the signal samples are passed

through D/A converters.

The outputs of the D/As are multiplied by the voiced or unvoiced signal sources.

The resulting signal are passed through bandpass filters.

The outputs of the bandpass filters are summed to form the synthesized speech signal.

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The Channel Vocoder (synthesizer block diagram):

D/AConverter

Decoder

D/AConverter

VoicingInformation

Pitchperiod

Pulsegenerator

Random Noise

generator

BandpassFilter

BandpassFilter

Switch

∑OutputOutput

speechspeech

FromFrom

ChannelChannel

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The Phase Vocoder : The phase vocoder is similar to the

channel vocoder.

However, instead of estimating the pitch, the phase vocoder estimates the phase derivative at the output of each filter.

By coding and transmitting the phase derivative, this vocoder destroys the phase information .

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The Phase Vocoder (analyzer block diagram):

nkcos nksin

nkcos

LowpassFilter

Encoder

LowpassFilter

DifferentiatorDifferentiator

DifferentiatorDifferentiator

Decimator

Decimator

ComputeShort-termMagnitude

AndPhase

Derivative

To

ChannelChannel

S(n)

nksin

nkcos

nak

nbk

Short-term magnitude

Short-term phase derivative

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The Phase Vocoder (synthesizer block diagram, kth channel):

nkcos

Interpolator

Decoder

∑

FromFrom

ChannelChannel Cos

Integrator

InterpolatorSin

Decimate

Short-term

amplitude

Decimate

Short-term

Phase

derivative

nksin

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The Formant Vocoder : The formant vocoder can be viewed as a

type of channel vocoder that estimate the first three or four formants in a segment of speech.

It is this information plus the pitch period that is encoded and transmitted to the receiver.

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The Formant Vocoder : Example of formant:

(a) : The spectrogram of the utterance “day one” showing the pitch and the harmonic structure of speech.

(b) : A zoomed spectrogram of the fundamental and the second harmonic.

(a) (b)

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The Formant Vocoder (analyzer block diagram):

F3

F2

F1

PitchAnd

V/UDecoder

F3B3

F2B2

F1B1

V/U

F0

Fk :The frequency of the kth formant Bk :The bandwidth of the kth formant

Input Input

SpeechSpeech

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The Formant Vocoder (synthesizer block diagram):

F3

F2

F1

ExcitationSignal

F3

B3

F2

B2

F1

B1

V/U

F0

∑

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Linear Predictive Coding : The objective of LP analysis is to estimate

parameters of an all-pole model of the vocal tract.

Several methods have been devised for generating the excitation sequence for speech synthesizes.

LPC-type of speech analysis and synthesis are differ primarily in the type of excitation signal that is generated for speech synthesis.

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LPC 10 : This methods is called LPC-10 because of

10 coefficient are typically employed.

LPC-10 partitions the speech into the 180 sample frame.

Pitch and voicing decision are determined by using the AMDF and zero crossing measures.

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Residual Excited LP Vocoder : Speech quality in speech quality can be

improved at the expense of a higher bit rate by computing and transmitting a residual error, as done in the case of DPCM.

One method is that the LPC model and excitation parameters are estimated from a frame of speech.

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Residual Excited LP Vocoder : The speech is synthesized at the transmitter and

subtracted from the original speech signal to form the residual error.

The residual error is quantized, coded, and transmitted to the receiver

At the receiver the signal is synthesized by adding the residual error to the signal generated from the model.

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RELPRELP Block Diagram :BufferAnd

window

LPanalysis

∑

Encoder

LPSynthesis

model

S(n)S(n)

To

ChannelChannelExcitation

parameters

LPLP

ParametersParameters

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Code Excited LP : CELP is an analysis-by-synthesis method

in which the excitation sequence is selected from a codebook of zero-mean Gaussian sequence.

The bit rate of the CELP is 4800 bps.

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CELP (analysis-by-synthesis coder) :

GaussianExcitationcodebook

PitchSynthesis

filter

SpectralEnvelope

(LP)Synthesis filter

∑

PerceptualWeightingFilter W(z)

ComputerEnergy

(square and sum)

Buffer andLP

analysis

Side

informationGain

LP

parameters

Speech samples

Index of

Excitation

sequence

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CELP (synthesizer) :

FromFrom

ChannelChanneldecoder

BufferAnd

controller

GaussianExcitationcodebook

PitchSynthesis

filter

LPSynthesis

filter

LP parameters,

gain and pitch estimate

updates

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Vector Sum Excited LP : The VSELP coder and decoder basically differ in

method by witch the excitation sequence is formed.

In next block diagram of the VSELP, there are three excitation source.

One excitation is obtained from the pitch period state.

The other two excitation source are obtained from two codebook.

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Vector Sum Excited LP : The bit rate of the VSELP is about 8000 bps.

Bit allocations for 8000-bps VSELP

Parameters Bits/5-ms Frame Bits/20msParameters Bits/5-ms Frame Bits/20ms

10 LPC coefficients - 38

Average speech energy - 5

Excitation codewords

from two VSELP

codebooks 14 56

Gain parameters 8 32

Lag of pitch filter 7 28

Total 29 159

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VSELP Decoder :

1

0

Long-termFilter state

Codebook1

Codebook2

∑Pitch

synthesisfilter

Spectralpost filter

Spectralenvelop

(LP)synthesis

filter

Synthetic

Speech

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