ch5.3 (Vocoders)

8/3/2019 ch5.3 (Vocoders)

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Vocoders


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The Channel Vocoder (analyzer):

The channel vocoder employs a bank ofbandpass 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 lowpassfiltered. The bandwidth of the lowpass filter is selected to

match the time variations in the characteristics of thevocal tract.

For measurement of the spectral magnitudes, avoicing detector and a pitch estimator areincluded 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 passedthrough D/A converters.

The outputs of the D/As are multiplied by the

voiced or unvoiced signal sources.

The resulting signal are passed throughbandpass filters.

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


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The Channel Vocoder (synthesizer block diagram):

D/AConverter

Decoder

D/AConverter

VoicingInformation

Pitchperiod

Pulsegenerator

RandomNoise

generator

BandpassFilter

BandpassFilter

Switch

Output

speech

From

Channel


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The Phase Vocoder :

The phase vocoder is similar to thechannel vocoder.

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

By coding and transmitting the phasederivative, this vocoder destroys the phaseinformation .


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The Phase Vocoder (analyzer block diagram):

nk

cos nk

sin

nk

cos

LowpassFilter

Encoder

LowpassFilter

Differentiator

Differentiator

Decimator

Decimator

ComputeShort-termMagnitude

AndPhase

Derivative

To

Channel

S(n)

nk

sin

nk

cos

nak

nbk

Short-termmagnitude

Short-term phasederivative


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

nkcos

Interpolator

Decoder

From

ChannelCos

Integrator

InterpolatorSin

DecimateShort-term

amplitude

Decimate

Short-term

Phase

derivative

nk

sin


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The Formant Vocoder :

The formant vocoder can be viewed as atype of channel vocoder that estimate thefirst three or four formants in a segment of

speech.

It is this information plus the pitch period

that is encoded and transmitted to thereceiver.


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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 ofspeech.

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

(a) (b)


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The Formant Vocoder (analyzer block diagram):

F3

F2

F1

PitchAnd

V/UDecoder

F3

B3

F2

B2

F1

B1

V/U

F0

Fk :The frequency of the kth formant

Bk :The bandwidth of the kth formant

Input

Speech


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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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LPC 10 :

This methods is called LPC-10 because of10 coefficient are typically employed.

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

Pitch and voicing decision are determinedby using the AMDF and zero crossingmeasures.


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Residual Excited LP Vocoder :

Speech quality in speech quality can beimproved at the expense of a higher bitrate 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 froma frame of speech.


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Residual Excited LP Vocoder :

The speech is synthesized at the transmitter andsubtracted from the original speech signal toform the residual error.

The residual error is quantized, coded, andtransmitted to the receiver

At the receiver the signal is synthesized byadding the residual error to the signal generatedfrom the model.


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RELP Block Diagram :

BufferAnd

window

LPanalysis

Encoder

LPSynthesis

model

S(n)

To

ChannelExcitation

parameters

LP

Parameters


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Code Excited LP :

CELP is an analysis-by-synthesis methodin which the excitation sequence isselected 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

Spectral

Envelope(LP)

Synthesis filter

PerceptualWeighting

Filter W(z)

ComputeEnergy of Error

(square and sum)

Buffer andLP

analysis

Side

informationGain

LP

parameters

Speech samples

Index of

Excitation

sequence

+

-


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CELP (synthesizer) :

From

Channeldecoder

BufferAnd

controller

GaussianExcitationcodebook

PitchSynthesis

filter

LPSynthesis

filter

LP parameters,

gain and pitchestimate

updates


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Vector Sum Excited LP :

The VSELP coder and decoder basically differ inmethod by which the excitation sequence isformed.

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

One excitation is obtained from the pitch periodstate.

The other two excitation source are obtainedfrom 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/20ms

10 LPC coefficients - 38

Average speech energy - 5

Excitation codewords

from two VSELP

codebooks 14 56Gain 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

2

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ch5.3 (Vocoders)