1 Vocoders
Jan 20, 2016
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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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