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Vocoders. 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.

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Vocoders


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the channel vocoder analyzer
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.
the channel vocoder analyzer block diagram

Encoder

Bandpass

Filter

Rectifier

Lowpass

Filter

A/D

Converter

Bandpass

Filter

Rectifier

Lowpass

Filter

A/D

Converter

To

Channel

S(n)

Voicing

detector

Pitch

detector

The Channel Vocoder (analyzer block diagram):
the channel vocoder synthesizer
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.
the channel vocoder synthesizer block diagram
The Channel Vocoder (synthesizer block diagram):

Decoder

D/A

Converter

Bandpass

Filter

Output

speech

D/A

Converter

Bandpass

Filter

From

Channel

Voicing

Information

Switch

Random

Noise

generator

Pitch

period

Pulse

generator

the phase vocoder
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 .
the phase vocoder analyzer block diagram
The Phase Vocoder (analyzer block diagram):

Compute

Short-term

Magnitude

And

Phase

Derivative

Short-term magnitude

Encoder

Lowpass

Filter

Decimator

Differentiator

S(n)

To

Channel

Differentiator

Lowpass

Filter

Decimator

Short-term phase derivative

the phase vocoder synthesizer block diagram kth channel

Decimate

Short-term

amplitude

Decoder

From

Channel

Cos

Interpolator

Integrator

Decimate

Short-term

Phase

derivative

Sin

Interpolator

The Phase Vocoder (synthesizer block diagram, kth channel):
the formant vocoder
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.
the formant vocoder1
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)

the formant vocoder analyzer block diagram
The Formant Vocoder (analyzer block diagram):

F3

F3

B3

F2

F2

B2

Input

Speech

F1

F1

B1

Pitch

And

V/U

Decoder

V/U

F0

Fk :The frequency of the kth formant

Bk :The bandwidth of the kth formant

the formant vocoder synthesizer block diagram
The Formant Vocoder (synthesizer block diagram):

F3

F3

B3

F2

F2

B2

F1

F1

B1

V/U

Excitation

Signal

F0

linear predictive coding
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.
lpc 10
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.
residual excited lp vocoder
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.
residual excited lp vocoder1
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.
relp block diagram
RELP Block Diagram :

Encoder

Buffer

And

window

S(n)

LP

Parameters

LP

analysis

To

Channel

Excitation

parameters

LP

Synthesis

model

code excited lp
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.
celp analysis by synthesis coder
CELP (analysis-by-synthesis coder) :

Speech samples

Side

information

Buffer and

LP

analysis

LP

parameters

Gain

+

Gaussian

Excitation

codebook

Pitch

Synthesis

filter

Spectral

Envelope

(LP)

Synthesis filter

-

Perceptual

Weighting

Filter W(z)

Compute

Energy of Error

(square and sum)

Index of

Excitation

sequence

celp synthesizer

Gaussian

Excitation

codebook

LP

Synthesis

filter

Buffer

And

controller

Pitch

Synthesis

filter

From

Channel

decoder

LP parameters,

gain and pitch estimate

updates

CELP (synthesizer) :
vector sum excited lp
Vector Sum Excited LP :
  • The VSELP coder and decoder basically differ in method by which 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.
vector sum excited lp1
Vector Sum Excited LP :
  • The bit rate of the VSELP is about 8000 bps.
    • Bit allocations for 8000-bps VSELP
vselp decoder

Long-term

Filter state

Pitch

synthesis

filter

Spectral

post filter

Spectral

envelop

(LP)

synthesis

filter

Codebook

1

Synthetic

Speech

Codebook

2

VSELP Decoder :