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. 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): Bandpass Filter A/D Converter Lowpass Rectifier Voicing detector Pitch Encoder S(n) To Channel

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): 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 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): 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): Interpolator Decoder ∑ From Channel Cos Integrator Sin Decimate Short-term amplitude Phase derivative

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 Vocoder : Example of formant: (a) (b) (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): 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): ∑ B2 F1 F1 B1 V/U Excitation Signal F0

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 : 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 : 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 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 : Buffer And window ∑ Encoder LP analysis S(n) To Parameters LP analysis To Channel Excitation parameters LP Synthesis model

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) : Speech samples Buffer and LP analysis Side information 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

gain and pitch estimate CELP (synthesizer) : From Channel decoder Buffer And controller Gaussian Excitation codebook Pitch Synthesis filter LP LP parameters, gain and pitch estimate updates

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 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 56 Gain parameters 8 32 Lag of pitch filter 7 28 Total 29 159

VSELP Decoder : Long-term Filter state Codebook 1 2 ∑ Pitch synthesis Spectral post filter envelop (LP) Synthetic Speech