Vocoders 1 The Channel Vocoder (analyzer) : The channel vocoder - PowerPoint PPT Presentation

Vocoders 1

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. 2

The Channel Vocoder (analyzer block diagram) : Bandpass Lowpass A/D Rectifier Filter Filter Converter Bandpass Lowpass A/D Rectifier Encoder Filter Filter Converter To S(n) Channel Voicing detector Pitch detector 3

The Channel Vocoder (synthesizer) :  16-20 linear-phase FIR filters  Covering 0-4 kHz  Each having a bandwidth between 100- 300 Hz  20-ms frames, or 50 Hz changing of spectral magnitude  LPF bandwidth: 20-25 Hz  Sampling rate of the output of the filters: 50 Hz 4

The Channel Vocoder (synthesizer) :  Bit rate:  1 bit for voicing detector  6 bits for pitch period  For 16 channels, each coded with 3-4 bits, updated 50 times per second  Then the total bit rate is 2400-3200 bps  Further reductions to 1200 bps can be achieved by exploiting frequency correlations of the spectrum magnitude 5

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. 6

The Channel Vocoder (synthesizer block diagram) : D/A Bandpass Converter Filter Output ∑ speech D/A Bandpass Converter Filter Decoder From Channel Voicing Switch Information Random Pitch Pulse Noise period generator generator 7

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 . 8

The Phase Vocoder (analyzer block diagram, kth channel)  cos n Short-term k magnitude a k n   Lowpass cos n sin n k Decimator k Filter Compute Differentiator Short-term Encoder S(n) To Magnitude Channel And Phase Differentiator Derivative Lowpass  Decimator cos n Filter k b k n Short-term phase  sin n derivative k 9

The Phase Vocoder (synthesizer block diagram, kth channel) Decimated Short-term amplitude  cos n k Decoder From Channel Interpolator Cos ∑ Integrator Interpolator Sin Decimated Short-term  sin n Phase k derivative 10

The Phase Vocoder :  LPF bandwidth: 50 Hz  Demodulation separation: 100 Hz  Number of filters: 25-30  Sampling rate of spectrum magnitude and phase derivative: 50-60 samples per second  Spectral magnitude is coded using PCM or DPCM  Phase derivative is coded linearly using 2-3 bits  The resulting bit rate is 7200 bps 11

The Formant Vocoder :  The formant vocoder can be viewed as a type of channel vocoder that estimates 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. 12

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) 13

The Formant Vocoder (analyzer block diagram) : F3 F3 B3 F2 F2 B2 Input F1 Speech F1 B1 Pitch V/U And V/U F0 Decoder Fk :The frequency of the kth formant Bk :The bandwidth of the kth formant 14

The Formant Vocoder ( synthesizer block diagram) : F3 F3 B3 F2 ∑ F2 B2 F1 F1 B1 V/U Excitation F0 Signal 15

Linear Predictive Coding :  The objective of LP analysis is to estimate parameters of an all-pole model for the vocal tract.  Several methods have been devised for generating the excitation sequence for speech synthesizes.  Various LPC-type speech analysis and synthesis methods differ primarily in the type of excitation signal generated for speech synthesis. 16

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. 17

A General Discrete-Time Model For Speech Production Pitch Gain s(n) Speech DT G(z) Signal Impulse Glottal Voiced U(n) generator Filter Voiced H(z) R(z) Volume V Vocal tract LP velocity U Filter Filter Uncorrelated Noise Unvoiced generator Gain 18

يطخ ييوگشيپ ييوگشيپ هبترم نييعت هحفص 19 زا 54

يطخ ييوگشيپ ييوگشيپ هبترم نييعت هحفص 20 زا 54

يطخ ييوگشيپ ييوگشيپ هبترم نييعت    m 2 s [ n ]       n m M 1 PG 10 log    m 2 e [ n ]      n m M 1 هحفص 21 زا 54

يطخ ييوگشيپ لاثم M=4 M=10 هحفص 22 زا 54

يطخ ييوگشيپ لاثم M=2 M=10 M=54 هحفص 23 زا 54

يطخ ييوگشيپ تدم دنلب يطخ ييوگشيپ هديا M=10 M=50 هحفص 24 زا 54

يطخ ييوگشيپ تدم دنلب يطخ ييوگشيپ هحفص 25 زا 54

ردكو LPC10 يمومع تاصخشم  LPC10     هحفص 26 زا 54

ردكو LPC10 هدننك دك PCM LPC LPC LPC Bit Encoder هحفص 27 زا 54

چيپ دويرپ صيخشت  m    R[l,m] s[n]s[n l]     n m N 1 m     MDF [ l , m ] s [ n ] s [ n l ]  YMC    n m N 1       s [ n ] b . s [ n N ] e [ n ], m N 1 m هحفص 28 زا 54

ردكو LPC10  MDF  T=20,21 ,…, 39,40,42 ,…, 80,84 ,…, 154 هحفص 29 زا 54

ردكو LPC10 هدننك دك LPC RC هحفص 30 زا 54

ردكو LPC10 راتفگ زتنس يلصا لانگيس هدننك دك شخب • رادادص نييعت / ف ندوب ادص يبمير • لاح يارب طثف ماگ هرود نييعت ترادادص • لانگيس هرهب هبساحم يرپ اب هبرض راطقدو V/U ماگ هرود رياري G هدش زتنس راتفگ زيون يفداصت هحفص 31 زا 54

ردكو LPC10 اهتيدودحم AR هحفص 32 زا 54

Residual Excited LP Vocoder :  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. 33

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. 34

Residual Excited LP Vocoder :  The residual signal is low-pass filtered at 1000 Hz in the analyzer to reduce bit rate  In the synthesizer, it is rectified and spectrum flattened (using a HPF), the lowpass and highpass signals are summed and the resulting residual error signal is used to excite the LPC model.  RELP vocoder provides communication-quality speech at about 9600 bps. 35

RELP Analyzer (type 1): Buffer S(n) f (n; m) e (n; m) ∑ And Residual window error LP Parameters ˆ { a (i; m)} stLP Encoder analysis To ˆ Θ , gain estimate Excitation 0 Channel V/U, decision parameters ˆ P , pitch estimate LP Synthesis model 36

RELP Analyzer (type 2): Prediction Residual Inverse Buffer To  f (n; m) S(n) Lowpass (n; m) Filter Decimator DFT Encoder And Channel Filter ˆ window A (z; m) LP Parameters stLP ˆ analysis { a (i; m)} 37

Synthesizer for a RELP vocoder Buffer Residual From Highpass And Rectifier Decoder Interpolator Filter Channel Controller ∑ LP model Parameter updates Excitation LP synthesizer 38

Multipulse LPC Vocoder  RELP needs to regenerate the high- frequency components at the decoder.  A crude approximation of the high frequencies  The multipulse LPC is a time domain analysis-by-synthesis method that results in a better excitation signal for the LPC vocal system filter. 39