MELP Vocoder Outline 1 Introduction MELP Vocoder Features - PowerPoint PPT Presentation

0 MELP Vocoder

Outline 1  Introduction  MELP Vocoder Features  Algorithm Description  Parameters & Comparison

Introduction 2  Traditional pitched-excited LPC vocoders use either a periodic train or white noise for synthesis filter  intelligible speech at very low bit rates  But sometimes results in mechanical or buzzy sound and are prone to tonal noise

Introduction 3  These problems arise from:  Inability of a simple pulse train to reproduce all kind of voiced speech  MELP vocoder uses a mixed-excitation model and it represents a richer ensemble of speech characteristic  Produce more natural sounding speech

MELP vocoder 4  Robust in background noise environments Mixed excitation  Based on traditional LPC model, also Aperiodic includes additional features pulses Pulse dispersion Adaptive spectral enhancement

ردكو MELP هدننك دك 5 زا54 ياطخ رتليف يدورو راتفگ ييوگشيپ ماگ يياهن هبساحم يراذگ هرجنپ گنيمه ييوگشيپ ياطخ هنماد هبساحم يادص تدش هبساحمي ماگ هبساحم هيروف ليدبت نازيم و جوا طاقن يگدنكارپ LPC هبساحم LPC ليدبت MSVQ LSF هب يانهپ هعسوت يا هيارآ يزاس بترم يسازيتناوكنو LPC دناب اهرادرب زا LSF يدبت ياه هنمادل رادرب رادرب هيروف هدش هزيتناوك LSF صاف داجيا متيروگلاهل صاف داجيا متيروگلاهل لقادح50زتره لقادح50زتره

ردكو MELP زيلانآ ياه هرجنپ تيعقوم زا54 6

ردكو MELP هيروف ليدبت ياه هنماد هبساحم • سلاپ داجيا رتليففيظو ه سلاپراطق ديلوتدراد ار . نيا FFT راك زا هدافتسا اب و200 و لانگيس زا هنومن هبرض خساپ شوپ جارختسا دريگ يم تروص. 7 زا54

ردكو MELP كيدويرپ ريغ مچرپ نييعت و ييادص ياه تدش هبساحم L=40,41, … ,160 ) 1- نيمخت لوا هلحرم( -نيياپ دناب ييادص تدش نبيعت 2 3- ييادص تدش نييعت4رگيد دناب 8 زا54

ردكو MELP جوا طاقن يگدنكارپ نازيم P=12.64 P=6.77 1  79 2 [ ] e n   n 80 160  p 1  79 e [ n ]   n 80 160 P=1.16 P=1.1 9 زا54

ردكو MELP جوا طاقن يگدنكارپ نازيم 1  79 2 e [ n ]   n 80 160  p 1  79 e [ n ]   n 80 160 10 زا54

ردكو MELP تيب صاصتخا لودج رتماراپرادادص تلاحيب تلاحادص LSF بيارض 25 25 هنمادهيروف ليدبت ياه 8- هرهب(2ميرف ره يازا هب راب) 88 VS1 ماگ هرود + 7 7 تدشييادص ياه 4- كيدويرپ ريغ مچرپ 1- اطخ زا تظفاحم -13 نويسازينوركنس تيب 11 تيب لكيصاصتخا ياه 5454 11 زا54

Mixed Excitation 12  Mixed-excitation is implemented using a multi- band mixing model  This model can simulate frequency dependent voicing strength  Using a mixture of Aperiodic/periodic and white noise as excitation  Primary effect of this unit is to reduce the buzz in broadband acoustic noise

Aperiodic pulses 13  When input signal is voiced, MELP vocoder can synthesize speech using either aperiodic or periodic pulses.  Aperiodic pulses used during transition regions between voiced and unvoiced segments of speech signal  Producing erratic glottal pulses without tonal noise

Pulse Dispersion 14  Pulse dispersion is implemented using fixed pulse dispersion filter based on a flattened triangle pulse  The pulse dispersion filter improves the match of bandpass filtered synthetic and natural speech waveforms in frequency bands which do not contain a formant resonance.  Spreading the excitation energy with a pitch period  Reduce harsh quality of the synthetic speech

Adaptive spectral enhancement filter 15  Based on the poles of the vocal tract filter  Is used to enhance the formant structure in the synthetic speech  This filter improves the match between synthetic and natural bandpass waveforms  more natural speech output

MELP Algorithm Description (Encoder) 16 filter out any low frequency noise 1. This filtered speech is again filtered in order to 2. perform the initial pitch search for the pitch estimation The next step is to perform the Bandpass voicing 3. analysis - In this step we decide to use periodic/Aperiodic train or white noise model

MELP Algorithm Description (Encoder) cont ’ d 17 In this stage A voice degree parameter is estimated in  each band, based on the normalized correlation function of the speech signal and the smoothed rectified signal in the non-DC band Let s k ( n ) denote the speech signal in band k , u k ( n ) denote  the DC-removed smoothed rectified signal of s k ( n ). The correlation function:  N 1   x ( n ) x ( n p )   n 0 R ( p )   x N 1 N 1    2 2 1 / 2 [ x ( n ) x ( n p ) ]   n 0 n 0 P – the pitch of current frame N – the frame length k – the voicing strength for band (defined as max( R sk ( P ) ,R uk ( P )))

MELP Algorithm Description (Encoder ) cont ’ d 18  The jittery state is determined by the peakiness of the fullwave rectified LP residue e ( n ):  N 1 1  2 1 / 2 [ e ( n ) ] N   n 0 Peakiness  N 1 1  e ( n ) N  n 0  If peakiness is greater than some threshold, the speech frame is then flagged as jittered (Aperiodic flag will be set)

MELP Algorithm Description (Encoder) cont ’ d 19 Applying a LPC analysis 4. 5. Calculating final pitch estimate 6. Calculating Gain estimate 7. quantize the LPC coefficients, pitch, gain and bandpass voicing 8. Fourier magnitudes are determined and quantized The information in these coefficients  improves the accuracy of the speech production model at the perceptually- important lower frequencies

MELP Encoder 20 Bandpass Gain Pitch Input Voicing Pre filter Calculator Search Decision signal Quantize LPC Final Pitch Gain, pitch, LSF Analysis And voicing Voicing, quantization Filter Decision jitter Fourier Apply Transmitted Magnitude Forward Bitstream calculation Error Correction

MELP Algorithm (Decoder) 21 Decoding the pitch 1. Applying gain attenuation 2. Interpolating linearly all of the synthesis 3. parameters pitch-synchronously Generating mixed-excitation 4.

MELP Algorithm (Decoder) cont ’ d 22 Applying an adaptive spectral enhancement 5. filter LPC synthesis and applying gain factor 6. Dispersion filtering 7.

MELP Decoder 23 Received Adaptive Noise Bitstream Decode Noise + Spectral Shaping parameters Generator Enhancement Filter Pulse Pulse Pulse Position Shaping Generator Jitter Filter Synthesized LPC Pulse Synthesis Dispersion gain Speech Filter Filter

Parameter Quantization Parameters Voiced Unvoiced LSF parameters 25 25 Fourier magnitudes 8 - Gain (2 per frames) 8 8 Pitch. overall voicing 7 7 Bandpass voicing 4 - Aperiodic flag 1 - Error protection - 13 Sync bit 1 1 Total bits / 22.5 ms 54 54 frame 24

Bit transmission order 25

Comparison of the 2400 BPS MELP with other Standard Coders 26 Diagnostic Acceptability  Measure Two Conditions  Quiet  Office  Continuously Variable Slope Delta Modulation (CVSD)  16,000 bps  Code Excited Linear Prediction (CELP)  4800 bps  FS1016  Mixed Excitation Linear Prediction (MELP)  2400 bps  FIPS Publication 137  Linear Predictive Coding (LPC)  2400 bps 

Comparison of the 2400 BPS MELP with other Standard Coders (cont ’ d) 27 Mean Opinion Score in Six Conditions  Quiet Anechoic Sound Chamber  Dynamic Microphone  Quiet - H250 Anechoic Sound Chamber  H250 Microphone  1% Random Bit Errors Anechoic Sound Chamber  Dynamic Microphone  0.5% Random Block Errors Anechoic Sound Chamber  Dynamic Microphone  50% Errors within a 35ms block  Office Modern Office Environment  Dynamic Microphone  Mobile Command Environment Field Shelter  EV M87 Microphone 

Comparison of the 2400 BPS MELP with other Standard Coders (cont ’ d) 28  Complexity with three Measurements  RAM  ROM  MIPS

Voice samples 29 LPC 10

Voice samples Original Sound MELP 1800 MELP 2000 MELP 2200 30