Speech Coding Basics

Speech Coding BasicsSpeech Coding Basics

Mahdi Amiri

Supervisor

Dr. H. R. Rabiee

April 2009

Sharif University of Technology

A TutorialA Tutorial

Page 2 of 30 Speech Coding Basics

Speech CodingSpeech CodingA road mapA road map

PCMDPCMADPCMLPCCELP


Pulse-code Modulation (PCM)Pulse-code Modulation (PCM)BasicsBasics

Digital Representation of an Analog Signal Sampling and Quantization

Parameters:– Sampling Rate (Samples per Second)

– Quantization Levels (Bits per Sample)

Pulse-code Modulation (PCM)Pulse-code Modulation (PCM)


Why Call it PCM?Why Call it PCM?

4-bit PCM4-bit PCM


How to choose proper…– Sampling Rate

• 8 Khz ?

– Quantization Level• 8 bit/sample ?

Bit per Second for 8000 Hz 8 bit PCM– 64 kbit/s


Bit per Second (bit/s)Bit per Second (bit/s)


Human Hearing Frequency Range– 20 Hz to 20 kHz– Play with “HearTest” to test your hearing– Most people will find that their hearing is most

sensitive around 1-4 kHz and that it is less sensitive at high and low frequencies.


Sampling RateSampling Rate



Hearing RangeHearing Range


Human Vocal Range– Normal: 80 Hz to 1100 Hz– Charles Kellogg (14 KHz) (not verified)

– Guinness Book of Records• Female: Georgia Brown

– (Eight octaves, 25087Hz)

• Male: Tim Storms– (Six octaves)


Sampling RateSampling Rate


8,000 Hz: Telephone, adequate for human speech 11,025 Hz 22,050 Hz – radio 32,000 Hz - miniDV digital video camcorder, DAT (LP mode) 44,100 Hz - audio CD, also most commonly used with MPEG-1 audio

(VCD, SVCD, MP3) 48,000 Hz - digital sound used for miniDV, digital TV, DVD, DAT, films

and professional audio 96,000 or 192,000 Hz - DVD-Audio, some LPCM DVD tracks, BD-ROM

(Blu-ray Disc) audio tracks, and HD-DVD (High-Definition DVD) audio tracks

2.8224 MHz - SACD, 1-bit sigma-delta modulation process known as Direct Stream Digital, co-developed by Sony and Philips”


Common Sampling RatesCommon Sampling Rates


Want to prevent human ear fatigue by minimizing quantization noise

Signal-to-Noise Ratio = 6.02B dBSNR is approximately 6 dB per bit.

– 16-bit => 96 dB– Above 36 dB is required


Quantization LevelsQuantization Levels


The average person cannot tell the difference between a bitrate above 192 kbit/s and the original CD/WAV.

Even if your headphones seal really well around your ears, they will probably only give you about 20 to 25 dB insulation from the external sound.


Good to KnowGood to Know



ImagesImages



u-law, a-lawu-law, a-law Nonuniform quantizers: Difficult to make, Expensive. Solution: Companding Uniform Q. Expanding



U-law, A-lawU-law, A-law



u-law, a-lawu-law, a-law

North America and JapanNorth America and Japan EuropeEurope


Differential PCM (DPCM)Differential PCM (DPCM)IdeaIdea

Differential PCM (DPCM)Differential PCM (DPCM)


Basic SchemeBasic Scheme

1Delta Modulation (DM): i n ia x z

Problem?Problem?

General Predictive CodingGeneral Predictive Coding

Differential PCM (DPCM)Differential PCM (DPCM)


Better StructureBetter Structure


Adaptive DPCM (ADPCM)Adaptive DPCM (ADPCM)IdeaIdea

Problem?Problem?

Adaptive DPCM (ADPCM)Adaptive DPCM (ADPCM)


Size of Quantization StepSize of Quantization Step

ADM: [ ] [ 1]n M n

12, 2P Q

1 if [ ] [ 1]

1 if [ ] [ 1]

M P c n c n

M Q c n c n


Speech Compression ConceptsSpeech Compression ConceptsSpectrogram, STFTSpectrogram, STFT

3D surface spectrogram of a part from a music piece.3D surface spectrogram of a part from a music piece.

Speech Compression ConceptsSpeech Compression Concepts


SpectrogramSpectrogram

Spectrogram of a male voice saying ‘nineteenth century’.Spectrogram of a male voice saying ‘nineteenth century’.



Spectrogram, DemonstrationSpectrogram, Demonstration

Bat Echolocation CallBat Echolocation Call Flute by Jean Pierre RampalFlute by Jean Pierre Rampal

Singing VoiceSinging Voice Face!Face!



FormantFormant


Linear Predictive Coding (LPC)Linear Predictive Coding (LPC)ModelingModeling

Linear Predictive Coding (LPC)Linear Predictive Coding (LPC)


Modeling (Hiss or Buzz)Modeling (Hiss or Buzz)

1

[ ] [ ]P

ii

x n a x n i

Predictor for each frame:Predictor for each frame:

Buzzer Buzzer Filter Filter

Speech = Formants + ResidueSpeech = Formants + Residue

Chuncks: 30 thr. 50 frames/sec.Chuncks: 30 thr. 50 frames/sec.

Linear Predictive Coding (LPC)Linear Predictive Coding (LPC)


Modeling (Hiss or Buzz)Modeling (Hiss or Buzz)


Code Excited Linear PredictionCode Excited Linear PredictionCELPCELP

Problem of LPC– Where there is both Hiss and Buzz

Solution– Encode residue

Method– Vector Quantization (Codebook)


ComparisonComparisonSample SpeechSample Speech

A lathe is a big tool. Grab every dish of sugar.A lathe is a big tool. Grab every dish of sugar.

ComparisonComparison


DemonstrationDemonstration

OriginalOriginal ADPCMADPCM

LPCLPC CELPCELP


Speech Coding BasicsSpeech Coding Basics

Thank You

FIND OUT MORE AT...

1. http://ce.sharif.edu/~m_amiri/

2. http://www.aictct.ir/dml/

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Definition ofDefinition ofVanishing Percentage (VP)Vanishing Percentage (VP)

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Speech Coding Basics