Lecture 15_fall_2010_6tp

Lecture 15_fall_2010_6tp - Analog-toAnalog-to-Digital...

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1 Digital Speech Processing Digital Speech Processing— Lecture 15 1 Speech Coding Methods Based on Speech Waveform Representations and Speech Models Models—Uniform and Non Uniform and Non- Uniform Coding Methods Analog Analog-to to-Digital Conversion Digital Conversion (Sampling and Quantization) 2 Class of “waveform coders” can be represented in this manner Information Rate , () Waveform coder information rate, of the digital representation of the signal, , defined as: w a I xt i 3 / 1 , where is the number of bits used to represent each sample and is the numbe wS S IB FB T B F T =⋅ = = r of samples/second Speech Information Rates Production level : – 10-15 phonemes/second for continuous speech – 32-64 phonemes per language => 6 bits/phoneme Information Rate =60-90 bps at the source Waveform level 4 – speech bandwidth from 4 – 10 kHz => sampling rate from 8 – 20 kHz – need 12-16 bit quantization for high quality digital coding Information Rate =96-320 Kbps => more than 3 orders of magnitude difference in Information Rates between the production and waveform levels Speech Analysis/Synthesis Systems 5 • Second class of digital speech coding systems: o analysis/synthesis systems o model-based systems o hybrid coders o vocoder (voice coder) systems • Detailed waveform properties generally not preserved o coder estimates parameters of a model for speech production o coder tries to preserve intelligibility and quality of reproduction from the digital representation Speech Coder Comparisons • Speech parameters (the chosen representation) are encoded for transmission or storage • analysis and encoding gives a data parameter vector data parameter vector computed at a sampling rate 6 • data parameter vector computed at a sampling rate much lower than the signal sampling rate • denote the “frame rate” of the analysis as F fr • total information rate for model-based coders is: • where B c is the total number of bits required to represent the parameter vector mc f r I BF =
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2 Speech Coder Comparisons 7 • waveform coders characterized by: • high bit rates (24 Kbps – 1 Mbps) •low complex ity • low flexibility • analysis/synthesis systems characterized by: • low bit rates (10 Kbps – 600 bps) • high complexity • great flexibility (e.g., time expansion/compression) Introduction to Waveform Coding 8 12 () ( ) ( ) ( ) ω π Ω =−∞ =−∞ = + =Ω ⇒ = + a jT a k j a k xn x nT jk Xe X j TT j TX e X T Introduction to Waveform Coding T=1/ F s 9 •to perfectly recover x a (t) (or equivalently a lowpass filtered version of it) from the set of digital samples (as yet unquantized) we require that F s = 1/T > twice the highest frequency in the input signal • this implies that x a (t) must first be lowpass filtered since speech is not inherently lowpass ¾ for telephone bandwidth the frequency range of interest is 200-3200 Hz (filtering range) => F s = 6400 Hz, 8000 Hz ¾ for wideband speech the frequency range of interest is 100-7000 Hz (filtering range) => F s = 16000 Hz Sampling Speech Sounds 10
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This note was uploaded on 12/29/2011 for the course ECE 259 taught by Professor Rabiner,l during the Fall '08 term at UCSB.

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Lecture 15_fall_2010_6tp - Analog-toAnalog-to-Digital...

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