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Lecture 17_winter_2012_6tp

Lecture 17_winter_2012_6tp - Waveform Coding versus Block...

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1 1 Digital Speech Processing— Lecture 17 Speech Coding Methods Based on Speech Models 2 Waveform Coding versus Block Processing Waveform coding – sample-by-sample matching of waveforms – coding quality measured using SNR Source modeling (block processing) – block processing of signal => vector of outputs every block – overlapped blocks Block 1 Block 2 Block 3 3 Model-Based Speech Coding we’ve carried waveform coding based on optimizing and maximizing SNR about as far as possible achieved bit rate reductions on the order of 4:1 (i.e., from 128 Kbps PCM to 32 Kbps ADPCM) at the same time achieving toll quality SNR for telephone-bandwidth speech to lower bit rate further without reducing speech quality, we need to exploit features of the speech production model, including: source modeling spectrum modeling use of codebook methods for coding efficiency we also need a new way of comparing performance of different waveform and model-based coding methods an objective measure, like SNR , isn’t an appropriate measure for model- based coders since they operate on blocks of speech and don’t follow the waveform on a sample-by-sample basis new subjective measures need to be used that measure user-perceived quality, intelligibility, and robustness to multiple factors 4 Topics Covered in this Lecture Enhancements for ADPCM Coders pitch prediction noise shaping Analysis-by-Synthesis Speech Coders multipulse linear prediction coder (MPLPC) code-excited linear prediction (CELP) Open-Loop Speech Coders two-state excitation model LPC vocoder residual-excited linear predictive coder mixed excitation systems speech coding quality measures - MOS speech coding standards 5 ] [ n x ] [ n d ] [ ˆ n d ] [ n c ] [ ~ n x ] [ ˆ n x Differential Quantization = = p k k z z P 1 ) ( ] [ n c ] [ ˆ n d ] [ ˆ n x ] [ ~ n x P : simple predictor of vocal tract response 6 Issues with Differential Quantization difference signal retains the character of the excitation signal – switches back and forth between quasi- periodic and noise-like signals prediction duration (even when using p =20) is order of 2.5 msec (for sampling rate of 8 kHz) – predictor is predicting vocal tract response – not the excitation period (for voiced sounds) Solution – incorporate two stages of prediction, namely a short-time predictor for the vocal tract response and a long- time predictor for pitch period
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