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IEEE Speech Coding Workshop Sept 17–20, 2000 Lake Lawn Resort Delavan, WI

IEEE Speech Coding Workshop Sept 17–20, 2000 Lake Lawn Resort Delavan, WI Jean-Marc Valin, Roch Lefebvre University of Sherbrooke Bandwidth Extension of Narrowband Speech for Low Bit-Rate Wideband Coding. Outline. Problem statement Proposed solution System performance Discussion.

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IEEE Speech Coding Workshop Sept 17–20, 2000 Lake Lawn Resort Delavan, WI

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  1. IEEE Speech Coding Workshop Sept 17–20, 2000 Lake Lawn Resort Delavan, WI Jean-Marc Valin, Roch Lefebvre University of Sherbrooke Bandwidth Extension of Narrowband Speech for Low Bit-Rate Wideband Coding

  2. Outline • Problem statement • Proposed solution • System performance • Discussion

  3. Problem Statement • Telephone Band: 300 - 3400 Hz • AM Band: 50 - 7000 Hz • How to make sound like with 500 bits/sec? • We need to recover information from both low and high-frequency bands (G.729)

  4. Proposed Solution • 1) Do our best to recover the wideband information from narrowband speech • 2) Use coding for the information that cannot be recovered • Recovered information : • Low-frequency band • High-frequency excitation • Coded information : • High-frequency spectral envelope

  5. Low-frequency regeneration Inverse IRM Filter 2 High-frequency regeneration System Overview • Inverse IRM filter is optional • produces a flat response from 200-3500 Hz 50-300 Hz band 8 kHz narrowband 16 kHz wideband 300-3400 Hz band 3400-8000 Hz band Side information

  6. Low-Frequency Regeneration (1/2) • Assumptions : • Only pitch harmonics need to be recovered • In general, no more than two pitch harmonics below 200 Hz • Absolute phase is not perceptually relevant • Frequency of harmonics determined from pitch analysis • Amplitudes determined from feed-forward multi-layer perceptron (output in log domain)

  7. 2 LP filter Low-Frequency Regeneration (2/2) Low frequencies 1st harmonic Low-frequency harmonic synthesis Scale and sum 2nd harmonic Narrowband speech Pitch delay (1) Pitch analysis Pitch gain Multi-layer Perceptron (1) (16) Scale factors MFCC calculation Cepstral coefficients

  8. 1 B(z) High pass High-Frequency Extension • Excitation-filter model (16 ms frames) • Problem is separated in two parts • Excitation extension (no side information) • Spectral envelope coding (side information) Narrowband speech High- frequency band Excitation extension A(z) Spectral envelope Extension LPC analysis Side information (High-frequency spectral envelope)

  9. 2 Absolute value Narrowband excitation wideband excitation Whitening filter High pass Excitation Extension

  10. Spectral Envelope Coding • Spectral envelope calculated from the wideband LPC coefficients • Quantization of the 3000-8000 Hz range (40 points) • Log domain • 8-bit Vector Quantization (500 bits/s side information, using 16 ms frames) • Concatenation with envelope obtained from LPC analysis on narrowband speech

  11. Objective results • Low-frequency band • 3 dB RMS error on harmonic amplitude • High-frequency band • 3.6 dB RMS error on envelope • No objective measure for excitation extension (perceptually very close to original)

  12. Subjective Results female male Original wideband Recovered from original IRM-filtered speech Recovered from G.729 coded speech

  13. Discussion • Highlights • Expand IRM-filtered telephone-band speech to AM band • Very low side information rate (500 bits/s) • Areas of improvement • Use high-band spectral estimation before coding • Use residual low-frequency information (below 300 Hz) • Noise robustness • Post-filtering

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