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Proposal for Adding Fixed Rate 2 kbps Vocoder to EVRC-B

Proposal for Adding Fixed Rate 2 kbps Vocoder to EVRC-B. Outline. EVRC-B Frame Type Classifications 2 kbps Vocoder Choices 2 kbps Vocoder Approach Encoder-Decoder Block Diagram Conclusions. EVRC-B Frame Type Classifications.

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Proposal for Adding Fixed Rate 2 kbps Vocoder to EVRC-B

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  1. Proposal forAdding Fixed Rate 2 kbps Vocoder to EVRC-B

  2. Outline • EVRC-B Frame Type Classifications • 2 kbps Vocoder Choices • 2 kbps Vocoder Approach • Encoder-Decoder Block Diagram • Conclusions

  3. EVRC-B Frame Type Classifications • In EVRC-B Vocoder, a 20 ms speech frame is classified into following frame types: • Silence Eighth-rate (16 bits) vocoder to transmit the background noise • Unvoiced Quarter rate NELP(Noise Excited Linear Prediction) (40 bits) • Voiced Quarter (40 bits) and Full rate (171 bits) PPP (Prototype Pitch Period) or half rate CELP (Codebook Excited Linear Prediction) • Up-transient Transition between voiced and unvoiced segments Full/half rate (171 bits) CELP • Transient Usually in the middle or towards the end of the voiced segment, where pitch is changing Full/half rate (171 bits) CELP • Down-transient Transition from voiced to unvoiced segment Half rate (80 bits) CELP

  4. 2 kbps Vocoder Choices • Not many choices! • Quarter rate NELP for unvoiced and may be for down transient frames • Only quarter rate PPP available for up-transient, voiced and transient frames • Can work with some changes, like non-differential encoding of pitch and prototype waveform parameters

  5. 2 kbps Vocoder Choices (cont.) • QPPP is ideally suited for purely voiced frame • Zero bits on the phase of the prototype waveform • QPPP can cope with gradual change of pitch over a voiced segment • Cannot handle large pitch change • Use of QPPP for coding all the transient frames result in poor quality

  6. 2 kbps Vocoder Approach • Need a new coding scheme to encode the transient frames at 2 kbps • The other areas that need to be worked or improved in order to have a good quality 2 kbps vocoder • Pitch estimation: Re-visit range and accuracy • Revisit QPPP to make it more robust in the absence of FPPP • Fine tuning of mode decision algorithm

  7. Top Level Changes • FCELP, HCELP or FPPP not used • New scheme for coding transient frames at 2 kbps • Enhanced pitch estimation • QPPP for voiced frame with modifications like non-differential quantization • Mode-decision tuning

  8. Decoder Block Diagram

  9. Encoder Block Diagram Green: unchanged blocks Blue: modified blocks Yellow: new block

  10. Conclusions • There is a need for a new scheme to encode transient frame at quarter rate • There is a need to improve the range and accuracy of pitch estimation • There is need to make some of the quantization in QPPP non-differential • There is a need to enhance the mode decision

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