1. A Very Low Bit Rate Protection Layer to Increase the Robustness of the AMR- WB+ Codec against Bit Errors Philippe Gournay Université de Sherbrooke Département de Génie Electrique et Informatique Sherbrooke, Québec, Canada

2. Outline Context The AMR-WB+ Codec Bit Sensitivity Study The Protection Layer Evaluation Results Audio Demonstration Conclusion

3. Context Challenging Applications for AMR-WB+ – Streaming and Multimedia Messaging in GSM, DVB-H Mobile TV, Digital Radio Channel Impairments – Lost Packets Concealment Procedure – Bit Errors Channel Coding (Forward Error Correction) Inherent Robustness of the codec Additional Protection Layer

4. Overview of the AMR-WB+ codec Speech and Music Mono and Stereo Inputs Wide Range of Audio Bandwidth – From 8 to 48 kHz Wide Range of Bit Rate – 6 to 36 kbps in mono, 7 to 48 kbps in stereo Hybrid Coding Model – Time (ACELP) and Frequency (TCX) Domain

5. The AMR-WB+ Encoder Mode Selection PACKETIZATION Bitstream ACELP TCX Audio 1, 2 or 4 frames 1 frame Mode Index, ISF

6. AMR-WB+ Frame Structure Three out of the 26 possible ACELP/TCX coding configurations ACELP Short TCXACELPMedium TCX Long TCX One super-frame = 80 ms @ 12.8 kHz (a) (b) (c)

7. AMR-WB+ Embedded Bitstream Structure Core Mode Core Bitstream (ACELP or TCX) BWE (mono/right) BWE (left) Low-band Mid-band Stereo Mode Stereo extension Core codec For medium and long TCX, the packetization procedure is responsible for distributing the bitstream among the required number of packets.

8. Bit Sensitivity Study Determine Individual Bit Sensitivities – Prior Knowledge (ACELP, AMR-WB Standard) – SNR Measurements Bit Sensitivity Classes – Class A: Highly sensitive – Class B: Moderately Sensitive – Class C: Not Sensitive Useful for U.E.P. (Unequal Error Protection)

9. 50100150200 0 20 40 60 80 100 ACELP SNR (dB) Bit position Segmental SNR (8.67 kbps mono) 50100150200 0 20 40 60 80 100 TCX20 100200300400 0 20 40 60 80 100 SNR (dB) TCX40 Bit position 200400600800 0 20 40 60 80 100 TCX80

10. The Protection Layer Additional Protection Layer – Standard AMR-WB+ Payload Kept Unchanged Very Low Bit Rate – 16 bits per frame (480 bits @ 32 kbps stereo) Performs Error Detection and/or Correction – Class A: Error Detection and Correction – Class B: Error Detection – Class C: Nothing Depends on the Core and Stereo Modes

11. Core Protection Layer (1) ACELPShort TCX Medium TCX Packet 1Packet 2 0 Mode redundancy 1 2 3 4Mean energyGlobal gain 3rd ISP subvector 51 st 2 nd ISP subvect. 1st ISP subvector 6Pitch SF1 7Gain SF1 8Pitch SF2 9Gain SF2 First 8 bits of AVQ codebook numbers 10Pitch SF3 3rd ISP subvector2nd ISP subvector 11Gain SF3 12Pitch SF4unused 13Gain SF4unused

12. Core Protection Layer (2) Long TCX Packet 1Packet 2Packet 3Packet 4 0 Mode redundancy 1 2 3 4Global gain 3rd ISP subvector First 8 bits of AVQ codebook numbers 5 1st ISP subvector 6 7 8 First 8 bits of AVQ codebook numbers Next 8 bits of AVQ codebook numbers 9 10 2nd ISP subvector 11 12unused 13unused

13. Stereo Extension Protection Layer Mode 0Mode 1 Mode 2 Packet 1Packet 2 14Mode ctrl. 15Bal. & gain BalanceGain Stereo mode 3 Packet 1Packet 2Packet 3Packet 4 14Mode ctrl. 15BalanceunusedGainunused

14. Subjective Evaluation (32 kbps stereo) Note: bit errors are also applied to the protection layer.

15. Audio Demonstration AMR-WB+ @ 32 kbps stereo – Mode Index=40, ISF=1.333 Original File Clear Channel Standard Codec, 0.5% BER Modified Codec, 0.5% BER

16. Audio Demonstration AMR-WB+ @ 32 kbps stereo – Mode Index=40, ISF=1.333 Original File Clear Channel Standard Codec, 0.5% BER Modified Codec, 0.5% BER

17. Audio Demonstration AMR-WB+ @ 32 kbps stereo – Mode Index=40, ISF=1.333 Original File Clear Channel Standard Codec, 0.5% BER Modified Codec, 0.5% BER

18. Audio Demonstration AMR-WB+ @ 32 kbps stereo – Mode Index=40, ISF=1.333 Original File Clear Channel Standard Codec, 0.5% BER Modified Codec, 0.5% BER

19. Conclusion AMR-WB+ Bit Sensitivity Study – Sensitivity Classes A, B and C – Useful for U.E.P. Additional Very Low Bit Rate Protection Layer – 16 bits per frame – Error Detection and Correction – Enhance Robustness against Bit Errors Significant Subjective Quality Improvement – Decoded Audio is Free of Major Artifacts at a Significant 0.5% BER