AMR-WB/NB

[kiah2008]

https://blog.csdn.net/ansondroider/article/details/115525041

amr-wb 发现mediaformat change 为单声道， 如何支持双声道

[kiah2008]

rfc3267,4867

传输反馈 码率调节 cmr codec mode request

[kiah2008]

https://en.wikipedia.org/wiki/Adaptive_Multi-Rate_Wideband

AMR-WB operates, like [AMR](https://en.wikipedia.org/wiki/Adaptive_Multi-Rate), with nine different bit rates. The lowest bit rate providing excellent speech quality in a clean environment is 12.65 kbit/s. Higher bit rates are useful in background noise conditions and for music. Also, lower bit rates of 6.60 and 8.85 kbit/s provide reasonable quality, especially when compared to narrow-band codecs.

The frequencies from 6.4 kHz to 7 kHz are only transmitted in the highest bitrate mode (23.85 kbit/s), while in the rest of the modes the decoder generates sounds by using the lower frequency data (75–6400 Hz) along with random noise (in order to simulate the high frequency band).[[8]](https://en.wikipedia.org/wiki/Adaptive_Multi-Rate_Wideband#cite_note-8)

All modes are sampled at 16 kHz (using 14-bit resolution) and processed at 12.8 kHz.

The bit rates are the following:

Mandatory multi-rate configuration
6.60 kbit/s (used for circuit switched [GSM](https://en.wikipedia.org/wiki/GSM) and [UMTS](https://en.wikipedia.org/wiki/UMTS) connections; should only be used temporarily during bad radio connections and is not considered [wideband](https://en.wikipedia.org/wiki/Wideband_audio) speech)
8.85 kbit/s (used for circuit switched GSM and UMTS connections; should only be used temporarily during bad radio connections and is not considered [wideband](https://en.wikipedia.org/wiki/Wideband_audio) speech; provides quality equal to [G.722](https://en.wikipedia.org/wiki/G.722) at 48 kbit/s for clean speech)
12.65 kbit/s (main anchor bitrate; used for circuit switched GSM and UMTS connections; offers superior audio quality to AMR at and above this bit rate; provides quality equal to or better than G722 at 56 kbit/s for clean speech)
Higher bitrates for speech in adverse background noise environments, combined speech and music, and multi-party conferencing.
14.25 kbit/s
15.85 kbit/s
18.25 kbit/s
19.85 kbit/s
23.05 kbit/s (not targeted for full-rate GSM channels)
23.85 kbit/s (provides quality equal to G.722 at 64 kbit/s for clean speech; not targeted for full-rate GSM channels)