Add AMR and AMR-WB codec modules supporting transcoding
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To add a codec for SIP/SDP (m=, rtmap, and ftmp), you create a format
module in Asterisk: `codec_amr.patch` (for m= and rtmap) and
`res/res_format_attr_amr.c` (for fmtp). However, this requires both
call legs to support AMR (pass-through only). If one leg does not
support AMR, the call has no audio. Or, if you use the pre-recorded
voice and music files of Asterisk, these files cannot be heard, because
they are not in AMR but in slin. Therefore, this repository adds not
just a format module for the audio-codecs AMR and AMR-WB but a
transcoding module as well: `codecs/codec_amr.c`.
This is an implementation of IETF
[RFC 4867](http://tools.ietf.org/html/rfc4867). Sometimes, AMR is
called AMR Narrowband (AMR-NB). AMR Wideband (ITU-T Recommendation
G.722.2) is sometimes abbreviated W-AMR
([GSA](http://www.gsacom.com/hdvoice/)). GSMA Mobile
is AMR-WB. Research papers comparing AMR and AMR-WB with other audio
Gbp-Pq: Name amr.patch