AMR bitrates
============

AMR has 8 distinct bitrates that it can switch between.
In GSM, only up to four rates can be selected; they are limited by the S bits.

In SDP:
-------

  m=audio 16010 RTP/AVP 112
  a=fmtp:112 mode-set=0,2,4,7

where

  7 = 12k20
  6 = 10k20
  5 = 7k95
  4 = 7k40
  3 = 6k70
  2 = 5k90
  1 = 5k15
  0 = 4k75

On A-interface:
---------------

"S-bits", S0 to S15, where each Sn defines a set of bitrates that are enabled.
For example, S0 means: allow only 4k75.

A particular S bit, S1, has a special meaning: when on a half-rate channel,
exclude 12k2 implicitly.

3GPP TS 28.062, Table 7.11.3.1.3-2

        S0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
  12.20    (x)                x                    x  x
  10.20                    x                 x  x
  7.95                  x                          x  x
  7.40      x        x                 x  x
  6.70            x                 x  x  x  x  x
  5.90      x  x                 x  x  x  x  x  x  x  x
  5.15
  4.75   x  x                    x  x  x  x  x  x  x  x

  OM     F  F  F  F  F  F  F  F  F  F  F  A  F  A  F  A

  HR     Y  Y  Y  Y  Y  Y        Y  Y  Y
  FR     Y  Y  Y  Y  Y  Y  Y  Y  Y  Y  Y  Y  Y  Y  Y  Y

The immediate question is, are the S0-15 bits a combinatory bitmask,
or a list of distinct rate selections?

Explained in an example:

Say S = 0x18 = S7 | S8

If it is combinatory, the SDP equivalent is a single resulting payload type:

  m=audio 1234 RTP/AVP 96
  a=fmtp:96 mode-set=0,2,7

If a list of distinct selections, the SDP equivalent is two distinct payload
types:

  m=audio 1234 RTP/AVP 96 97
  a=fmtp:96 mode-set=7
  a=fmtp:97 mode-set=0,2