70 lines
1.6 KiB
Plaintext
70 lines
1.6 KiB
Plaintext
AMR bitrates
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============
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AMR has 8 distinct bitrates that it can switch between.
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In GSM, only up to four rates can be selected; they are limited by the S bits.
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In SDP:
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-------
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m=audio 16010 RTP/AVP 112
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a=fmtp:112 mode-set=0,2,4,7
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where
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7 = 12k20
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6 = 10k20
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5 = 7k95
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4 = 7k40
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3 = 6k70
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2 = 5k90
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1 = 5k15
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0 = 4k75
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On A-interface:
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---------------
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"S-bits", S0 to S15, where each Sn defines a set of bitrates that are enabled.
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For example, S0 means: allow only 4k75.
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A particular S bit, S1, has a special meaning: when on a half-rate channel,
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exclude 12k2 implicitly.
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3GPP TS 28.062, Table 7.11.3.1.3-2
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S0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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12.20 (x) x x x
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10.20 x x x
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7.95 x x x
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7.40 x x x x
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6.70 x x x x x x
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5.90 x x x x x x x x x x
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5.15
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4.75 x x x x x x x x x x
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OM F F F F F F F F F F F A F A F A
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HR Y Y Y Y Y Y Y Y Y
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FR Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
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The immediate question is, are the S0-15 bits a combinatory bitmask,
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or a list of distinct rate selections?
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Explained in an example:
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Say S = 0x18 = S7 | S8
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If it is combinatory, the SDP equivalent is a single resulting payload type:
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m=audio 1234 RTP/AVP 96
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a=fmtp:96 mode-set=0,2,7
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If a list of distinct selections, the SDP equivalent is two distinct payload
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types:
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m=audio 1234 RTP/AVP 96 97
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a=fmtp:96 mode-set=7
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a=fmtp:97 mode-set=0,2
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