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Harald Welte laforge at gnumonks.orgHi Neels, in terms of the actual payload, I believe (IIRC), that AMR over IuUP used three sub-flows. They are encoded after each other, see TS 25.415 6.6.3.27 / Figure 27a for an illustration. Thse SDUs of each sub-flow contain the bits of one class. The rationale for teh above lies in the structure of the RAB and the channel bundles on the Iu radio layer. Both on the radio and on the IuFP side between NodeB and RNC, there are actually three independent streams of bits, one for each of the classes. For IuUP, it seems they are then re-combined in the RNC into one IuUP payload, but still the three separate chunks accordign to their class. In order to get the regular RTP-AMR payload, they need to be mixed/mangled into the order specified by whatever RFC specifies the AMR payload format, I think it as https://tools.ietf.org/html/rfc3267 - whcih then refers to the IF1 format as specified in 3GPP TS 26.101, where IF1 is described in Section 4. The order of the fields here is "logical", so if there's a given parameter that has 6 bits, then those 6 bits are consecutive. In the air interface, you may want to transmit the first two (MSB) bits in class A, the middle two in class B and the last two bits in class C. This is so the most improtant bits for speech recovery are given higher amount of forward error correction than the less important bits. I believe the tables in Annex B of 26.101 is what's needed in terms of re-ordering. Still, there's plenty of things that can go wrong in terms of bit-order within a byte, byte ordering, ..., and hence I think it's good to start with writing functions and unit tests for this first. GAPK might be a good place for prototyping, as it already understands the concept of different representations/bit-orders/formats for one given codec, and it has support for playback via alsa, ... -- - Harald Welte <laforge at gnumonks.org> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)