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Sylvain Munaut 246tnt at gmail.comHi, > I am looking for a way to disable the vocoder for voice calls, i.e., > supply my own 260 bits every 20 ms to be sent in the uplink TCH > instead of the output from the vocoder in the Calypso DSP, and on the > downlink, receive the bits which would otherwise go into the vocoder. > > My question is: is it possible to do what I seek using the know-how of > Calypso DSP black magic that has already been amassed by the OsmocomBB > project? I am thinking of the following 3 possible starting points: > > 1. LCR integration: the mobile app can be configured to route voice > call audio to the Linux host instead of the phone's earpiece and > mic, right? At which point does it intercept the standard voice > path? Does it intercept right where I want it, passing raw > over-the-air TCH bits to the external host, such that Asterisk or > whatever has to run the GSM codec, or is the intercept happening at > the point of linear PCM samples, such that the uplink TCH bits are > still generated by the DSP black box? It's neither PCM audio, nor raw TCH bits. They're the raw codec data ( so 260 bits / 20 ms for FR codec ). I say they're not the raw TCH bits because the DSP is still doing all the channel coding / error correction and this process is codec specific. The bits will be re-ordered and some of them will be protected by ECC and some won't because the channel coding excpect to find GSM FR codec bits there. > 2. The burst_ind branch lets the Linux host see every burst that is > received on the downlink, right? It would therefore include TCH > bursts during voice calls, right? This way I should be able to > capture all of the raw TCH bits on the downlink - but what about > the uplink? > > 3. I've also read about the Calypso-as-BTS hack - way cool! In order > to work, this hack must support both receiving and transmitting > arbitrary bursts, right? If neither option 1 nor option 2 would > work, do you guys think the Calypso-as-BTS implementation code > would serve as a starting point for what I seek? I do need to run > the OsmocomBB phone in the standard MS role, not in the BTS role, > and I need to place voice calls on the network in the standard > manner - but with TCH rerouted to my own source and sink for raw > over-the-air bits. Both these method provide access to the raw bursts bits, but : 1) that means you need to do all the channel coding yourself 2) they screw so much with the DSP that it can't act as a phone at all anymore ( FCCH sync broken for instance ) Cheers, Sylvain