FreeCalypso > hg > gsm-codec-lib
comparison doc/Calypso-TCH-downlink @ 136:8eb0e7a39409
doc: document command line utilities
| author | Mychaela Falconia <falcon@freecalypso.org> |
|---|---|
| date | Sun, 11 Dec 2022 22:20:36 +0000 |
| parents | |
| children | 486e19acc66f |
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| 135:22601ae99434 | 136:8eb0e7a39409 |
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| 1 It has been discovered that the DSP ROM in the Calypso GSM baseband processor | |
| 2 implements one nifty feature which is not used at all in standard phone or modem | |
| 3 operation, but which can be used for all kinds of interesting hacks: the traffic | |
| 4 channel (TCH) bits coming out of the GSM 05.03 channel decoder in the downlink | |
| 5 direction (to be fed to one of several speech decoders) can be read out of the | |
| 6 DSP's API RAM in real time, and in the uplink direction the user can feed her | |
| 7 own bits to the input of the GSM 05.03 channel encoder, effectively suppressing | |
| 8 the output of the internal vocoder. | |
| 9 | |
| 10 I (Mother Mychaela) previously played with this functionality back in 2016, and | |
| 11 it's been mostly shelved since then. This functionality is now getting renewed | |
| 12 attention in late 2022: now that we have a proper set of codec libraries (the | |
| 13 present package) and a proper understanding of Rx DTX handling requirements, we | |
| 14 can take another shot at decoding TCH downlink captures taken from Calypso GSM | |
| 15 MS. | |
| 16 | |
| 17 For reasons of project resource planning, the work to study Calypso DSP downlink | |
| 18 bits in EFR mode is currently postponed until after we build our long-overdue FC | |
| 19 Venus board, but in the case of FR1 codec, we have a TCH downlink recording that | |
| 20 was captured back in 2016, and by analyzing this recording, we can make the | |
| 21 following observations: | |
| 22 | |
| 23 * DSP status word 0 contains various flags; we already knew some of them, but | |
| 24 now we got additional knowledge: | |
| 25 | |
| 26 bit 2 is BFI | |
| 27 bits 4:3 give a 2-bit SID flag per GSM 06.31 section 6.1.1 | |
| 28 | |
| 29 Unfortunately I couldn't find TAF anywhere - but oh well. | |
| 30 | |
| 31 * DSP status word 1 appears to be garbage, DSP status word 2 appears to be some | |
| 32 kind of error count - no new knowledge here. | |
| 33 | |
| 34 * By heeding the newly-understood BFI flag in DSP status word 0, we can do a | |
| 35 much cleaner decoding of TCH downlink capture than we did back in 2016. | |
| 36 | |
| 37 The new gsmfr-cvt-dlcap utility in the present package replaces the old | |
| 38 fc-tch2fr utility from FC host tools - the latter should now be considered a | |
| 39 bogon. The output from gsmfr-cvt-dlcap is in our gsmx format, containing | |
| 40 Themyscira BFI markers in those frame positions where Calypso DSP indicated BFI | |
| 41 on TCH downlink, and it needs to be fed to gsmfr-decode from the present | |
| 42 package; our FR1 Rx DTX preprocessor will take care of SIDs and BFIs, and the | |
| 43 complete decoding chain should then closely replicate what must be happening | |
| 44 inside the original DSP. |