FreeCalypso > hg > gsm-codec-lib
annotate doc/Calypso-TCH-downlink @ 139:be57e06bed84
factor out common part of gsmfr-cvt-dlcap, in prep for EFR
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Tue, 13 Dec 2022 07:03:55 +0000 |
parents | 8eb0e7a39409 |
children | 486e19acc66f |
rev | line source |
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8eb0e7a39409
doc: document command line utilities
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1 It has been discovered that the DSP ROM in the Calypso GSM baseband processor |
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2 implements one nifty feature which is not used at all in standard phone or modem |
8eb0e7a39409
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3 operation, but which can be used for all kinds of interesting hacks: the traffic |
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4 channel (TCH) bits coming out of the GSM 05.03 channel decoder in the downlink |
8eb0e7a39409
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5 direction (to be fed to one of several speech decoders) can be read out of the |
8eb0e7a39409
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6 DSP's API RAM in real time, and in the uplink direction the user can feed her |
8eb0e7a39409
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7 own bits to the input of the GSM 05.03 channel encoder, effectively suppressing |
8eb0e7a39409
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8 the output of the internal vocoder. |
8eb0e7a39409
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9 |
8eb0e7a39409
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10 I (Mother Mychaela) previously played with this functionality back in 2016, and |
8eb0e7a39409
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11 it's been mostly shelved since then. This functionality is now getting renewed |
8eb0e7a39409
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12 attention in late 2022: now that we have a proper set of codec libraries (the |
8eb0e7a39409
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13 present package) and a proper understanding of Rx DTX handling requirements, we |
8eb0e7a39409
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14 can take another shot at decoding TCH downlink captures taken from Calypso GSM |
8eb0e7a39409
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15 MS. |
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16 |
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17 For reasons of project resource planning, the work to study Calypso DSP downlink |
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18 bits in EFR mode is currently postponed until after we build our long-overdue FC |
8eb0e7a39409
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Mychaela Falconia <falcon@freecalypso.org>
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19 Venus board, but in the case of FR1 codec, we have a TCH downlink recording that |
8eb0e7a39409
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20 was captured back in 2016, and by analyzing this recording, we can make the |
8eb0e7a39409
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21 following observations: |
8eb0e7a39409
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22 |
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23 * DSP status word 0 contains various flags; we already knew some of them, but |
8eb0e7a39409
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24 now we got additional knowledge: |
8eb0e7a39409
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25 |
8eb0e7a39409
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26 bit 2 is BFI |
8eb0e7a39409
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27 bits 4:3 give a 2-bit SID flag per GSM 06.31 section 6.1.1 |
8eb0e7a39409
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28 |
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29 Unfortunately I couldn't find TAF anywhere - but oh well. |
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30 |
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31 * DSP status word 1 appears to be garbage, DSP status word 2 appears to be some |
8eb0e7a39409
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32 kind of error count - no new knowledge here. |
8eb0e7a39409
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33 |
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34 * By heeding the newly-understood BFI flag in DSP status word 0, we can do a |
8eb0e7a39409
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Mychaela Falconia <falcon@freecalypso.org>
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35 much cleaner decoding of TCH downlink capture than we did back in 2016. |
8eb0e7a39409
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36 |
8eb0e7a39409
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37 The new gsmfr-cvt-dlcap utility in the present package replaces the old |
8eb0e7a39409
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Mychaela Falconia <falcon@freecalypso.org>
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38 fc-tch2fr utility from FC host tools - the latter should now be considered a |
8eb0e7a39409
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39 bogon. The output from gsmfr-cvt-dlcap is in our gsmx format, containing |
8eb0e7a39409
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40 Themyscira BFI markers in those frame positions where Calypso DSP indicated BFI |
8eb0e7a39409
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41 on TCH downlink, and it needs to be fed to gsmfr-decode from the present |
8eb0e7a39409
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Mychaela Falconia <falcon@freecalypso.org>
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42 package; our FR1 Rx DTX preprocessor will take care of SIDs and BFIs, and the |
8eb0e7a39409
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43 complete decoding chain should then closely replicate what must be happening |
8eb0e7a39409
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44 inside the original DSP. |