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annotate doc/Calypso-TCH-downlink @ 166:500f3e93964f

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gsmfr-dlcap-gsmx: handle DSP flags consistently with EFR version
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 16 Dec 2022 08:23:32 +0000
parents 8eb0e7a39409
children 486e19acc66f
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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
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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
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5 direction (to be fed to one of several speech decoders) can be read out of the
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6 DSP's API RAM in real time, and in the uplink direction the user can feed her
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7 own bits to the input of the GSM 05.03 channel encoder, effectively suppressing
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8 the output of the internal vocoder.
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9
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10 I (Mother Mychaela) previously played with this functionality back in 2016, and
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11 it's been mostly shelved since then. This functionality is now getting renewed
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12 attention in late 2022: now that we have a proper set of codec libraries (the
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13 present package) and a proper understanding of Rx DTX handling requirements, we
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14 can take another shot at decoding TCH downlink captures taken from Calypso GSM
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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
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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
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20 was captured back in 2016, and by analyzing this recording, we can make the
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21 following observations:
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22
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23 * DSP status word 0 contains various flags; we already knew some of them, but
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24 now we got additional knowledge:
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25
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26 bit 2 is BFI
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27 bits 4:3 give a 2-bit SID flag per GSM 06.31 section 6.1.1
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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
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Mychaela Falconia <falcon@freecalypso.org>
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32 kind of error count - no new knowledge here.
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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
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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.
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36
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37 The new gsmfr-cvt-dlcap utility in the present package replaces the old
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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
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39 bogon. The output from gsmfr-cvt-dlcap is in our gsmx format, containing
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Mychaela Falconia <falcon@freecalypso.org>
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40 Themyscira BFI markers in those frame positions where Calypso DSP indicated BFI
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41 on TCH downlink, and it needs to be fed to gsmfr-decode from the present
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42 package; our FR1 Rx DTX preprocessor will take care of SIDs and BFIs, and the
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43 complete decoding chain should then closely replicate what must be happening
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44 inside the original DSP.