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doc/AMR-library-API: describe the decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 21:32:31 +0000
parents 019eed8b1948
children 516e84085a15
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1 The first Themyscira library for GSM-FR speech codec was libgsmfrp, an add-on
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2 to legacy libgsm from TU-Berlin. The current offering is libgsmfr2, which
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3 supplants the earlier combination of libgsm+libgsmfrp. This document details
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4 the change history in this continuum of GSM-FR codec libraries.
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6 Changes from libgsmfrp version 1.0.2 to libgsmfr2 version 2.0.0
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7 ===============================================================
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9 * Dependency on <gsm.h> defined types abolished, the entire library uses
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10 <stdint.h> types instead.
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12 * The Rx DTX handler component of the new library is unchanged from libgsmfrp
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13 version 1.0.2, aside from the use of new types: uint8_t instead of gsm_byte,
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14 explicit arrays of uint8_t instead of gsm_frame.
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16 * In addition to this Rx DTX handler component, the new library includes the
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17 GSM 06.10 encoder & decoder component (ported from libgsm pl22), the new
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18 full decoder wrapper with decoder homing, an encoder homing function, and
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19 new stateless frame packing and unpacking functions.
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21 libgsmfrp change history: version 1.0.1 to version 1.0.2
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22 ========================================================
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24 There are only two changes, both involving corner cases with invalid SID frames
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25 being received:
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27 1) An invalid SID frame was received immediately following a good speech frame.
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28 In this case we start CN generation, but we take the needed LARc and Xmaxc
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29 parameters from the last speech frame, instead of the usual procedure of
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30 extracting them from a valid SID frame. The change from 1.0.1 to 1.0.2
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31 concerns the Xmaxc parameter in this corner case: in 1.0.1 we took Xmaxc
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32 from the last subframe and used it for ensuing CN generation, but in 1.0.2
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33 we compute a more proper mean Xmaxc from all 4 subframes, by dequantizing,
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34 summing and requantizing.
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36 2) An invalid SID frame was received in the speech muting state. The sequence
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37 of inputs would have to be:
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39 - a good speech frame;
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40 - one or more BFIs, but not too many, so that the cached speech frame
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41 does not decay fully by Xmaxc reduction;
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42 - an invalid SID frame.
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44 In version 1.0.1 we handled this even more obscure corner case by entering
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45 the CN muting state, i.e., the state that is normally entered upon the
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46 second lost SID. In version 1.0.2 we ignore invalid SID in the speech
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47 muting state and act as if we got BFI, i.e., continue speech muting rather
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48 than switch to CN muting.
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50 libgsmfrp change history: version 1.0.0 to version 1.0.1
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51 ========================================================
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53 Version 1.0.0 exhibited the following defects, which are fixed in 1.0.1:
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55 1) The last received valid SID was cached forever for the purpose of
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56 handling future invalid SIDs - we could have received some valid
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57 SID ages ago, then lots of speech or NO_DATA, and if we then get
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58 an invalid SID, we would resurrect the last valid SID from ancient
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59 history - a bad design. In our new design, we handle invalid SID
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60 based on the current state, much like BFI.
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62 2) GSM 06.11 spec says clearly that after the second lost SID
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63 (received BFI=1 && TAF=1 in CN state) we need to gradually decrease
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64 the output level, rather than jump directly to emitting silence
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65 frames - we previously failed to implement such logic.
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67 3) Per GSM 06.12 section 5.2, Xmaxc should be the same in all 4 subframes
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68 in a SID frame. What should we do if we receive an otherwise valid
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69 SID frame with different Xmaxc? Our previous approach would
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70 replicate this Xmaxc oddity in every subsequent generated CN frame,
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71 which is rather bad. In our new design, the very first CN frame
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72 (which can be seen as a transformation of the SID frame itself)
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73 retains the original 4 distinct Xmaxc, but all subsequent CN frames
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74 are based on the Xmaxc from the last subframe of the most recent SID.