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doc/Codec-utils: add note about PCM file format issue
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 14 Dec 2022 07:25:46 +0000
parents 8eb0e7a39409
children 04936af99fc8
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8eb0e7a39409 doc: document command line utilities
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1 Standalone command line utilities for FR and EFR codecs
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2 =======================================================
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3
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4 The pre-existing FOSS opencore-amr package includes amrnb-enc and amrnb-dec test
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5 programs: the first reads linear PCM from a WAV file and emits AMR encoder
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6 output in a .amr file (RFC 4867 AMR storage format), the second reads this .amr
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7 format and emits AMR decoder output as WAV. Inspired by these simple test
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8 programs, the present package offers equivalent command line utilities for GSM
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9 FR and EFR codecs. Here they are:
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10
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11 gsmfr-encode This utility reads linear PCM from a WAV file, runs libgsm
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12 06.10 encoder and writes the output in the classic .gsm format
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13 (directly abutted FR codec frames of 33 bytes each). We don't
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14 currently have a Tx-side DTX implementation (VAD etc) for GSM
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15 FR, hence the output from gsmfr-encode will always consist of
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16 good speech frames only.
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17
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18 gsmfr-decode This utility reads our gsmx format (see Binary-file-format
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19 article), which is a superset of the classic libgsm format.
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20 The input to gsmfr-decode may be a pure .gsm recording as
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21 produced by gsmfr-encode or toast from libgsm package, or it
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22 can also contain SID frames and/or BFI markers. The processing
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23 performed by gsmfr-decode begins with our FR1 Rx DTX handler
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24 preprocessor, which will be an identity transform for pure .gsm
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25 input but becomes important for real-world input containing SIDs
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26 and BFIs, and is followed by gsm_decode() from libgsm. The
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27 decoded output is written as WAV.
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28
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29 gsmefr-encode This utility reads linear PCM from a WAV file, runs our EFR
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30 encoder (Themyscira libgsmefr) and writes the output in our gsmx
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31 format. There is an option to enable or disable DTX: -d enables
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32 DTX, otherwise it is disabled. (This option mirrors amrnb-enc.)
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33
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34 gsmefr-decode This utility reads our gsmx format (which must be EFR, not FR1)
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35 and feeds all frames and BFIs to our EFR decoder. The decoded
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36 output is written as WAV.
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37
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38 WAV reading and writing functions used by the above utilities have been lifted
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39 out of opencore-amrnb test code.
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41 Now that we are starting to encounter use cases that involve more than feeding
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42 a PC microphone recording to test encoders and playing decoder output on PC
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43 speakers, many of those other use cases call for using raw format instead of WAV
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44 for 16-bit linear PCM. While it would be philosophically most proper to add an
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45 option to gsm[e]fr-encode and gsm[e]fr-decode utilities to read/write raw PCM
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46 files instead of WAV, doing so would complicate the code too much. As a
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47 compromise between code simplicity and functionality, we provide lossless
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48 conversion utilities - see PCM-file-formats article.