FreeCalypso > hg > gsm-codec-lib
view doc/Binary-file-format @ 604:54f0f1b74c25
libgsmhr1 TFO: require BFI=0 and SID=0 for homing
In order for a received frame to be recognized as DHF, we need
not only the correct bit pattern, but also BFI=0 and SID=0.
The BFI=0 requirement should be obvious, while the SID=0 requirement
is needed only for HR codec. With FR and EFR, SID classification
comes from the payload bits and no separate check is needed -
but in HR we get an out-of-band SID ternary flag. When SID=1,
no payload bits are used at all; when SID=2, we use only the first
33 bits of the payload. Therefore, it is proper to conditionalize
DHF acceptance on SID=0.
We already implemented this logic in the just finished full decoder;
now bring TFO code into agreement.
| author | Mychaela Falconia <falcon@freecalypso.org> |
|---|---|
| date | Thu, 04 Dec 2025 19:40:35 +0000 |
| parents | f469bad44c0e |
| children |
line wrap: on
line source
We (Themyscira Wireless) define our own binary file format for testing of GSM 06.10 (FR) and EFR codec functions; this format of ours is an extension of classic .gsm format from libgsm/toast. The original libgsm file format is a directly abutted sequence of 33-byte libgsm frames, equivalent to RTP frames for GSM FR, with the upper nibble of the first byte in each frame equal to 0xD, serving as a signature. We simply extend this idea: our version is still a directly abutted sequence of binary records, but each record is now one of 3 possibilities: - a 33-byte GSM FR frame in libgsm/RTP format, 0xD signature - a 31-byte GSM EFR frame in RTP format (ETSI TS 101 318), 0xC signature - a 2-byte Themyscira-extension BFI marker, 0xBF signature, see below File reading functions begin by reading only one byte; this byte, once decoded, tells us how many more bytes need to be read, and frame synchronization is thus maintained. The recommended filename suffix for extended-libgsm binary files in the present format is .gsmx; of course dot-separated filename suffixes hold absolutely no special meaning on Unix systems, but many developers still strongly prefer to have them for psychological comfort. Any gsmx file (FR or EFR) can be dumped in human-readable form with our gsmrec-dump utility. This utility turns every read frame from bytes into codec parameters with gsmfr_unpack_to_array() or EFR_frame2params(), and then displays those parameters in a sensible manner, with a per-frame header line followed by 4 lines of subframe parameters. FR and EFR frames are not expected to be mixed in the same stream recording; our low-level binary file reading function and gsmrec-dump will grok such mixing just fine, but each higher-level test program (beyond gsmrec-dump) is expected to be written for only one codec, either FR or EFR. BFI marker format ================= Every 20 ms frame in our gsmx files is either a good FR/EFR frame or a BFI (Bad Frame Indication) marker. The BFI marker format used in our gsmx file format is the same format which we (Themyscira Wireless) previously used in our GSM RAN RTP transport, before switching to our current TRAUlike RTP format. This BFI marker format is quite simple: byte 0: 0xBF signature; byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81, section 6.1.1 in both documents; other bits are reserved.