gsm-codec-lib-r2 new release

Hello GSM community, I just put a new release of Themyscira GSM codec libraries and utilities package: ftp://ftp.freecalypso.org/pub/GSM/codecs/gsm-codec-lib-r2.tar.bz2 ftp://ftp.freecalypso.org/pub/GSM/codecs/gsm-codec-lib-latest.tar.bz2 (symlink) The two libraries in this package (libgsmefr and libgsmfrp) are intended for people who develop gateway software interconnecting Osmocom-based GSM networks to PSTN or other networks, gateways which include a speech transcoding function that terminates the GSM codec leg. If anyone is currently interconnecting an Osmocom GSM voice network to the outside world using software which you did not write yourself (Asterisk, FreeSwitch, Kamailio, whatever) and you care about the plain old FR codec, and/or care about EFR, beyond just AMR, I encourage you to investigate your current non-Osmocom gateway software to see exactly how it implements FR and EFR. Because there were NO pre-existing FOSS libraries that correctly implement FR and EFR decoding prior to my Themyscira gsm-codec-lib development, most pre-existing gateway software probably implements these codecs in a flawed manner: FR codec: Everyone to my knowledge implements this codec using classic libgsm, a library that dates back to 1990s. It's a good library, it's a fully correct implementation of GSM 06.10 spec, and I use it too. However, it implements _only_ a bare 06.10 encoder and a bare 06.10 decoder, without any DTX functions of GSM 06.31 and related specs. In the encoder direction having no DTX isn't really a problem (you won't be able to do DTXd anyway unless you got lots of spectrum and are running multi-ARFCN cells), but the lack of an Rx DTX handler per GSM 06.31 *is* a real problem: if you feed the uplink from a GSM call (RTP stream from a BTS) to a bare GSM 06.10 decoder such as gsm_decode() function in libgsm, you won't get correct handling of SID frames, which every standard GSM MS will transmit, and you won't get correct handling of BFI frame gaps, which will always occur. The correct solution is to insert a call to a GSM 06.31 Rx DTX handler (it is more than an ECU) just before the call to gsm_decode(), and my libgsmfrp offering is that GSM 06.31 Rx DTX handler. EFR codec: Everyone other than me implements EFR (if they support it at all) using an AMR library such as libopencore-amrnb. I have seen totally broken implementations that schlep 244-bit payloads directly between supposed-to-be-EFR RTP and the AMR library, without reordering those bits per gsm690_12_2_bitorder[] - those implementation have exactly zero chance of ever actually working with a real GSM-EFR MS on the other end - and I've also seen implementations that do perform this bit reordering and are thus closer to correct. But even the latter implementations are still wrong when it comes to SID handling: EFR is equivalent to the highest MR122 mode of AMR only for regular speech frames, but not for SID. There does exist a special encoding format for representing GSM-EFR SID in AMR frame interfaces, but libopencore-amrnb does not support GSM-EFR SID in any way at all. If you take the uplink from a GSM-EFR call and feed it to libopencore-amrnb decoder, any time the GSM MS emits a SID frame, strange noise sounds will appear at the output of that decoder, instead of the correct comfort noise. Themyscira libgsmefr is a proper encoder and decoder library for EFR, based on the EFR reference code from ETSI, in exactly the same way how libopencore-amrnb is based on the AMR reference code from ETSI/3GPP. It still has some performance problems which I will be working on later (the goal of getting it to perform no worse than libopencore-amrnb has not been achieved yet), but at least it is correct. Hasta la Victoria, Siempre, Mychaela aka The Mother