Hi Stanislav, On Wed, May 16, 2018 at 05:15:07PM +0200, Stanislav Sinyagin wrote: This is the "PCM" interface I was referring-to. This means you need a PCM slave interface to interface with it, as the modems typically all insist on being a PCM master. The intention here is that you attach some external audio codec IC that converts from the PCM to analog audio. This means you cannot interface this PCM interface e.g. with standard USB-Audio bridge ICs, as all those USB-Audi bridge ICs (basically "USB soundcard" ICs) all also only implement the "master" of the PCM interface and not the slave. See http://laforge.gnumonks.org/blog/20170902-cellular_modems-voice/ In the end, you have to use something like the SSC (synchronous serial) peripheal of Atmel SAM3/SAM4/... devices, or an XMOS device in order to interface with that audio. Needless to say, the lack of a standard for where PCM lines are on mPCIe slots means that you cannot build any base board that will interoperate with mPCIe modules from different vendors. The fact that a PCM bus is present on the hardware pins of the mPCIe socket or the pads of a LGA module also still doesn't mean that you actually will have working audio. Many modem module maker do not obtain patent licenses for audio/voice, as they know/expect their modems are typically only used in machine2machine or other data-only applications. Finally, even if the firmware and hardware interface is present, in many cases the modem manufacturers make you sign a declaration that you will only use the voice interface as some kind of "emergency communication" only, and not as part of your normal product. Once again, patent licensing differences for voice vs. data-only use cases are to be blamed for that. Oh, and I'm not even touching the question on whether audio will work in circuit-switched 2G/3G only, or whether it will also work with VoLTE, and particularly not whether they will do SRVCC, etc. That adds yet another dimension to the problem. Regards, Harald -- - Harald Welte <laforge(a)gnumonks.org&gt; http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)