One option is using mini-PCIe WAN cards that are VoLTE capable, using the I2S bus for bidirectional digital audio. The sticking point with this approach is that you need a backplain card with an I2S to USB bridge in order to capture the audio. This would probably work best with mod-gsmopen. I searched high and low for such a thing, and managed to find one Russian firm that had a single slot device. The downside is that it is horrendously expensive (about $400 euros from memory), considering it only has about $5 worth of components on it. There are proprietary backplains, but so are their connectors, which limits you to their own line of daughter cards. Francis On 5/12/2016 9:29 PM, Oivvio Polite wrote:
I'm building a call recording tool (outgoing calls only) where the prime considerations are 1) audio quality and 2) ease of use.
During the last years many mobile carriers have enabled VoLTE calling that operate at higher bitrates with better audio quality than traditional telephony.
A common patterns seems to be that VoLTE is only enabled when the phone handsets on both ends supports it and both parties are on the same mobile carrier.
To get that sweet HD audio in my call recordings I can't just run FreeSwitch with a regular SIP trunking provider. I've googled extensively and can't find any SIP trunking provider that claims VoLTE capability.
A primitive way recording VoLTE calls with FreeSwitch would be to cook something together with mod_portaudio kind of like this:
The user dials into FreeSwitch with a some custom HD audio client, probably using WebRTC. We capture this call leg at the server. Hooked up to the FreeSwitch box we have 3 audio cards. Connected to each audio card is a VoLTE capable cellphone on a diffrent mobile carrier (We pick the three most popular). The phones connect via the cellphone jacks. Each phone is also running a custom app that listens (via Wifi) for instructions from our FreeSwitch box.
When the call arrives to FreeSwitch we check what carrier the the user is dialing. Hopefully one of our 3 connected phones also use that carrier. If so when instruct that phone to call the destination number and we connect the phone via mod_portaudio as the second leg of our call.
This scheme as kind of Rube Goldberg and has many limitation.
1. If we want to support simultaneous calls on the same carrier we need more phones.
2. We loose audio quality in the recording when going from digital to analog.
3. Cellphones error prone, subject to freezes and what not. This is not something you want to put in a datacenter.
So I'm wondering what other options might be for recording VoLTE calls.
kind regards, Oivvio Polite
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