EC25, Voice over USB

Hi Experts

I am testing LTE Voice Call using Voice over USB.
And I referred to Quectel_EC2x_EG9x_Voice_Over_USB_and_UAC_Application_Not_V1.0.pdf.

in.s16 is a PCM read from the USB NMEA port.
I looped back PCM to USB. but I can’t hear any sound on the speaker phone.
in.s16 plays well.

I set it up according to the manual and the settings are as follows.


Is there any setting I missed?

Thank you.

Dear Ihuijae,
Please change the command AT+QAUDMOD=2 to have a try. Thanks!

It’s the same.
How can I debug this?

Could you help to provide the method to loop back PCM to USB ? And whethe there have any output information via NMEA port ?
How about to check the sound parameters such as AT+CLVL? AT+QSIDET? Thanks!

You can download PCM audio files from the URL below.
in.s16 is PCM audio read from USB.
The sound is good. No noise is included.

How does the loopback work?

I have also changed the CLVL and QSIDET parameters, but I still can’t hear the sound.

I would like to implement a voice call in this way(voice over usb, not UAC), do you recommend this?

Thank you.

Please note that module only support mono, and it also have the requirement for the PCM data. Please double check it.

If all can confirm are correct, it may contact our local FAE to check it on site, you can email to Thanks!

When I read PCM audio on USB, it is 8K 16bit mono.
and if I send it back to USB, it is also 8K 16bit mono.
So, the PCM audio I read/write is meet the specifications.
in.s16 is PCM audio that I read on USB. and I sent it back to USB.

Thank you.