Telit LE910C4-EU USB audio voice latency around 480 ms

Raspberry Pi Base HATs 3G/4G&LTE Base HAT
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Hardware:

  • Module: Telit LE910C4-EU (firmware 25.21.670 [Mar 04 2021])

  • Raspberry Pi model: Tested on Raspberry Pi and also on another Linux computer directly connected to the Sixfab/Telit module

  • Connection method: USB

  • Power supply:

    Two setups were tested:

    1. Raspberry Pi setup:

    The Sixfab/Telit module is connected through an externally powered USB hub. The USB hub has its own power supply, so the modem is not powered only from the Raspberry Pi USB port.

    1. Linux computer setup:

    The Sixfab/Telit module is connected directly to the Linux computer over USB and powered from the computer’s USB port.

    In both setups, the module enumerates correctly, registers on the network, answers GSM calls, and exposes the USB audio device.

System:

  • OS & version:

For Raspberry: Raspberry Pi OS Bookworm 64-bit

Computer: Linux Mint 22.3, Kernel:

  • Kernel version:

For Raspberry: 6.12.75+rpt-rpi-2712

Computer: 6.17.0-22-generic

SIM & Connectivity:

  • SIM card type: Lifecell
  • APN configured: Not relevant for this voice/audio test
  • Connection protocol: QMI is detected for data, but the issue is with USB audio voice path

Modem Recognition Check:

  • lsusb output:

1bc7:1230 Telit Wireless Solutions LE910C4-EU

  • dmesg | grep ttyUSB output:

    option 2-1.2:1.0: GSM modem (1-port) converter detected

    usb 2-1.2: GSM modem (1-port) converter now attached to ttyUSB0

    option 2-1.2:1.6: GSM modem (1-port) converter detected

    usb 2-1.2: GSM modem (1-port) converter now attached to ttyUSB1

    option 2-1.2:1.7: GSM modem (1-port) converter detected

    usb 2-1.2: GSM modem (1-port) converter now attached to ttyUSB2

    option 2-1.2:1.8: GSM modem (1-port) converter detected

    usb 2-1.2: GSM modem (1-port) converter now attached to ttyUSB3

    option 2-1.2:1.9: GSM modem (1-port) converter detected

    usb 2-1.2: GSM modem (1-port) converter now attached to ttyUSB4

LED Status:

  • PWR LED (red): on
  • SGNL LED (blue): blinking

AT Command Outputs:

  • AT+CREG? result: +CREG: 0,1
  • AT+CPIN? result: READY
  • AT+CSQ result: +CSQ: 25,4

Issue:
I am trying to use the Sixfab Raspberry Pi 4G/LTE Modem Kit with the Telit LE910C4-EU module as a GSM voice gateway. The module is connected over USB and appears as a USB audio device on Linux. Voice USB audio is configured with:

AT#USBCFG=11

AT#ADSPC=6,0

Linux sees the audio device as ALSA hw:1,0. The required audio format is mono, 16-bit, 16 kHz PCM.

To isolate the latency, I removed my application completely and tested only a direct ALSA loopback during an active GSM call.

I answered the incoming call with AT/ATA and then ran:

arecord -D hw:1,0 -f S16_LE -r 16000 -c 1 --period-time=5000 --buffer-time=10000 -t raw | \ aplay -D hw:1,0 -f S16_LE -r 16000 -c 1 --period-time=5000 --buffer-time=10000 -t raw

When I speak into the phone, Linux receives the audio from the Telit USB capture device and immediately sends it back to the Telit USB playback device. The returned echo delay is consistently around 0.45-0.48 seconds.

Because this test uses only direct ALSA loopback with a 10 ms buffer and no application/gateway code, I am trying to understand whether this latency is expected for the LE910C4-EU USB audio voice path, or whether I configured something incorrectly.

Full error output:
No application error. The direct loopback works, but the round-trip audio delay is around 0.45-0.48 seconds.

Relevant AT outputs:

AT#USBCFG?

#USBCFG: 11

AT#ADSPC?

#ADSPC: 6,0

AT#DVI?

#DVI: 1,2,1

AT#CAP?

#CAP: 0

AT#SRP?

#SRP: 0

What you have already tried:
* Tested my full gateway bridge and noticed voice delay.

* Removed the gateway application and tested direct ALSA loopback only.

* Tested on Raspberry Pi during an active GSM call: delay around 0.45-0.48 seconds.

* Tested on another Linux computer with the Sixfab/Telit module connected directly: same delay around 0.45-0.48 seconds.

* Confirmed USB audio routing with AT#ADSPC=6,0.

* Confirmed GSM-side voice reaches Linux capture only after setting AT#ADSPC=6,0.

* Swapped/check antennas and confirmed the modem is registered and has usable signal.

* Tried low ALSA buffer settings: --period-time=5000 --buffer-time=10000.

Since the ALSA buffer is only 10 ms and no gateway/application code is involved, it looks like the delay may be in the Telit USB audio voice path or modem voice processing.

Questions:

1. Is around 480 ms round-trip latency expected for LE910C4-EU USB audio voice?

2. Is USB audio intended for real-time voice gateway use?

3. Are there any AT commands or firmware settings that can reduce this latency?

4. Could this indicate that I configured something incorrectly?

Any guidance would be appreciated.