UAC pop/crackling noise on some laptops or USB cables

[Leo_Lee]

Hi NXP team

Issue: UAC data with pop or crackling noise on some laptops or USB cables.

Environment: i.MX8MP with A11 or A13 image.

We've enabled the UAC and UVC function on imx8 platform.

Currently UVC function can play video streaming from imx8 to PC without any problem,

But UAC function may hear audio noise from PC to imx8 with specific combination of laptops or cables.

We use tinycap to access UAC sound card and record the audio data to a file in imx8 directly and confirm it has noise.

Another strange thing is that if we open a camera App on laptop side (video start streaming from imx8), the audio noise will disappear.

Any information will be appreciated.

[jimmychan]

Could you show me the test cases? how to connect the devices? so we could try to reproduce the issue on our evk.

[Leo_Lee]

Hi Jimmy

We set imx8 as an UVC/UAC device by below procedures.

And please note the audio noise may only happen on specific laptops or cables.

or may happen while switching camera source on the laptop.

Again, once the camera App is opened on the laptop side and select imx8 as a source, audio noise will disappear.

1. Apply kernel_config.patch kernel config
2. Download userspace uvc-gadget tool from https://github.com/wlhe/uvc-gadget and compile
3. adb push uvc_gadget.sh and uvc-gadget to device
4. open uart console
5. below command run on uart console
6. run uvc_gadget.sh //enable uvc gadget kernel driver
7. run userspace tools ./uvc-gadget -v /dev/camera_id -u /dev/uvc_device_id -f 1

[jimmychan]

Thanks for the information reply.

Which board are you using? i.MX8MP EVK? or your own design board?

It is hard to determine where is the noise occur. It is better to narrow down the issue.

When you plug/unplug the camera, is there any error in log?

you may add some code in the driver (audio) and camera App to check where is the noise sound occur?

[Leo_Lee]

We are using our own board and we didn't see any error logs while connecting the board with host PC via USB cable.

Because we config the board as an UVC/UAC device, you can play an audio file from host PC to the devices, then using tinycap program to record it and check the noise.

We just don't know why the noise will be disappear if oping a camera App on the host PC side.

[jimmychan]

It is hard to determine where is the problem. You may do more different test to narrow down the issue first.

[Leo_Lee]

Could you give us some suggestions about the test we can narrow down this issue?

And any USB or UVC parameter may affect the UAC transmission?

[jimmychan]

As you mentioned, audio noise may only happen on specific laptops or cables. You may check what is the different between the "good" one and the "bad" one. Seems it is more like hardware issue. You may check the hardware signals or voltage different when noise happen.

Also, you mentioned the noise will disappear if running a camera App on host PC side. If you have source code of the camera App, you could add some tests in the source code to find out why the noise disappear.

[Leo_Lee]

For audio noise disappear after a running camera App on host PC issue, we are hoping NXP can provide some clues in driver level, and why UVC affect UAC? is there any USB/UVC/UAC parameters we can try?