Power line hum on Lime2 with mainline kernel

[Fex]

After figuring out how to enable analog audio on the Lime2 (https://forum.armbian.com/index.php?/topic/4224-analog-audio-on-lime2-with-stable-kernel-version-49/), I now have the problem that there is a clearly noticeable mainline hum whenever no sound is playing. The hum goes away as soon as any sound is playing on the device, and it returns a couple seconds after the sound stopped. It appears like some kind of power saving feature kicks in.

 

With the legacy kernel the hum goes away during boot and does not return.

 

Any ideas?

 

[Igor]

Try changing CPU governor.

[Fex]

Thanks for the tip! Still no luck :-(

I changed the governor from "ondemand" to "performance", but the problem persists. I also noticed that the hum stays during playback, if either "Power Amplifier DAC" or "Power Amplifier Mute" is muted.

[Fex]

I found a workaround!

 

After looking at some kernel stack traces, I found that this guy was responsible: https://www.kernel.org/doc/html/latest/sound/soc/dapm.html

 

When the PCM device is closed the following happens in the kernel:

snd_pcm_release() -> soc_pcm_close() -> delay of `pmdown_time` -> close_delayed_work()

The last function powers down the audio subsystem, causing the hum. To circumvent this behavior, I simply set `pmdown_time` to -1:

echo -1 > /sys/devices/platform/soc@01c00000/1c22c00.codec/cdc/pmdown_time

Now, once the sound subsystem is powered on, it stays powered on. Not ideal but it seems to work.

[Tido]

Bernie mentioned in this - post - that it is caused by "a voltage difference between ground and AV Jack" so this is probably a HW design mistake.

As this workaround also seems to work on Banana Pi, what is the possible problem in doing as suggested above?

 

[Bernie_O]

The only disadvantage I can think of is a slightly higher power consumption.

Or am I missing something?

Cheers, Bernie_O.