VU meter for line in

[vanmanda]

Hi all,

I'm building an audio recording appliance. I use LINE IN to record sounds from various sources. Sometimes these sources are microphone ampliefiers oversteering the signal because someone is screaming too loud in the mic. Therefore I would like to add a VU meter to my software in which I can show the user that the signal is over 0dB and he/she should decrease that capture level. In python I am using sounddevice.inputStream to read the RMS value of line in.
I did the following test:
I applied an 0dB signal to the line in (1,4V)
I placed the capture value in alsamixer at 56 which corresponds to 0dB.
I used the corresponding RMS value as 0dB value for my VU meter.
Then I increased the capture value in alsamixer to 100 - which should oversteer the signal by +12dB.
Using the bypass I can clearly hear that the signal is much louder.
The RMS-value remains however unchanged, so my VU meter does not surpass 0dB.
When I do the same with a less powerfull input signal, I see that changing the capture value in alsamixer does have an effect on the RMS-value.

Does some have a better idea on how to create a VU meter that works together with the capture value in alsamixer?