Because of the continuous background noise of Cardputer ADV, I really don't know what caused these problems. I have done a series of extreme modifications to my firmware and done the following four tests:
Taken together, these tests basically rule out the high-pass filter settings, app-level gain, playback chain, and simple mic magnification as the main cause.
In other words, this looks more like a hardware/input-chain noise issue than something caused by the recorder's software processing.
1. I pushed the high-pass filter all the way up to 1000 Hz, but the noise was still there. This suggests it was not low-frequency rumble or DC offset.
2. I disabled the app-level recording gain and soft compression by switching to Raw Capture. The noise was still there, so it was not caused by my own 2x PCM gain amplifying the noise.
3. I lowered M5Unified's mic magnification from the default 16 down to 1. The recording became much quieter overall, so the setting was definitely taking effect, but the noise was still audible. That means the issue was not simply excessive mic software gain.
4. I copied the recorded WAV files to a computer and played them there. The noise was still present, so it was not coming from the Cardputer's speaker or playback chain.
Taken together, these tests basically rule out the high-pass filter settings, app-level gain, playback chain, and simple mic magnification as the main cause.
In other words, this looks more like a hardware/input-chain noise issue than something caused by the recorder's software processing.
