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I have a cheap, simple recording setup: a Behringer UMC204HD DAC interface and a Behringer C-1 microphone connected over XLR.

For some reason the input from the microphone peaks (clips) at precisely -6 dB in Audacity and other audio software, so I have to manually add gain in postprocessing to it to make it peak at 0 dB. The DAC doesn't have any sort of microphone boost feature that I know of that would affect this.

Is this some sort of a hardware limitation or a software incompatibility? I have tried the setup under both Windows and Linux so I suspect it's not a software thing.

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    Are you bringing a mono source into a stereo input/channel? Best guess is you've got 6dB pan law happening somewhere. – Tetsujin Jul 5 at 9:58
  • I think that's precisely what's going on. The input looks to be coming to my computer in stereo, but only the left channel has anything on it. And it looks like that at least on my current setup the left channel is actually peaking at 0dB, and the -6dB thing only happens when I remix it to mono in Audacity or PulseAudio. I think you can go ahead and claim "mono source into a stereo input" as the answer here :-) – burneddi Jul 5 at 14:21
  • I could put it in as an answer, but I don't know your hardware, software or indeed either of your tested OSes, so I have no idea how to fix it. – Tetsujin Jul 5 at 17:29
  • Well, the question wasn't so much about fixing it as it was about why it's happening in the first place ;-). Anyway, I ended up fixing it by using a PulseAudio remapping that remaps the front-left channel (and only that channel) into mono. Though for some reason I had to resample it to 48kHz from 192kHz or else it would record at 4x the speed, even though the interface is supposed to be 192kHz! – burneddi Jul 6 at 10:57
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The answer is, as Tetsujin suggested in the comments above, the result of mixing mono into stereo or vice versa. It seems most digital audio tools I used automatically apply pan law in these situations, which results in -6 dB of gain when mixing mono into stereo. In my case, my audio interface's inputs are stereo but my microphone is mono, so the audio was only on the left channel, and mixing this into mono also resulted in the -6 dB pan law being applied automatically by both PulseAudio and Audacity.

In my case, I fixed this by remapping only the left channel into mono in PulseAudio, which avoided the automatic application of pan law: load-module module-remap-source master=alsa_input.usb-BEHRINGER_UMC204HD_192k-00.analog-stereo master_channel_map=front-left channel_map=mono rate=48000 format=s24le.

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