When recording in Mono, all available channels on the input device are mixed together

[jasonrohrer]

This has been a known issue in Audacity for years:

When you record in Mono, your single input (like a mic) is recorded at half volume, if your input device has two channels.

This is a minor annoyance when you have a 2-channel input device. It’s even described in the Audacity manual here:

https://manual.audacityteam.org/man/faq_recording_troubleshooting.html#How_can_I_prevent_a_half-volume_mono_recording.3F

But on a many-channel input device, it makes mono recording totally unusable. For example on the Motu 624, which has 24 channels, your single input is recorded at 1/24 volume. See here:

https://www.reddit.com/r/linuxaudio/comments/rewiqv/quiet_mic_audio_when_trying_to_use_mono_source_on/

But in testing today… it’s even worse than just low volume.

If you have MULTIPLE mics plugged into your interface, and pick “mono” to only record the first one… well, ALL mics are recorded and mixed together into one track.

This is probably not what most people would ever want to do.

Most people don’t notice it, because they only have one mic plugged in.

But it’s definitely not good or sensible behavior.

Even if Audacity wants to support a “mix every channel together as Mono” mode, that should be a separate entry on the channel-select list, along with, well… just recording Channel 1 at full volume.

It’s also not clear to me if this is really an Audacity problem, or something deeper with Pulse or ALSA. For example, I have used the command-line utility arecord, and at least at one point, the same problem was present there. Also, when using Zoom for calls, I found that the 24-channel hardware was unusable, because the one mic channel would be mixed down to 1/24 of its actual loudness.

However, I just tried arecord on a new version of Ubuntu, and it doesn’t seem like it’s mixing multiple mics together in the same way as Audacity.

I have the Audacity source code here, and I’m building it now… but before I dig in, is there any institutional memory about why Audacity works this way, or what the source of the problem is?

[kozikowski]

@jasonrohrer

your single input (like a mic) is recorded at half volume, if your input device has two channels.

Right. That’s mono mix-down. The assumption is made that there are two full volume performances, one at each input. When you record a mix-down it is assumed each input has to be reduced by half to leave room.

You can record a perfect single channel by recording the performance in stereo and then slicing off the dead channel and convert to Official Mono later. It’s more work, but that would give you an exact, correct mono recording.

I did wonder why provision wasn’t made to ignore all the inputs other than one. That should be a software design, right?

I can think of one Promotion and Publicity reason not to do that. Having all your recording channels drop dead but one can be seen as damage. No maker would volunteer to do that.

“They keep sending it back saying only one channel was working.”

You might be able to do that with a third party driver or something. Can you program audio driver software?

Koz

[kozikowski]

There’s the policy that Audacity doesn’t apply filters, effects, or corrections on recording. This instance is a little fuzzy. The two conditions are full correct stereo recording, and half-volume stereo mix-down.

If full volume mono recording was available, you could have the condition of double volume and serious distortion.

You need the driver which intentionally ignores all the channels but one—and then gives you a separate control panel to choose which one.

I want to record channel 19 in your multi-channel example.

Koz