How to check if an audio file is mono mixed?

Question

I connect Zoom h4n Pro to my Mac via USB as audio interface. I then record audios with QuickTime.

Because I put the h4n on my left hand, the right & left inputs don't have the exact same volumes. There is a way to set mono mix on as presented here. I do find this setting when using h4n as a recorder, whereas I don't find it when using it as audio interface. Please correct me if I'm wrong.

When I record audios with QuickTime, it seems that the software automatically mixes the left & right channels. Does anyone know how to seriously check if an audio file generated by QuickTime has exactly same volumes from left and right?

Answer 1

If I understand you correctly, what you really want to know is just if your stereo file has the exact same signal on the left and the right channel.

If that it what you want to know you can phase invert one of the channels, then mix them and listen to the result or bounce the result to a track and look at the waveform. If the result is nothing, the left and right channel of the original recording was identical. Otherwise you will hear/see the difference between the two channels.

As some other people suggested you can also use a goniometer (for example in RME Digicheck if you have a RME interface on you computer) to see phase differences between the two channels, but I'm not sure if that will also indicate a difference in signal level, the test above (to mix a phase inverted copy with the original signal) however will.

Answer 2

The program I use has a phase scoop. Mono is shown as a straight line, stereo as a "cloud". Maybe you could find something like that in another program. One suggestion is to download Reaper, you are allowed to test the program for free, I suggest you pay for it if you keep on using it.

Answer 3

Theoretically, a solution would be:

1. Record the exact same signal twice, one time stereo and one time mono.
2. Pick a mixing plugin of your choice (e. g. this free https://www.maat.digital/2buscontrol/)
3. Mixdown/export the stereo recording as mono.
4. Compare the mono export to the mono recording, e. g. by phase-inverting the export's signal and summing both.

In case of two identical recordings this should result in a zero-line signal. Of course, you will never have two absolutely identical recordings. There will be differences. But you can come close:

• For this test, put your device on a fixed and stable stand.
• Record a simple signal from another electric device, like maybe a tone from a speaker.
• Use high volume to increase the Signal-to-environmental-Noise-Ratio.
• Take care of low environmental noise, of course.

The standard comparing procedure with phase invertion (described under 4.) would probably not give a clear result in this case. But comparing both signals by ear should at least give you the idea, if left and right are levelled out equally enough for your recording purposes.

After all, you might probably cross-check using a third recording of the same signal, if you manage to flip the device perfectly by 180°, swapping left and right channel thereby. Of course, the same limitations apply again. Plus some physical differences of the two mic capsules.

So after all, this remains to be an approach, but maybe the best one I can think off.