4-channel input

[Noah Graham]

Thanks for this awesome program, which I am using to teach Fourier analysis and signal processing.

I have an unusual setup that gives behavior different from what I expected in a minor way, but perhaps it is correct.  In my application I connect four separate audio channels to a Behringer UMC404HD audio interface, which then connects to the computer as a USB audio device.  When I do this, what Friture displays (I am using the spectrogram) corresponds to the sum of channels 1, 3, and 4.  If I change the first input channel, it's instead the combination of channels 2, 3, and 4.  Ideally I would like to see the result of all four channels put together, but I couldn't figure out how to do this, even with two-channel input.