Minor Brainwave

The problem with my originally proposed method may have been convolving with a sine wave that does not have a DC offset? Because of 0s...

Idea

Maybe this is total nonsense...
What if I found the THD% of a nonlinear device for a bin of frequencies...
I then convolved that behaviour with the Frequency domain version of some music.
I would then have a signal with a known average thd% per f bin.

I can then add and remove that harmonic content as a function of a slider, and know the THD%av. The thing to remember is that, if I add x much energy over the frequency response at that frequency, and then reduce that by half, that is still a known amount of energy added. Der Untergang is the IMD and other artifacts possibly caused by adding this content. That said, an amount of energy + an amount of energy is still that given amount, in this instance. I would no longer be applying the NLD to the signal, just its effects and thus I may avoid some of the other effects of the NLD on a boradband basis.

EDIT: Reality check

Some of the fundamental properties of signals.

So a signal may have multiple channels, but each channel will effectively be just a set of number values through time (in matlab). This change of numbers will infer spectral content throughout a time period, but will in effect just be an intensity value at any single point. in time.

Continuous Time & Discrete Time

In continuous time, this would mean that THD should only be there for the response time of the device in the instance of an impulse, and by extension the THD pattern will change with continuous time. This does not have to imply IMD, as the distortion characteristic (DC) of the device should imply the DC output with signal instantaneously (assuming it is a memoryless device such as those that I am using). The problem may become more apparent, when taking time variant devices into account such as capacitors and inductors.

So if you chop the signal into small packets of time, the same must be true?
This is why the signal must be processed in the time domain. If you did it in the frequency domain it wouldn't work.

If you were really clever, you would find a way to separate out the THD effects over time, and see the difference, sample to sample. There must be a way to take an NLD, and simply get a numerical output for the distortion value, without the music? Because you loose frequency information when you look at small segments of the time domain.

Here is a concept for you

Is it not possible to comprehend that the difference in sample time, may allow for time variable devices to correct themselves, if you placed a small enough get in-between audio samples. The gap would have to be pretty small, or in a digital amplifier.

After a couple days to think on it, I think that a THD% study using music will not work.
Why?
Because signals with multiple frequencies at once, in