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Old 03-25-2020, 05:09 PM   #1
phoo
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Join Date: Dec 2010
Location: Hampstead, NC
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Default Inharmonic or Intermodulation Distortion Issues

I have a very complex mix in the works. It has a lot of high frequencies that are much like sibilances, but not all are from voices. Some are "airiness". Yes, these are up there, and yes they have been EQed as much as would normally be done. The problem is not tone, nor them occupying the same space. This has turned onto a technical issue. Compromise is expected, but I'm trying to understand why this is happening.

The problem is when some tracks are mixed there is what sounds like intermodulation distortion being produced to the point it sounds like inharmonic distortion. Two clean smooth tracks mix and sound like they are being run through a clipper. The sound is grainy at best, and has crackles at worst.

Tracks are 24 bit float, playing as 32 bits - 44.1K sample rate. Mixing to the same.

This is not a level issue as all tracks are well below any kind of clipping. The mixed tracks are around -12db. For that matter, since mixing is done in 64 bits I don't see how it would be possible to clip during the mix.

I'm guessing intermodulation, which will create frequencies not present in any of the source. This could and would cause what I'm hearing. Lower "new" frequencies would sound grainy. Higher "new" frequencies could and would cause terrible distortion is those frequencies are above the nyquist frequency. I can justify (via my ignorance) that this is what's happening.

It's also similar to dropping to 8 bits, or lower.

Solutions? Well, yes, EQ out the HF of the sources until it goes away. That's impractical, as I'm having to EQ out everything above 5k or so for it to be completely avoided. That's OK for some tracks, but completely ruins the sound of others, but to cut off the cymbals at 5K, or vocal sibilance? Not really.

Solutions that seem to have an effect, after EQ had failed is quite interesting. These have worked with a good bit of success. Each causes a good number of questions to arise. These don't work at all on some tracks and with more than two tracks isn't much more complicated when any pair of tracks is OK.

Solution:

1) Flip the phase of one track.

2) Shift the timing of a track a few msecs, or samples.

3) Use the RePitch plug-in to change the pitch just a little, or just the formant.

Think of this problem as being "how to mix multiple tracks of white noise" in such a way as to not create the artifacts that intermodulation can cause.

In the real world this is happening when the snare hit, there is a cymbal crash, or there sibilances, all while a smooth airy mellotron is sustaining chords.

What is actually causing this, and are there better solutions to avoid it?

I know this is a lot to read, but I hope I explained the problem well enough for discussion.

THANKS.

(please forgive any typos)
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