Q: Why I shouldn’t use more than 4-5 models for UVR ensemble (in most cases)

A: It's easier to get, when you separate the same song using some models. Get the best 4-5 models out of the most recommended currently, plus make some more separations, using some random ones. Then try to reflect avg spec from UVR by importing all of these results to your DAW.

 

You'll do it by decreasing volume by 3dB per one stem, so for a pair you need to decrease the volume of two stems by 6dB (possibly 6.02 as well). Decrease the volume by the same value further for more than a pair for all stems accordingly, so you'll get pretty much similar result like avg spec in UVR.

 

You can also maybe apply a limiter on the master. In the second variant, manipulate the volume of all stems by your taste instead of keeping the same volume. By this process, you can observe that the more results imported above 4-5 results, the worse result you have when you don't decrease volume of worse results. When you have control over the volume of single results, you'll end up decreasing the volume of bad results (or deleting them completely). You don't have this opportunity in UVR using avg spec - so like in the first variant in your DAW when you set the same volume for all results. The only way to not deteriorate the final result further, is to delete such worse results from the bag entirely, to not worsen the final outcome when you have too many models ensembled. Without the possibility of decreasing volume of such a result when all volumes are equal, the more results you'll import to the bag of the 4-5 the best models, the worse final result you'll get. Because you cannot compensate for bad results in the bag by decreasing their volume like in avg spec - all tracks are equally loud in the bag of avg to the models with good results - hence, good models sound quieter if they are in minority and the final outcome is worse.

The 4-5 max models ensemble rule is taken from long-conducted tests of SDR on MVSEP multisong leaderboard. When various ensembles were tested in UVR, most of these combinations didn't consist of more than 4-5 models, because above that, SDR was usually dropping. Usually due to all the reasons I mentioned.

Even using clever methods of using only certain frequencies of specific models, like in ZFTurbo, jarredou and Captain FLAM code from MDX23 (don't confuse with MDX23C arch) and its derivations, which minimize the influence of "diminishing returns" when using too many models I think they never used more than 4-5 in their bags, and they conducted impressive amount of testing, and jarredou even focused on SDR during developing his fork (actually OG ZFTurbo code too).

 

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For vocal popping in instrumental issue, read about chunks or update UVR to use a better option used automatically (called batch mode) if you didn't update to 5.6/+ for a long time already, but the issue might still occur on GPUs with less than 11GB VRAM (and earlier patches doesn’t have Roformers support).

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