What does 'inverted oscillator' mean?

It's a cool option. NN-XT has no 'invert sample' option, that's why we included inverted versions of 5 oscillators to the refill (see picture).

If you load both the normal AND the inverted version of a sample into NN-XT, you will hear nothing, because the 2 samples will extinguish each other. But if you start changing any parameters (volume, filter, fine tuning, etc.) in one of the sample (and leave the other pair unchanged), you will hear a very interesting high-pass filter effect: and that's the point!
Using a normal-inverted oscillator pair you can get an extra High-Pass filter that works independently of the NN-XT's other filters and offers many new routing options, interesting digital effects. Check it out! For example, load the 'PT Digital Distortion' or the 'PT Alien Arp2' patches!

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