Controlling harsh high end

Balance a cut with a boost, e.g. a cut at 5kHz and a boost at 15kHz.¹ You can also use a simple low-pass filter to emulate the lower bandwidth of analog gear.²

For vocals, use a multiband compressor or spectral shaper on the harsh zone (Nova GE, TB MBC, or DSEQ3).³ Follow that up with a de-esser (Darrell Thorp uses Pro-DS) and then boost with a shelf.⁴ Darrell Thorp then adds a compressor and a Parallel compression track (sometimes with another multiband compressor after the broadband compressor).³ Darrell uses this same kind of chain for other instruments too. A boost around 250Hz can also help mask harshness (see How to use complementary frequencies).⁵

Make sure your nonlinear processing also has oversampling. If not, it can contribute to more harshness.

Mike Senior mentions using a transient designer to address pokey digital recordings.⁶ Softening digital transients follows this same principle, along with tape emulate and dynamic EQ.

“Dark” tape emulators, like the FATSO, can help de-harsh. They warm the sound a bit but don’t remove too much energy, since they add harmonics.⁷ Try cranking up the bias on a Tape saturation plugin.

If you need to boost without it getting harsh, try one of the following. Dan Worrall says some engineers may like linear phase EQ boosts for the way the pre-ringing smoothes transients.⁸ Use upward compression on the high band. Emiliano Caballero does this in parallel.⁹