Limiting "ess" sounds in speech with ADAU1701

Discussion created by KJBob on Jun 23, 2014
Latest reply on Jun 29, 2015 by mmmike

     Some folks speak with unusually loud "ess" syllables -- a loud hiss, or even a whistle.  Combine this with certain microphones that peak at 4KHz, and they can peg the VU meter!  This really grates on my ears.  Commercial "de-essers" are of course available, but if you're already building an audio project such as an EQ, might as well include this function.


     This one works by splitting the audio into three frequency bands.  Low and high frequencies pass through unaffected, while the center band (where the nasty syllables occur) are limited if too loud.  Using a ratio approach helps isolate the effect from overall level changes over a fairly wide level range -- the only adjustment is for how aggressive you wish the "ess"-killing effect to be.  As it turns out, the RMS versions of the compressor and envelope algorithms better differentiate between  "esses" compared to vocal harmonics that happen to fall within the mid-frequency band, so that's what I used for the compressor and the envelope follower that drives the Real-Time Display.  Of course the latter two blocks are only needed for development purposes.  The RMS external input compressor is only available in stereo.  The -1701's divide algorithm doesn't mind division by zero should that occur, it effectively clamps its divisor input to 0.0625 if lower than that.  If you're using a -145x, add a small offset to the divisor to prevent divide by zero.




     My tests show this ess-limiter is safe -- virtually inaudible for normal speech such as a podcast.  It's also effective -- it cuts the nastiest "esses" down to human size.  The attached project runs on a -1701 mini eval board.