Bass Clippers — an overview

By Cornelius Gould

Whenever audio is “clipped”, it is literally “distorted”.  This distortion is very similar to what is used for that big LOUD rock guitar sound.  The key to broadcast processing is to do this without the distortion being audible.   Clipping in broadcast audio give the audio more “impact” and in most cases also boosts perceived loudness.

The Bass clippers probably came to prominence with the introduction of Bob Orban’s Optimod 8100 audio processor.

In the 8100, the purpose of the bass clipper in the 8100 is to more-or less allow the bass processor to run at a more natural rate.  This rate means that the attack time is somewhat slow.  Slow attack times means that you sometimes get large peak excursions that must be dealt with to control modulation.  The cleverness of the 8100 is this:  The peak is allowed to happen, but it is “chopped” off by the bass clipper. This provides instantaneous compression of the bass audio.   Without losing bass “impact”.  This is cool, but that’s not all!

The action of the bass clipper would be quite audible if left in that configuration. To some listeners, it may sound as if they have blown a woofer, and an annoying rattle would be heard whenever bass thumps were in the program audio.  The solution is to have the bass clipper followed by a low pass filter.  This effectively removes all the distortion products that produces the “rattling sound”, leaving only clean tightly controlled bass.

Since the bass clipper reduces bass peaks to a known manageable level, the 8100’s overall program clipper can be set to provide more loudness without much concern to loud bass peaks that can cause annoying distortion to singer’s voices, or other similar sounds.

Since the time of the 8100, many manufacturers have provided a host of user controls to adjust the bass clipping network to suit their tastes.  In the 8100, the bass clippers weren’t user adjustable, so they  “were what they were”.  They were adjusted for minimum audible effect, and left at that.  The difference now is that the end users are given the ability to “drive the bass” at a much “hotter” level into the bass clipper. In some instances, the user is given control over how the lowpass filter following the bass clipper works.

Manipulating this filter alters the “texture” you get from the bass clipper.  Other schemes involve changing from hard to soft clipping with the goal of finding the appropriate “texture” for the bass to further tailor your on-air sound.