Category: Audio Processing Fundamentals

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! Continue reading “What does a “Bass Clipper” do?”

Audio Processing University – Lessons for your ears!

We audio processing mainiacs refer to all kinds of “distortion” that only we seem to be able to hear.   You will hear us toss around all kinds of wierd terms to describe these seemingly mythical sounds we hear when evaluating audio processing systems.

This page is designed to give you an audio class for your ears, because it is all about how to listen for these sounds.

Listening to these files will also help you with some of the audio concepts described in my Audio Processing 101 page .

We’ve taken a small piece of audio from Joan Armatrading’s song “Lover’s Speak” to use for today’s lesson.  I will use grossly exaggerated audio to show what we are hearing when we use terms such as “harmonic Distortion”, “Pumping”, etc. Continue reading “Audio Processing U”

Audio Processing 101

By Cornelius Gould

What Is Audio Processing?
To help with understanding some of the terms used here, I suggest you check out Audio Processing University part One.

Audio processing has its roots in the early days in radio stemming from the desire to automatically control peak levels in a broadcast chain.   The reason for this came about as a way to assist radio console operators.  In this case, the console operators literally controlled the modulation of a broadcast station by manually turning up and / or down the levels on their console.  If the levels were to peak above 100% on the board, then the transmitter would operate illegally.   So, the operators had to act quickly when this situation arose to keep distortion and illegal over-modulation to a minimum.

This is where the automatic peak limiter came into play.  Physically, the peak limiter would be a device with audio inputs and outputs.   The unit operates by monitoring the levels feeding into it, and make audio level corrections anytime the levels exceeded a pre-set reference point (or threshold). Usually this reference point was set to 100%.

If program levels remain below 100%, the limiter assumes a “unity gain” state. That is, the audio levels on the input equal the output.   If the audio exceeds 100%, then the output of the limiter is reduced by an equal amount.  That is, if the input is, say, 175%, then the output of the limiter is 75% less than the input. Continue reading “Audio Processing 101”

The U.S. 75 microsecond Pre-Emphasis Curve used for FM Broadcasts in the Americas and South Korea.

 

 

 

 

 

 

 

 

 

 

What is Pre-Emphasis?

Pre-Emphasis is an early form of noise reduction.  It is where the high audio frequencies are boosted by some amount and transmitted over a delivery system.  At the receive point, the high frequencies are attenuated (De-Emphasized) at a porportional amount as the boost.  By doing this, noises in the transmission system are also reduced by that amount. Continue reading “The 75 Microsecond Pre-Emphasis Curve”