The idea behind perceptual CODECs

Question: If you were to drop a rock and a tiny pin onto a table, and both land at the same time…do they both make a sound?

Yes they do.

Can you hear them both?  Probably not…


Logic would dictate the reason why is because the sound of the rock is so loud that it would “cover” those of the tiny pin. This concept is known as masking.  The really loud sound of the loud rock masked the tiny sound that the pin made. The Rock And The Pin

This is a nice introduction to how perceptual audio coding works.  Continue reading “The idea behind perceptual CODECs”

Listening for Coding Artifacts

Listening for coding artifacts

When you hear audio engineers talk about coded audio, inevitably you’ll hear us talk about “coding artifacts” in one way or another.What are “coding artifacts”, and what exactly is it that we are hearing?Listening for artifacts involves knowing where to listen.  In this case, one must listen int he background.  It is also easier to hear artifacts if you have a non-coded version of the audio compare with. Continue reading “Listening for Coding Artifacts”

Audio Processing U

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

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 75 Microsecond Pre-Emphasis Curve

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











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”