NGWave Audio/Sound/MP3 Editor: Compression Part I

This article is an introduction to Dynamic Compression. We will attempt to cover the basics behind compression, what it is, what it's used for, and how NGWave's Dynamic Compressor works with these principals.

Automatic Gain Control

You've likely heard the term AGC before: Automatic Gain Control. Many consumer products have some form of AGC built-in. Audio cassette recorders and video cameras often incorporate an AGC circuit in the audio recording, to keep volume levels within the appropriate range.

Generally you can hear the AGC at work if you pay attention to it. A loud sound "kicks out" the volume, and you hear the background noise slowly creep back up. You can think of AGC as having a guy at the Level Control at all times, trying to keep the levels correct for recording.

AGC circuits are found in other, non-audio applications. The automatic iris (brightness control) on a consumer video camera is an AGC circuit. Radio receivers usually have an AGC to control the input signal level.

Uses in Audio Fields

Radio stations have to sound consistant in order to sound professional. At the same time, the output levels to the transmitter must be limited to a specific range to avoid over-modulation. How do they do it? How is it that, no matter what they play, the volume level is extremely consistant?

Dynamic Compression is essentially AGC, but typically with more features to help make the adjustments more transparent, or unnoticable.

Settings Explained

There are many settings on a typical compressor. We'll cover the basic settings that most compressors offer, and explain what they are used for.

The Threshold is the level at which compression is applied. Audio that is below the threshold does not affect the compression. Once the audio reaches or exceeds the threshold, the compressor kicks into action.

The Attack Rate adjusts how fast the compressor reduces the level. Since the goal is to keep the audio at or below the Threshold level, anything above that results in a decrease in volume. Attack controls how fast the level is decreased.

The Release Rate is just the opposite: when the audio is below the threshold, we need to bring the gain back up. The Release Rate controls how fast this occurrs.

The Gain control simply lets you increase the overall gain after the compression stage. This lets you make up for the compression.

The Ratio control affects how much the output is affected as a result of the input. At inf:1 (infinite ratio), we apply infinity decibels of compression for every 1 decibel over the threshold. The output audio is compressed at exactly the threshold level. At 4:1, for every 1 decibel over the threshold, we apply 4 decibels of compression.

An easier way to think of the Ratio control is that it mixes the raw, uncompressed audio with the compressed audio. A ratio of inf:1 means you're hearing only fully compressed audio, and 1:1 would be only raw audio. At a rate of 2:1, you're mixing the two proportionally.

Note that this isn't entirely accurate, but is an easy way to understand how the Ratio control affects the resulting audio. Higher ratios lead to more compression.