Compression is a form of dynamics processing, meaning that it deals with the overall levels in a file. Compression automatically turns down sections of your audio file, based on settings you specify. Think of it as an automatic volume control. In fact, you may have run into Automatic Gain Control (AGC) on a piece of hardware you own. Many portable tape recorders have an AGC circuit built in to the microphone. This is a fancy marketing term for a compressor. The AGC circuit ensures that all sounds are picked up and that sounds don't get so loud that they distort the microphone input.
Most compressors offer a number of controls that enable you to set where the compression effect kicks in and how drastic it is. But before we get into the details of how compression works, let's talk a little bit about why compression is useful.
Why use compression?
At its most basic, compression is useful as a safeguard against distortion. A compressor automatically adjusts the signal level when it exceeds a certain threshold, so that a guest who suddenly gets excited won't send your levels into the red. For this simple reason, compressors are very useful in live situations, when you may not be able to control situations as tightly as you'd like to.
Compression also is useful when working in lower fidelity environments, because it allows you to match the dynamic range of your production to the available dynamic range of the equipment on which it is played back. CD-quality audio has a very large dynamic range, and provided you're listening in a quiet environment, you can hear very quiet sounds as well as very loud sounds coming off a well-produced CD. This simply isn't the case for most podcasts.
First, most podcasts are played back on desktop systems, very often in slightly noisy environments. So if your podcast has very quiet sections, people will have a hard time listening, because they'll either have to turn the volume up temporarily or shush the people around them. And if you're encoding at a lower bit rate (96Kbps, 64Kbps, or even 32Kbps), the dynamic range simply isn't that great. The encoding software uses volume as a determinant of importance, so quieter sections won't sound as good as the louder sections.
If the quieter sections of your podcast are going to be difficult to hear, then you want to turn those sections up, right? Sure you do. You want the overall level of your file evened out, so you don't have large differences between the loud sections and the quiet sections. You want the dynamic range compressed. This is precisely what a compressor does.
Figure 1 illustrates the dynamic range and headroom of a file. We'd like to turn up the quiet sections of this file, but at a certain point, the louder sections will go into distortion. Using a compressor, we can turn the file up and ensure that the loud sections don't go into distortion, because the compressor turns those sections down automatically.
The final reason to use compression is because we're used to hearing compressed audio on all the traditional broadcast mediums. Radio and television use compression liberally. This is partially for the protective reasons discussed previously, but also because of a particular side effect of compression. Compression tends to make things sound fuller, because it brings up the bottom end of the audio signal. Between the protective qualities of compression and the added warmth, this combination is hard to beat.
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