AF’s Weblog

December 30, 2010

Compressors: How They Really Work

It’s one of the most used, and most misunderstood, signal processors. While people use it to make a recording “punchier,” it often ends up dulling the sound instead because the controls aren’t set optimally. And it was supposed to go away when the digital age, with its wide dynamic range, appeared.

Yet the compressor is more popular than ever, with more variations on the basic concept than ever before. Let’s look at what’s available, pros and cons of the different types, and applications.

Introduction

Compression was originally invented to shoehorn the dynamics of live music (which can exceed 100 dB) into the restricted dynamic range of radio and TV broadcasts (around 40-50 dB), vinyl (50-60 dB), and tape (40dB to 105 dB, depending on type, speed, and noise reduction used). As shown in Fig. 1, this process lowers only the peaks of signals while leaving lower levels unchanged, then boosts the overall level to bring the signal peaks back up to maximum. (Bringing up the level also brings up any noise as well, but you can’t have everything.)

Fig. 1: The first, black section shows the original audio. The middle, green section shows the same audio after compression; the third, blue section shows the same audio after compression and turning up the output control. Note how softer parts ot the first section have much higher levels in the third section, yet the peak values are the same.

Even though digital media such as the CD have a decent dynamic range, people are accustomed to compressed sound. Compression has been standard practice to help soft signals overcome the ambient noise in typical listening environments; furthermore, analog tape has an inherent, natural compression that engineers have used (consciously or not) for over half a century.

There are other reasons for compression. With digital encoding, higher levels have less distortion than lower levels—the opposite of analog technology. So, when recording into digital systems (tape or hard disk), compression can shift most of the signal to a higher overall average level to maximize resolution.

Compression can create greater apparent loudness (commercials on TV sound so much louder than the programs because they are compressed without mercy). Furthermore, given a choice between two roughly equivalent signal sources, people will often prefer the louder one. And of course, compression can smooth out a sound—from increasing piano sustain to compensating for a singer’s poor mic technique.

Now let’s look at some compressor basics…

Compressor Types

Compressors are available in hardware (usually a rack mount design or for guitarists, a “stomp box”) and as software plug-ins for existing digital audio-based programs. Following is a description of various compressor types.

  • “Old faithful.” Whether rack-mount or software-based, typical features include two channels with gain reduction amount meters that show how much your signal is being compressed, and most of the controls mentioned above.
  • Multiband compressors. These divide the audio spectrum into multiple bands, with each one compressed individually. This allows for a less “effected” sound (for example, low frequencies don’t end up compressing high frequencies), and some models let you compress only the frequency ranges that need to be compressed.
  • Vintage and specialty compressors. Some swear that only the compressor in an SSL console will do the job. Others find the ultimate squeeze to be a big bucks tube compressor. And some guitarists can’t live without their vintage Dan Armstrong Orange Squeezer, considered by many to be the finest guitar sustainer ever made. Fact is, all compressors have a distinctive sound, and what might work for one sound source might not work for another. If you don’t have that cool, tube-based compressor from the 50s of which engineers are enamored, don’t lose too much sleep over it: Many software plug-ins emulate vintage gear with an astonishing degree of accuracy.

Whatever kind of audio work you do, there’s a compressor somewhere in your future. Just don’t overcompress—in fact, avoid using compression as a cop out for bad mic technique or dead strings on a guitar. I wouldn’t go as far as those who diss all kinds of compression, but it is an effect that needs to be used subtly to do its best.

To read the full article see:  Compressors Demystified

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