COMPRESSION

A signal compressor is an audio effect, like delay, reverb, or chorus. However, unlike most effects, compressors are not intended to cause a distinctive change in the character of a sound. Instead they enhance the character of a sound. But Used properly, compression is invisible.

These days, anything that has been professionally mixed is liable to be compressed. Even acoustic recordings are mastered with light compression. A compressor can tighten up a mix, expand a stereo field, bring vocals to the top, or add punch to both bass and drums. It can make your music sound warm and fat, or bright and edgy. If you're tricky, you can even use compressors to create subtle distortion and overdrive effects.

There are two standard compressor types - upward and downward. Downward compressors adjust the volume of an audio signal so that the loud parts are reduced relative to the quiet ones. Upward compressors do the opposite and raise the volume of the quiet parts relative to the loud. (In case you are wondering, compressors are called 'compressors' because the original hardware models were all of the downward variety. That is to say, they 'compressed' loud signals down to quieter levels.)

Think of a downward compressor as your own personal studio engineer. He monitors your song on a VU meter and always has one hand resting on a master volume control. If the level on the VU meter stays above a certain point (the threshold) the engineer does not adjust the volume. However, as soon as the VU meter drops below that point, he pushes the volume control up. Then, when the original signal gets louder again, he slides the volume control back down.

The following 'before and after' waveform diagrams demonstrate the effect of compression on a signal:

Picture one shows the original signal; the quieter section and the louder section; In picture two (after compressing), notice how the quiet section of this waveform, which falls beneath the threshold, is amplified during compression so that it is no longer as quiet relative to the loud section. However, the 'peaks' of the loud section, because they are all above the threshold, remain untouched. This 'transformed' signal will have a much louder, fuller sound. (Why shouldn't you simply use a gain change to make things louder? Because master gain adjusts the amplitude of every sample, loud or soft, equally. Using it to approximate the 'loudness' you are accustomed to hearing in music that is compressed will probably cause 'clipping' - when the highest peaks of a waveform are pushed beyond the maximum allowable volume level. This, in turn, will result in a scratchy sound, like static, during the loudest parts of your song. Compression is the only real way to achieve the even, smooth and controlled quality of loudness that is one of the hallmarks of professionally produced music.)

There are no hard and fast rules to go by when you're figuring out what compression settings will work best on a recording. As with all audio work, your ears are going to tell you whether or not you are using compression properly. However, understanding what each of the settings in a compressor does will help you to experiment more effectively.

There are 4 basic settings - Threshold, Ratio, Attack and Release:

We have already touched on threshold. It tells the compressor how low the VU level can drop before a signal needs to be amplified. If the threshold setting is cranked to the left, only the very softest sounds will be raised. If it's cranked to the right, almost everything will be affected.

Ratio tells the compressor how much to increase the volume of a signal by when that signal drops below the threshold level. A small ratio will increase the volume marginally. A larger ratio will raise it more dramatically.

Attack and Release control the compressor's response time. A short attack means that the volume level will be increased almost instantly after a signal drops beneath the threshold level. A slow attack will cause the compressor to wait longer before adjusting anything. (In general, the shorter the attack, the more 'punch' your song will have.) Release, by comparison, tells the compressor how quickly to slide the volume back down to normal after the original signal has again risen above the threshold. A short release will drop the volume down almost instantly. A slow release will drop it more gradually.

Upward compressors preserve signal resolution better than downward compressors. However, one of their drawbacks is that they also amplify background noise. (The Gate / Expander diminishes, instead of amplifying, sounds that are beneath the threshold level. A low Gate / Expander threshold will cut noise artifacts without noticeably affecting the rest of your music.)

Finally, be wary of too much compression. It can cause distortion in the form of "pumping and breathing," where the volume level of a signal is briefly but noticeably quieter immediately after a loud section, and quiet sections are too loud in comparison with loud sections. Unless you are aiming for compression-specific effects, like overdrive, it is always advisable to err on the side of caution and use lighter settings.

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