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Compression: What It Is and How to Use It

What is compression?

Compression reduces the dynamic range available to an audio signal - the space between the softest and loudest sounds. Using a compressor can transform your music, bringing out otherwise hidden parts, by controlling the maximum levels of the piece and keeping a higher average loudness.

Basic compression parameters

These are common controls on all compressors. An understanding of these parameters should help you improve your audio recordings almost immediately.

Ratio: The degree of reduction of the dynamic range caused by the compressor.

The ratio represents the difference between the audio signal increase coming into the compressor, and the increase at the output. As an example, a 5:1 ratio would mean that a 5dB increase in the audio signal coming into the compressor would result in a 1dB increase in the output. It is a constant in that the ratio of input change to output change will always be the same regardless of how much compression is taking place.

Threshold: The level at which the compressor takes affect on the incoming audio signal. The compressor will have no effect on the signal below this level.

When a signal reaches or exceeds threshold level, the signals gain will be reduced by the amount the signal is greater than the threshold in relation to the ratio setting. Simply put, the threshold is the sensitivity of the compressor.

Knee: The precise point at which the compressor begins to reduce gain.

Hard Knee - the moment is sudden and definite

Soft Knee - a smooth transition from amplifier to compressor, allowing a wider range of threshold values to be effective.

Attack: The time the compressor takes before it affects the signal when it has breached threshold level.

Generally these times range from 1 millisecond up to a comparably slow 100 milliseconds. These settings are important in determining the final sound quality with respect to its brightness or high frequency content.

Fast attack - compresses the signal very quickly, affecting the gain at waveform level. The information transients at the front of the waveform provide affect the brightness of the sound, and so as a result using a fast attack can result in dulling the sound.

Slow attack - allows the front, transient section of the sound to pass through the compressor before it begins having too large an affect. If the setting is too slow, the compressor may be ineffective.

Release: The time it takes the signal to return from a compressed one into a normal, amplified signal once the audio drops below threshold level.

Release can range from 20 milliseconds to over 5 seconds generally speaking. It is important to use release carefully ? too fast a release with a short attack time can distort low frequency sounds.

Examples of compressor usage

Bass guitar: A compressor to add to the overall warmth of a bass sound. As a starting point try out a ratio of 5:1 with a medium threshold, medium attack and slower release.

Vocals: It is important to understand that each vocalist will require carefully tailored compressor settings to bring out the nuances in their singing. Try starting with threshold around 0dB, ratio of 4:1 and attack and release approximately in the middle. Adjust the output gain as necessary.

Hip-hop style compression: To get hip hop sounding bass and percussion sounds try a ratio of between 4:1 and 7:1 with a threshold of around 10dB to 15dB of gain reduction. Attack should have a different setting for each track in the mix. Begin by reducing it to its fastest level. Slowly increase the attack until the audio?s timbre dulls a little. Then, slowly again, decrease attack time until the original timbre has been restored. This is reversed for release settings. Again applying the release to each individual track, increase the release time to its longest level. Then reduce the release until the compressor can audibly recover before each note or hit. Too large a release will affect the gain of the next note.

Posted on 30 Apr 2009 11:06 to category : Tips and advice

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