Mixing Tips & Tricks #1: Dynamic Compression

Everyone loves dynamic compression. But how many of those compression lovers know exactly what dynamic compression is doing to their audio signal? I recall mentionioning my favorite soft-compressor, Sonalksis SV-315MKII, in a audio forum last week and a member said,

“i use this Plugin too and it works great. But sometimes this compressor is a lil bit to hard.”

I couldn’t believe my ears, because as I have previously stated, this the most ‘versatile and flexible soft-compressor’ that I have worked with to date. But then I was reminded that most musicians and hobbyists using compression use it very much like any other effect, twisting and turning knobs; pushing buttons and flipping switches to see which one sounds the best. Which is fine, and can come up with very artistic and even enviable results. But there is something to say about very deliberate and precise application of dynamic compression.

So what is dynamic compression? Dynamic compressors make the loud parts of audio softer, and softer parts of audio louder. Its also used to shape the audio signal in complex and interesting ways. Sound shaping occurs by increasing or decreasing the length of the audio’s attack and decay. Most compressors have similar characteristics. Wikirecording.org has a good overview of compression that sums up the settings of a compressor quite nicely saying,

Threshold. This is the level at which gain reduction begins to happen. Usually measured in dB. Lower threshold values increase the amount of compression, as less signal is required for gain reduction to occur. A threshold of -20 dB would initiate compression any time the signal went above it.

Ratio. This is the ratio of change between input level and output level once the threshold is reached. For example, a ratio of 4:1 means that an input level increase of 4 dB would only result in an output level increase of 1 dB. Example: if you take the -20 dB threshold and take your input signal that reached -12 dB with no compressoin (8 dB above the threshold), if you compress it at 4:1 (8 / 4 = 2) the signal would only reach -18 dB (-20 + 2 = -18).

Attack. The amount of time it takes for gain reduction to take place once the threshold is reached. The ratio is not applied instantaneously but over a period of time (the attack time) Usually measured in microseconds or milliseconds. Use longer attack times when you want more of the transient information to pass through without being reduced (for example, allowing the initial attack of a snare drum).

Release. The amount of time it takes for gain to return to normal when the signal drops below the threshold. Usually measured in microseconds or milliseconds.

Makeup Gain. Brings the level of the whole signal back up to a decent level after it has been reduced by the compressor. This also has the effect making quiet parts (that are not being compressed) louder. (wikirecording.org)

One mistake people make is to think that there is a magic formula that will work for every song. This is just not true, each song has a different attitude and groove. If you cannot find the groove of a song you may have a hard time using compression for mixing or mastering. One way I like to look for the right settings is to start out with medium ratio of 4:1, however this ratio is considered quite high for mastering. Then I set the compressor for a fairly fast release between 50 and 100 milliseconds. Next, I’ll adjust the threshold and try to find where the gain reduction meter (GR) bounces with the groove of the song or whatever you want the compressor to affect. This way you know that the compressor is effectively responding to the musical accents of the song and not other factors of audio signal.

Thereafter, I will play around with the ratio and release time until I find the sound desired. And that is how I start out with the compressor, I hope you find this small article helpful. And be sure to approach the attack time with care. Making the attack time of the compressor too fast causes elements of distortion to creep into your signal. So if you are looking for an unusually fast attack compressor my suggestion is to use a VCA modeled soft-compressor as opposed to an optical, FET or variable gain compressor. The circuitry of the VCA in hard-compressors responds more quickly to fast attack audio signal as a result of many detector looking at the same signal. And VCA modeled soft-compressor’s algorithms are patterned after this and should respond similarly.