Friday, May 11, 2012

Stereo Compression

Since the title of this blog is “Tips and Tricks for Audio Engineers” here’s a tip on linking compressors in stereo mode. A lot of inexperienced engineers and some “experienced” ones too expect when they link 2 compressors in stereo mode for the left or the first one to become “The Master”. There are very few stereo compressors out on the market that truly link all of their controls when in stereo mode. The dBx166XL is one of them. 

The more common linking is the side chain link. In this case 2 or more compressors are considered linked even though their individual controls still work independently so there is no "master". You would say, how are they linked if they are still independently controlled. Merging or linking the side chains of compressors makes all of them compress at the same time according to the highest gain reduction at any given time of any of the linked compressors. In other words you can link 2 compressors in a stereo pair and have one of them set to fast attack and fast release while the other is set to the slowest attack and the slowest release. The thresholds can be set at different values too, but whichever one of the 2 signals going through the 2 compressors ends up triggering one of the compressors (i.e. the signal crosses above the set threshold level for that channel) it will make not only that channel of the compressor react to it, but both channels simultaneously will reduce the gain with the exact amount. For example the channel with the fast attack and fast release may have a sustained bass notes going through it and if the threshold is set really high the bass may not even get compressed at all. On the other channel you may have a kick drum with the same peak level as the bass and threshold set really low so that every kick hit crosses way above the threshold. Because the attack of the kick compressor is set to the slowest possible setting for the given compressor, the kick will not trigger the compressor either, so both the bass and the kick will stay unaffected by the linked compressors. Lets say the drummer plays a slow cymbal roll which gets picked up by the outside kick mic, and even though the level of it is much lower than the kick itself, it is loud enough to cross above the low setting of the threshold on that channel of the compressor and because it is sustained it will trigger the compressor even with the slow attack setting and it will make it reduce the gain of the cymbal roll. At the same time because the side chains of the 2 compressors are linked it will also reduce the gain of the bass as well (probably not what you were going for). Every compressor will react differently in such a situation depending on the ratio and makeup setting as well as the attack and release time settings. The point is: Experiment, and figure things out on your own for every compressor you own. Don’t assume they all react the same. A lot of times you may find linking functions where the thresholds and/or makeup gain controls of the right/second compressor get overwritten by the settings of the first one while the attack and release controls may still be independent.

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