In our latest mastering workshop Mike Hillier turns his attention to the use and misuse of compressors in mastering…
For many engineers, compression is one of the toughest tools to crack. The basic concepts are easy enough to grasp, but in inexperienced hands it can be all to easy to ruin a mix with over-use of compression, or by dialling in the wrong type of compression.
For this reason, while they may be happy using compression on individual instruments, many mix engineers shy away from compression across the mix-buss, preferring instead to leave this task to the mastering engineer, who is trusted to have a better understanding of how buss-compression affects the mix as a whole.
This is a bold decision though, as it leaves a major mix decision to a third-party. Other mix engineers may make judicious use of buss-compression, and of course, some experienced mix engineers choose not to use buss-compression at all for creative reasons.
As the mastering engineer, your first task before using compression is to listen to the dynamics of the track as a whole and decide what, if anything, needs to be controlled. The amount and type of compression already used will have an impact on what more compression, if any, you
will want to use.
When listening for buss-compression, listen to how well the mix as a whole gels together. Are there any instruments or frequencies that poke out? Do any of the transients, particularly those that aren’t percussive, sound too spiky?
Fast transients can be dealt with using a fast attack, fast release compressor. Set this with a fairly high threshold, so as not to compress anything beyond the transients. This kind of compression can help to control a track, and push transients back into the mix, but it can also kill any sense of punch, clarity and dynamic in the mix, so be careful when applying it to only compress transients that are causing problems in the mix.
For fast compression, VCA-style compressors are usually the best tool, as they can be set with very fast settings without introducing audible distortion. FET compressors, while fast, are less common in mastering, due to the distortions associated with these units.
If the transients aren’t causing a problem, or have already been dealt with, and the mix still isn’t gelled together like you might want, a compressor with a slower attack and release can be used with a lower threshold to gently glue the mix together.
Very low ratios are usually used for this task, with 4:1 being on the heavy end, while 2:1 and even 1.5:1 are common choices. Optical and Vari-Mu compressor types are the most common choices for this type of compression, but VCA compressors can also be used.
One of the common mistakes with buss-compression is considering it to be a set and forget tool. If the song has varying dynamics as it progresses, you may need to automate the threshold, to try and get a more solid compressor response.
Similarly, you can use automation to make certain sections stand out, perhaps pushing the gain into the compressor a little harder on the final chorus. Subtle, half-decibel changes are usually all that is required, but can make a huge difference.
Different compressors will have different tonal applications and even very similar units – such as two different optical compressors – can sound quite different, even before they begin to compress, so it can be useful to try different models if you are ucky enough to have the options available
Focus On Chaining Up
While we’re displaying the use of two compressors chained in serial throughout this workshop, often the material sent to us for mastering won’t require both compressors to be engaged.
We may choose to use both anyway, with the threshold on one set so as to not compress at all, to add the tonality of the compressor. But most often we will simply prefer to only use a single compressor.