How to set a mastering compressor
What are the goals of compression (and limiting)?
In the most basic sense, compression is the reduction of the crest factor of audio. Crest factor is the difference between the average amplitude and peak amplitude of the measured content. In the digital domain, average amplitude is most commonly measured as RMS, and peak amplitude as read by your daw or conversion meter is a negative number descending from an upward limit of 0.0db. It’s important to note that any peak content exceeding 0.0db will be digitally truncated, introducing high-order harmonics and intermodulation distortion. While this type of digital clipping is often used by mastering engineers to achieve extreme loudness, this technique is outside of the purview of this article. Whats important to keep in mind is that any processing that reduces crest factor, whether compression or clipping, will also increase perceived loudness.
While buss compressors function by reducing crest factor, their practical use in mastering is quite different. After all, the non-linearity of digital limiting plugins allows them to increase loudness with more transparency than is possible with a compressor. Because of this, mastering engineers mainly use buss compression to precisely control the dynamic envelope of the content being mastered. The term “glue,” which is frequently used to describe professionally-mastered content, is essentially describing content with a properly controlled dynamic envelope. Once a mastering compressor is properly set, you’ll also notice your content is more exciting, has more depth, and a more pleasing stereo image.
Mastering compressors are tweaky pieces of gear with widely overlapping controls. To achieve more gain reduction, for example, one can increase the compression ratio, decrease the threshold, shorten the attack or lengthen the release. While all of these changes will increase gain reduction, they will each impart a very different character to the compressed content. For this reason, it can be helpful to ‘reset’ your mastering compressor before adjusting it to your content. In my opinion, an ideal starting point is setting your mastering compressor to its lowest ratio (above 1:1), highest threshold, slowest attack and fastest release.
From here, decrease the compressor’s threshold until you achieve several db of gain reduction. This makes it possible to hear what the compressor is doing and therefore the kind of character its adding to the track. While perfecting your settings, it's helpful to temporarily over-compress your content. To adjust the dynamic envelope, start by reducing attack times until your compressor starts to ‘grab’ immediately after major transients. Next, lengthen release time until your gain reduction just barely returns to zero before the next transient. This is a good starting point for a mastering compressor. How you adjust from these settings will depend on the type of dynamic effect you’re looking for from your compressor.
From this basic starting point, raise the threshold until your maximum gain reduction is about .5db. This is a typical amount of gain reduction for dance, rnb or hip-hop content. In my work flow, I typically make a test print of about 32 bars with these settings. Next, I incrementally increase the compression ratio while simultaneously increasing the threshold to maintain the same .5db of gain reduction, printing another test at each new ratio. This allows me to a/b compare the track with different ratios at the same perceived volume. Higher ratios will make the track feel closer to you relative to the speakers, yet at the same time more processed. Lower ratios feel more transparent with less harshness. Excessive ratios are characteristically distorted, with louder elements ducking away quieter elements.
After you’ve selected the ratio which best suits your goals and content, the attack and release times can be further tweaked. With dance genres where closeness and depth are desirable traits, slower attacks and faster releases are generally most suitable. Vocal content such as hip-hop and r&b often requires a bit more control from the compressor. As attack times shorten, your mix will sound more controlled and farther from the listener. Attack times that are too slow will sound bouncy, pumpy or even distorted. As release times lengthen, your track will have more roundness, smoothness and glue. Excessively long release times will sound squashed, hazy, or muddy. Release times that are too short will sound pumpy and distorted. Again, it can be helpful to run test prints with slight variations in attack and release times. Even the most experienced engineer can benefit from the a/b comparison this allows.
It’s my opinion that a capable analogue compressor is necessary for creating the highest quality audio masters. However, plugins have gained significant ground over the last few years, and now allow engineers to achieve very acceptable results without the cost or headaches of breaking out of the box. In fact, my favorite mastering compressor plug-in, the Tokyo Dawn Records ‘TDR Kotelnikov,’ is available for free download.
In the analogue realm, a great starter compressor is the Foote Control Systems P3s. While Foote Control Systems makes a slightly tweaked mastering edition of this compressor, even the base model is well suited for mastering and an outstanding value. The p3s is exceptionally transparent and it's versatile controls offer a high level of dynamic control. Foote Control Systems also makes a higher end compressor called the p4s me. While it's controls are a bit less extensive, it imparts an aesthetic analogue enhancement to your content due to its very large transformers. The p4s me is one of my favorite analogue compressors and gets used on a daily basis in my studio. My other go to compressor is the fairchild inspired Knif Vari Mu II. Its amazing amorphous core transformers offer detailed and transparent compression thats unmatched by any other piece of analogue gear. Its secondary release curves provide a familiar Fairchild characteristic that sounds right on nearly everything. Keep in mind when using outboard analogue compression, you'll also need high quality d/a and a/d conversion. Consumer grade audio-interfaces may get you by for your monitoring chain, but they will not pass for your mastering signal.