The Reason Why You Don’t Want to Compress the Master Bus

The Reason Why You Don’t Want to Compress the Master Bus

You've often been told not to do it...but here's the technical reason why

by Craig Anderton

Traditionally, mixing and mastering were separate processes. The object of mixing was to provide the ideal balance among tracks, with mastering adding the final polish to make the most out of that balance as well as insure consistency over the course of an album. Mix engineers did not add final compression or EQ on the master not just out of respect for mastering engineers, but because mastering suites often had the best possible tools to complement the engineer's expertise.

The uncompressed pre-master is at the top, the compressed version below.  Whether you place EQ before or after compression makes a big difference in the final sound.

This tradition continued into the early days of DAWs for aesthetic but also practical reasons. Plug-ins intended for mixing had to be light on CPU consumption if you were going to have multiple instances in a multitrack project. Mastering plug-ins were designed for maximum fidelity and accuracy, not conservation of CPU cycles. So, you would bring them into a program like Sound Forge or Wavelab that specialized in mastering.

With today’s more powerful DAWs and computers, more people are adding “mastering” processors to the master bus and mastering as part of the mixing process. The main advantage to this is you don’t get “surprises” should mastering change a mix’s delicate balance. If a mastering engineer’s expertise isn’t required, then this is a valid way to work. (I still think it’s better to treat mixing and mastering as separate processes even if you’re doing both yourself, but to each his own.)

If you do separate mixing and mastering, then the standard advice is “don’t add any processors to the master bus” but there are seldom practical reasons given as to why this is important, aside from the usual caveat of not tying the mastering engineer’s hands. So, let’s look at the technical reason why this is important. (Incidentally I’m not just talking about brickwall limiting; a lot of folks think “well I’ll add just a little ‘glue’ to the master bus so the tracks sit together better” but I even recommend against that until the actual mastering process.)

For me, mastering is primarily about EQ and to a lesser extent, dynamics. However EQ before dynamics or EQ afterward produces very different results. A good example is wanting to bring up a kick drum in an EDM song during the mastering process. If you insert EQ after limiting, the kick will sound big but the extra level is now above the limiting threshold. This restricts the amount of available headroom, so now you have to lower the overall level, or add a second stage of limiting to try and re-gain the additional level.

If the EQ is before limiting, then you’re “pushing” the kick into the limiter. While this doesn’t produce as much actual level as EQ after limiting, it gives the psycho-acoustic impact of more level because the kick “pushes” the rest of the audio out of the way to make room for the kick. The music sounds like it’s straining a little more, and has an added feeling of power.

Another consideration works in reverse. If a master needs to be brighter, I tend to add that processing post-dynamics. Brightness can lead to ear fatigue, so EQ before compression can bring up the brightness to an unnatural degree. But also note that boosting treble frequencies, unlike boosting a kick drum, doesn’t add a lot of energy to the master. So to maintain headroom, you may need to reduce the overall level by only a fraction of a dB or so.

The bottom line is that not processing the master bus isn’t just about tradition—there’s definitely a valid reason to create pre-masters with sufficient headroom and no compression in the master bus, then apply the processing needed to produce a quality master recording.

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 Craig Anderton is Editorial Director of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.