Mix bus processing is using EQ, compressors and limiters on the output bus of an audio mixer. It can be stereo or mono, real or virtual and is used on the entire audio mix. These processors are used to “glue” the mix together, to make it punchier and to fix any overall frequency imbalances.
Sweetwater, a pro audio dealer based in Fort Wayne, Indiana, tells us that approaches to mix bus processing vary. Some engineers prefer to place processors on the output bus (also known as the “2-mix”) before the mix begins and then mix into those processors for a more “finished” sound.
Since adding mix processors may change the perception of balances, dynamics and EQ in the mix, this allows the mix to with clearly audible results.
Some engineers use no mix bus processing at all, preferring to leave all post-mix processing in the hands of the mastering engineer. This would arguably be the purist approach, but it also allows all mix processing to be handled by the (presumably) super-high-quality mastering gear. Others apply mix bus processors late in the mixdown to lightly “touch up” the mix before mastering.
Whichever way mix bus processing is approached, Sweetwater recommends using caution with how heavily EQ and dynamics processing is applied. Over EQ or compression of audio can tie the hands of the mastering engineer. In particular, mix bus processing should not be used to make the mix louder. Compression and limiting should be used at this stage to glue and control the mix, not to increase level.
Individual hardware devices or plug-ins can do mix bus processing. The stereo output from the 2-Bus+ routed through a Dangerous Music Bax EQ, a Maag Audio EQ-4M and a Manley Variable Mu limiter/compressor on the stereo mix bus can work wonders.
Manley Variable Mu Limiter/Compressor
Another analog approach is to use an all-in-one mix bus processor, such as the Rupert Neve Designs Portico II Master Bus Processor or the new SSL Fusion. These combine EQ, compression/limiting, stereo field processing, and more into one package. Another option is a mix-processing plug-in, such as iZotope Ozone or Zynaptiq Intensity, which allows similar processing (and a variety of other things) in the virtual domain.
Mix bus processing is the last opportunity to get things right in an audio mix before it moves on to mastering. Choose the processors carefully and use them intelligently. This way you’ll have a excellent sounding mix that will only sound better once it has been mastered.
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