For those of us who have dived headfirst into immersive mixing (Atmos or Sony 360RA), there comes a vexing moment where we realize that there is no simple way to apply bus compression for the mix glue we’ve become dependent upon in stereo. In an Atmos mix, our groove elements may be assigned to a 7.1.2 or 9.1.6 bed, while other tracks may also be assigned to that same bed or to objects that can be placed or moved freely outside the bed. This mixing style provides unprecedented possibilities but also upends the way we think about bus compression! This issue becomes highlighted when a song that already has a great-sounding stereo mix needs to be converted into an immersive mix. Imagine trying to replicate Nirvana’s “Smells Like Teen Spirit” without bus compression!
Over the course of a few dozen Atmos mixes, I’ve experimented with several methods to replicate the effects of mix bus compression in the multichannel environment. All my efforts felt like compromises and left me thinking there should be a better way. Well, PSPaudioware have addressed this problem with PSP auralComp, a compressor and brick wall limiter plug-in capable of processing multichannel busses up to 16 channels wide. As I was in the midst of mixing an Atmos album project, it was the perfect time to give the plug-in a workout.
PSP auralComp grew out of a collaboration between PSPaudioware and Ronald Prent, a pioneering mixer, consultant, and thought leader in surround and immersive music. The plug-in is designed for multichannel bus compression, but a stereo instance of the plug-in can also be used for mid/side compression. Each of the plug-in’s channels hosts a fully featured compressor, with controls for Threshold, Ratio, Attack, Release, Make-Up gain, wet/dry Mix, and sophisticated sidechain linking options. The heart of the compressor emulates an opto-type circuit, but the magic of this processor lies in the way the user can customize the sidechain/key signal for each channel’s compressor. Despite its sophistication, the plug-in feels fresh and easy to navigate, with elegant and easy-to-read graphics. The LED-style meters are responsive and the clearly labeled controls behave well.
Typically, a stereo bus compressor links both channels so that either the sum of the left and right channels (or the louder of the two) triggers the compression behavior and both are affected similarly. For immersive mixing we might want more flexibility regarding which channels have linked compression behavior. For instance, the front left and right speakers may contain content that should not affect the dynamics of the overhead channels but should affect the rear channels. With the PSP auralComp, up to 16 channels in a bus can be linked with almost limitless options as to which channel(s) trigger other channels' dynamic processing.
PSP auralComp lets the user create eight independent sidechain submixes, any of which can be assigned to trigger any of the plug-in’s channel compressors. Channels can also remain independent, with only their own content controlling their respective compressor. A sidechain submix is a blend of any (or all) of the channel’s signals; a submix can be further modified by applying one of four adjustable filters. The submixes are, by default, named with Greek letters but can be freely renamed. For quick editing, channels can be grouped, allowing their controls to be simultaneously modified, and parameters (such as filter settings) can be copied and pasted between submixes.
The best way to explain the application of this plug-in is to give an example. Let’s put the drums, bass, and lead vocals in the front left and right channels and send a bit of the lead vocal and lead guitar to the center channel and front height speakers. The left and right channels could benefit from stereo-linked compression, but the center and heights might pump too much if their compression is simply linked to the left and right channels. With auralComp, the center and overhead channel compressors can be triggered by a submix that includes a mix of the left and right channels that is filtered to exclude the low frequency information. This way, the center and overheads could be triggered by the average level of the drums/bass/vocal blend, but without excessive pumping caused by low frequency energy of the kick. This method provides glue and control to the rhythm tracks yet allows greater dynamics for the vocals and guitar in the other channels. Similarly, rear channel percussion can be linked to the dynamics of the drums in the front channels to maintain a solid and immersive groove.
A brickwall limiter in the output section of the plug-in can be used to control the overall bus level. This feature is a huge benefit for Atmos mixing, which is a constant balancing act of building a powerful mix while trying to maintain the required mix level of -18 LUFS. PSP auralComp has the ability to generate the sidechain blend using different summing methods including the RMS sum of the submix signals, the sum of the rectified submix signals, or the actual sum of the submixed channels. The differences between these methods may not seem intuitive, but the user manual clearly describes the different use cases of the sidechain submix methods. In fact, a quick read through the manual will enlighten you about the possibilities and strategies of the PSP auralComp. To help get us started, the plug-in also includes a set of useful factory presets.
This compressor allows me to replicate the bus compression techniques I’ve spent years working on in the stereo realm and expand them into the immersive realm. I no longer have to forego bus compression in my Atmos mixes or create tortuous routings to create linked compressor channels. Even if you only use the PSP auralComp for stereo compression, you will find it offers more creative options than almost any other bus compressor. A few years ago, I hoped that plug-in designers would push past simply replicating traditional compressors and develop new paradigms for dynamic controllers, and the auralComp is a great leap forward in that regard.
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