Essentially bx_XL use M/S or Mid/Side processing to split the incoming stereo single into two components for separate compression and limiting. The Mid or mono center channel signal is all audio signals common to both the left and right channels and the Side signal is everything else--all signals that are not correlated between the two channels. The Side signal conveys the width information that makes sound into stereo sound.

XL goes further by splitting the Mid signal into Mid Lo and Mid Hi frequency bands and provides a crossover frequency adjustment to set the exact frequency where they divide. These components, the Mid Lo, Mid Hi, and the Side are each processed with individually adjustable limiters and XL harmonic maximizers and then combined back together and coded back into a conventional stereo signal and finally routed to a peak stop "brick wall" limiter.

All of this technical hard work is done in the background and controlled by a powerfully detailed GUI that allows for using only a few or as many parameters as you chose.

I liked the Master section that groups all sections with a single set of controls. I found the Master section the best place to start using this plug-in. I would get my mix loud just using the Gain Boost and Threshold knobs in this section and then (usually) raise the Threshold in the Mid Hi to "unclamped" the center channel that contains vocals, snare, guitars, keys etc. As needed, I would also tweak other parameters to see if I could further refine the processing.

I liked the effect of the XL controls on each section and, again, I usually went for the "ganged" Master XL control to set them globally for my mix. If you want more harmonic coloration in the bottom end of your mix, it is easy to go and crank that section's XL up.

The M and S controls on the I/O section enabled me to set the center channel Mid level of my mastered mix in relation to the overall stereo width. Stereo width or the S level is also adjustable and I liked to automate that parameter for certain sections of the mix such as a breakdown where there are no vocals in the center.

Lastly, there are four meters at the top of XL for checking: the level relationship between the Mid Lo and Mid Hi, the ratio or proportion of Mid to Side levels, the Correlation meter measures the aggregate in-phase component of the left and right channels' program audio when summed in mono, and Balance shows the instantaneously left channel versus right channel levels. Cool!