Brainworx bx_XL

THE BRAINWORX bx_XL mastering plug-in (AU, RTAS, VST2.4, and VST3)—also suitable for mixing—converts a stereo signal into separate mid and side (M/S) channels for separate dynamics processing.

Fig. 1 The bx_XL offers sophisticated splitband M/S processing, monitoring, metering, and sidechain


Powerful split-band mid/side limiter plug-in for mastering


THE BRAINWORX bx_XL mastering plug-in (AU, RTAS, VST2.4, and VST3)—also suitable for mixing—converts a stereo signal into separate mid and side (M/S) channels for separate dynamics processing. It supports sampling rates up to 192kHz and uses 64-bit internal processing, giving a huge internal dynamic range.

The mid channel contains all mix elements common to left and right channels (e.g., center-panned kick drum, lead vocals), while the side channel carries the difference signal—mix components with dissimilar left and right channels, like stereo reverb. But the bx_XL further splits the mid channel into two frequency bands (Mid Hi and Mid Lo) for independent limiting, with a Crossover Frequency slider to set the desired split point; see Figure 1. (For simplicity, we’ll call these two bands—along with the broadband Side channel—“channels.”)

Imagine being able to limit a centerpanned lead vocal’s high frequencies without affecting the dynamic range of the kick drum’s bottom end, or the dynamics of hard-panned guitars. Powerful stuff, but it’s just the tip of the XL iceberg.

Image Maker Left- and right-channel input levels are independently adjustable, both to tweak the stereo image and to change how hard you drive the downstream limiters. You can link levels to preserve the stereo image; if you set a level offset between channels, linking preserves it.

The mid and side channels have separate pan controls so you can, for example, center the kick, snare, bass, and lead vocal in your mix without skewing the image for reverbs and stereo-miked guitars. And if, say, the mix’s right channel is higher than the left, you can re-balance it using the side channel’s pan control—without disturbing the center image!

bx_XL also includes a Mono Maker control, a powerful feature found in other Brainworx plug-ins, that centers the low end by converting all signal below its corner frequency into mono for the plug-in’s input signals. You can link the Mono Maker control to the afore-mentioned Crossover Frequency slider.

bx_XL’s Mid Lo, Mid Hi, and Side channels each have the same types of (independent) controls—XL, attack, release, bypass, solo, gain boost, threshold, fader link, and sidechain (see next section). Raising a channel’s gain boost or lowering its threshold increases that band’s amount of limiting. If you activate the fader link button, the threshold lowers as you increase the gain boost control, keeping the channel’s output level more consistent.

Fig. 2 The Sidechain Mix slider adjusts the relative balance of two sidechain signals. The thin vertical line of “LEDs” between input and output meters indicates the combined signals’ level in a keyed channel’s sidechain (with the Sidechain button activated).


Limiter attack times for each channel range between 0.1 and 1.5 ms, with release times from 0.01 to 999 ms. A band’s solo button lets you monitor it in isolation, although you can’t solo more than one channel at once.

Raising a channel’s XL control adds 3rdand 5th-order harmonics to its signal, postlimiter; XL processing can be bypassed for each channel. Meters for each channel show their input, output, gain-reduction and, if the sidechain button is activated, sidechain levels.

Sidechain Acclaim Powerful sidechain features populate bx_XL’s thoughtful interface. Assuming your DAW supports sidechaining, you can choose two sidechain sources from Mid Lo, Mid Hi, Mid (both Lo and Hi), Side, and External (see Figure 2). The Sidechain Mix control sets the relative signal balance between the two sources.

bx_XL facilitates sidechain setup with a solo (key-listen) button for the sidechain. In fact, you can also solo the left, right, mid, and side channels in turn in mono (routed to both left and right outputs)—a great aid for centering a mix’s imaging and hunting down phase and distortion problems. A defeatable Auto Solo feature automatically solos any channel when you mouse-grab any of its controls (until you let go).

Yes, Master Additional Master-section controls for XL (level and bypass), gain boost, and threshold, provide simultaneous control over their counterparts in Mid Lo, Mid Hi, and Side channels, preserving any inter-channel control off sets. Linkable mid- and side-channel output-level controls follow in the signal path. The signal then runs through a defeatable stereo brickwall limiter (which sports gain-reduction meters), followed by a master output-level control. Innovative L/R output meters show peak and RMS levels (after all processing) simultaneously with your mix’s overall dynamic range. Other meters show correlation (L/R phase coherence), L/R balance, and the ratio of mid-to-side and Mid Lo-to-Mid Hi content compared to that in your original mix.

bx_XL provides 32 steps of undo and redo. Four workspaces allow comparing and switching among different setups (great for mixes that require, for example, different treatments in verse, chorus, bridge, and coda). The states of all four workspaces are stored in custom presets saved using your DAW.

I’ve Got a Crush A single instance of bx_XL consumed about 15 to 20 percent of my 8-core Mac Pro’s CPU resources with DP’s buffer set to 1024 samples.

I first used bx_XL on an overly dynamic instrumental mix that lacked low end (web clips 1 and 2). It was easy to add more bottom by cranking the Mid Lo channel’s Gain Boost, and lower the channel’s threshold to control the largest peaks. XL-harmonics processing applied to the Side channel added wonderfully musical presence to a hard-panned synth pad; adjusting the side channel’s output gain restored some width to the mix that had been diminished by my mid channel adjustments, and gave the bass and guitar more room in the center. The peak stop limiter was effective and sounded quite forgiving—it made the mix sound just a hair less open and supple. However bx_XL lacks dithering facilities, an odd omission for a “finishing” limiter.

The meter sections show up to four different levels (input, output, gain reduction, and sidechain) simultaneously, and I quickly became hooked on the wealth of information. When I stopped DP, all meters held their last levels until they received new input signals, and the L/R output meters held full-scale “overs” until I stopped and restarted DP’s transport (oddly, they indicated clipping around 1.3 dB below DP’s full-scale readings). To my amazement and joy, the correlation and balance meters showed the original mix’s associated attributes when the plug-in was bypassed (whether using the plug-in’s or DP’s bypass button), greatly facilitating comparisons to the unmastered mix.

bx_XL v1.1.1 was a little buggy. Gain-boost tweaks didn’t always revert and restore when using Undo and Redo, and using any of the solo buttons (except the sidechain’s) almost always caused DP to freeze. My fail-safe workaround was to activate the Auto Solo function and mouse-grab any control for the channel I wanted to solo; setting the Crossover Frequency slider to 20Hz allowed monitoring the mid channel’s full audio spectrum.

Although the GUI’s layout is positively faultless, most of the solo functions are buggy and the documentation is confusing enough that the learning curve is unnecessarily steep. I’m sure the bugs will be fixed, though, and this is a tremendously powerful plug-in as is. I use Brainworx’s other plug-ins on virtually all of my mastering sessions; bx_XL is next on my must-have list.


STRENGTHS: Superb sound quality. Powerful, deep feature set. Extremely well-organized GUI.

LIMITATIONS: No dither. A little buggy. Documentation needs improvement

$419 MSRP