Mix-bus compression is a double-edged sword. Used properly, it can deliver additional loudness, and help to tackle problems commonly associated with mixing in DAWs, applying coherence and 'glue' — yet it can just as easily ruin your mix completely. Most of us have probably found ourselves wondering whether putting a compressor across the mix bus will make things better or worse, and wondering which compressor to use. Slate Digital's answer to this dilemma is to give you not just one compressor but three.
Virtual Buss Compressors (VBC) is a software bundle comprising three plug-ins created in the image of some of the most sought-after hardware mix-bus compressors, plus a fourth plug-in that combines all three models in one interface. The VBC bundle is authorised to a second-generation iLok USB key and is available for both Mac OS and Windows in Audio Units, VST and VST3 formats (32- and 64-bit) as well as RTAS and AAX1 (32-bit only). A 64-bit AAX2 version is in development. In the initial release, the VST3 versions did not work in Cubase 7 (64-bit), but this was soon fixed in the 1.0.1.6 public beta. Aside from that, there have been no stability issues.
FG-Grey
Like all of the VBC plug-ins, FG-Grey offers a variable high-pass filter in the side-chain, which lets you decide how much influence low-frequency mix elements such as bass and kick drum should have over the compression: depending on the musical style, some pumping can actually feel quite nice. Another addition to all three plug-ins, not present in the original hardware, is a Mix knob for instant parallel compression.
The set of controls is standard for a VCA compressor — attack, release, threshold and ratio — and FG-Grey is probably the easiest of the three to configure, because the parameter ranges are sensibly chosen. However, the black knobs with their thin markers are quite hard to read against an almost black background. It's easy to overlook the Auto release button, which typically gets a lot of use in mix-bus applications. Unlike the hardware, which only has three ratio settings, the lowest being 2:1, FG-Grey can do 1.5:1 for a softer type of compression. For accuracy's sake, I should mention that FG-Grey also omits one feature of the original hardware: the auto-fade button. I'm sure it had its uses in analogue times, but now that we have DAW automation, I doubt that many engineers will miss a knob for automated fade-outs. One thing I dislike, however, is the fact that FG-Grey isn't level-matched. Just plugging it in without any processing going on increases the level by 1.5dB.
FG-Red
The Red 3 Compressor/Limiter may not be as popular as the SSL Buss Compressor, but as mix god Chris Lord-Alge is a prominent user, chances are you've heard it in action a million times.
he parameter set, again, is typical for a VCA design: attack, release, threshold and ratio. But while FG-Grey has stepped attack and release controls showing actual values in milliseconds, FG-Red has continuous controls going from 0.0 to 10.0. This makes the Focusrite emulation a bit more difficult to operate than the SSL clone, at least for me. On the plus side, as well as the variable side-chain high-pass filter and Mix knobs, Slate have added a third feature. Apparently, many users of the Focusrite hardware like the unique saturation characteristics of its output stage when driven hard, and FG-Red's Drive knob allows you to add some grit without readjusting input and output levels.
FG-Mu
FG-Mu is the only VBC plug-in that bears a passing resemblance to its hardware counterpart. The VU meters are similarly shaped, and there are big, vintage-style knobs. Yet again, however, Slate have incorporated extra features. While the original Fairchild 670 merely had a couple of controls per channel for input gain and threshold, plus a six-position time-constant switch, the FG-Mu has separate knobs for attack and release time, as well as an additional output-level adjustment. The latter can be used in conjunction with the input-level knob, to set the right amount of (virtual) tube saturation.
Like the original, FG-Mu has no ratio control. The Fairchild 670 was originally designed as a limiter, but — typically for a variable-mu tube design — its compression curve has a very soft knee, ranging from about 2:1 at the onset of compression to full limiting (about 30:1). Its soft characteristics, along with its rather fast attack and longish release times, mean that it is not always ideally suited to today's hard-hitting pop music. After all, it was designed before rock & roll made any real impact. However, FG-Mu performs nicely on everything for which people typically use a Fairchild these days, which is mainly vocals, string sections and slow-moving stuff, and it does work well as a mix-bus compressor on songs without drums, or vintage-flavoured tracks with the percussive elements mixed further back.
Conclusion
In action, I was quite impressed by the Slate collection. Each of the Virtual Buss Compressors comes with a distinct sonic personality. FG-Grey is the snappy one, excellent for in-your-face pop; FG-Red would be my first choice for rock tracks, due to its saturation function; and FG-Mu provides a lush bloom with a vintage flavour for slow tracks. None of the compressors is entirely neutral. They all introduce some degree of harmonic distortion, and while the two VCA designs are fairly linear in the frequency domain, FG-Mu displays a gentle bump in the 2kHz region.
The Virtual Buss Compressors bundle may not be as radically new as some of Slate Digital's previous offerings, but it's rare to find so much sonic detail and realism in a native plug-in. Even though Slate have taken some liberties, the emulation quality is exceptional: these plug-ins sound and feel like working with analogue equipment, and the included Focusrite Red 3 emulation is a rarity in the plug-in world.
What are the down sides? With their carefully modelled non-linearities, which cannot be turned off, the VBC plug-ins may not be ideally suited to ultra-clean processing such as is required in some mastering applications, as they will always impart a certain flavour.
Read the full review at Sound On Sound.