Building A Multi-Band Compressor In Reason

Reason doesn't have a dedicated multi‑band compressor, but that doesn't mean you can't build your own.

This month we go all Blue Peter — you get to build your own multi‑band compressor, and here's one I prepared earlier.

As we've seen in recent Reason columns, there are some useful audio treatments and effects that appear to be missing in Reason, but are achievable when you know where to look — or if you think a little outside the box. Examples include comb filtering and ring modulation, courtesy of Malström's filters, the multi‑tap and analogue‑style delay algorithms in RV7000, and pseudo-stereo techniques via signal splitting and short delays. This month we're going to turn our attention to multi‑band processing with a hands-on example.

Multi‑band Dyamics

The wiring for the home brew dual‑band compressor described in the main text.

It's quite surprising, I think, that neither Reason nor Record has a dedicated multi‑band dynamics processor. These are frequently employed for mastering purposes, splitting a mix into separate frequency bands, and providing separate compressors for each. This ensures that compression on cymbals (which consist mostly of high frequencies) isn't triggered every time a loud kick-drum (predominantly low frequency) comes along, to give just one common example. The overall outcome is much greater control over compression with less obvious pumping and 'surging' than could be achieved with a single broad‑band compressor.

So how do you get this in Reason (and Record)? Well, without too much fuss you can have yourself a dual‑band compressor, and that's often a whole lot better than a broad‑band compressor, or nothing at all, for slapping onto a main mix. It's possible thanks to a sneaky little technique involving the MClass Stereo Imager device. This has a crossover that can be brought into play independently of any stereo processing, and splits a signal into low‑ and high‑frequency bands. Then a bit of jiggery‑pokery separates those bands into discrete audio feeds.

Home Brew Compressor

The front and rear of the MClass Stereo Imager, when set up to work as a crossover. The combination of the front‑panel 'Solo Lo Band' and rear‑panel 'Hi Band' switch setting causes low‑ and high‑frequency components to be handled separately by its two stereo outputs.

Here's a quick walk‑through of how to set up a simple dual‑band compressor for a drum loop playing in a Dr Rex, in Reason 4. To make the whole thing neater, and recallable, we're going to build it in a Combinator:

1. With your Dr Rex already in place, create a new Combinator. Do this by right‑clicking on the Dr Rex and the two devices will be next to each other, which is handy.

2. View the rear of the rack and do a bit of audio re‑routing. We need the Dr Rex's audio output patched into the Combinator's Combi Input. The Combi Output should then be hooked up to a mixer channel.

3. Now for the crossover. Make sure the Combinator's 'Show Devices' button is selected. Then in the Combinator's Device area, currently empty, right‑click with the shift key held down, and choose an MClass Stereo Imager from the Create sub‑menu. The shift key prevents it being automatically wired up.

4. Now go on and create two compressors in a similar way. For simplicity here I'm using COMP‑01s, but you could just as easily choose MClass Compressors, which have more bells and whistles.

5. We need to re‑combine the signals from these two low- and high-band compressors, so in readiness for that, create a Line Mixer 6:2, still with the shift key held down.

6. Now, a cable‑fest. Patch the Combinator's 'To Devices' to the Stereo Imager's audio input, then the Stereo Imager's Audio Output to the L/R input of one of the compressors, and its 'Separate Out' to the other. Hook up the compressors' outputs to input channels on the Line Mixer. And finally take the Master out of the Line Mixer to the 'From Devices' input on the Combinator. Phew! The results should look like the screen above.

Crossover

You just built a dual‑band compressor — well done! But there's one final step, to configure the crossover. The Stereo Imager device wasn't strictly built for this role, but it does a great job. The trick is to engage one of its front‑panel solo modes (in this example I chose to solo the 'Lo Band') and that causes its main outputs to carry only frequencies below the 'X‑over Freq' knob setting. But we're also using the separate outputs, and on the rear of the Imager there's a switch to choose what emerges from those. In this case I chose 'Hi Band'. So we have low frequencies coming from the audio output, and high frequencies coming from the separate output, and the front‑panel 'X‑over Freq' knob sets the crossover point between the two. Cunning.

That's the last step. Now I've got hugely increased flexibility over the compression characteristics of my Dr Rex loop. Whacking up the ratio on my Lo Band compressor makes the bass pump and grind, but my Hi Band cymbals, which aren't being compressed very much at all, still sound completely natural. Just tweaking compressor settings makes a huge difference to the sound of the loop, and the crossover frequency knob on the Stereo Imager allows the effect to be 'tuned' to include or exclude different parts of the frequency spectrum. What's more, the Line Mixer allows the relative levels of each band to be adjusted, acting almost like a bass/treble EQ. After that, it's all about experimentation, and articles like SOS's own Multi‑band workshop (/sos/aug02/articles/multiband.asp) can give you some more ideas. Incidentally, the best way to apply a multi‑band compressor like this to an entire mix in Reason is to patch the Combinator that contains it between your main 14:2 mixer and the Hardware Interface. Then everything goes through it.

One More Band

A three‑band compressor on paper...

The dual‑band compressor I described above is a useful thing, but how much more useful would it be to have a three‑band compressor? These allow even more subtle and natural‑sounding treatments, and can particularly keep vocals (which are normally very much 'mid'-band) sounding really natural while energetic compression occurs in the low and high bands. You can also compress just a narrow middle band of frequencies (try working outwards from about 5kHz) to suppress sibilance when you've only got a whole mix to work with.

Such a thing is possible, by going further down the dual‑band route, but you have to work hard not to get confused! First, you use a Stereo Imager to split off low- and high-band signals as before. But you feed the upper-band signal directly into another Stereo Imager, and its 'X‑over Freq' control then effectively sets the crossover point between the resulting mid‑range and high bands. The Imager outputs then feed three compressors, which are subsequently combined by a line mixer.

... and wired together in Reason.It's a bit of a Heath Robinson thing to build from scratch, and to look at, and some potential confusion surrounds the crossover frequency settings of the two Stereo Imagers — you have to make sure the mid/high-band frequency is set higher than the low/upper‑band frequency, or you'll get some sort of parallel compression occurring. But actually it's a superbly useful thing to have around, and it's easy to master after a minute or two. You can see how it works in the diagram above and, in the screen below, how it appears in Reason. Despite somewhat scary appearances, this only takes a couple of minutes to build, and it's well worth it.