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Q. Could I create my own multi-band compression?

Q. Could I create my own multi-band compression?

I have been considering buying a Focusrite Mixmaster for its analogue multi‑band compression. However, before I do, I would appreciate your views on the following.

Before the days of digital and 'finalising' products, 'multi‑band' compression was presumably all but non‑existent. However, look at the equipment list of any top studio and you will find a number of compressors, obviously for different applications. I assume, using group inserts, that these could be utilised to provide a sort of multi‑band compression, if it was required, by grouping instruments according to their frequency ranges.

I have always thought the problem with 'finalising' products is that the very thing they set out to achieve is actually their shortcoming, in that all the sounds that fall into a specific frequency range will have the same amount of processing applied to them. For example, the low band would treat a bass guitar, the kick drum and the low notes of synths in the same way and, this may be the last thing you want.

Despite this, there is no doubt that devices such as the TC Finalizer and Dbx Quantum provide a very convenient package for the small studio, and it occurred to me that in these days of 'valve everything', an affordable multi‑band valve compressor would be very nice to have. Indeed, some time ago I sent TL Audio an email suggesting they put three of their FAT1 units in a box with a crossover and give us the very thing. They said they would think about it.

Given the affordability of valve compressors from the likes of TLA and Behringer, I have been considering the practicalities of creating my own multi‑band compression setup. Three‑way crossovers, with user‑defined crossover points, are readily available from Samson, Soundtech and others for about £150. Add three valve compressors, at about £250 each, and for less than a thousand pounds you have a three‑band valve compressor, with all the parameters for each band having instant control via knobs. It would be less expensive than a Quantum or Finalizer, though it would have no presets or memories. But do we really need them? A pen and paper does the same thing.

I realise that you could just use the three compressors on different groups, as previou sly mentioned. It's the idea of a multi‑band unit that is of interest. I'm not particularly technical, so there may be other issues that would need to be addressed.

John Ward

Technical Editor Hugh Robjohns replies: You have raised several different issues here, so I'll try to address them in order. The first thing to say is that multi‑band compression is almost always used as a mastering process, to smooth out the overall spectral content of a mix, and to optimise its perceived loudness. It is a final polishing process, not a part of mixing. Radio broadcasters use heavy multi‑band compression, principally to make their stations sound louder than their competitors. By using multiple frequency bands (five bands are typically used for pop music stations, and three for classical music), a greater degree of compression can be applied than could be acheived with a single, overall compressor, and with less obvious side effects (if set up correctly, anyway). Of course, this does inherently change the balance of the mix in a very dynamic way, and the heavier the compression settings, the less subtle the effect.

The technique of using different compressors in different groups (or on individual instruments) during the mix is, of course, widely used but this is not the same as multi‑band compression. Kick drums and bass guitars both contain frequencies way outside the 'low frequency' band of a typical multi‑band compressor, so using an overall compressor on these instruments changes level but not spectral balance, whereas a multi‑band compressor changes spectral balance. By compressing groups of instruments in the way you suggest, you may well achieve a tighter sound, but you would be unlikely to obtain the same kind of overall spectral smoothing or increased volume that a multi‑band compressor could provide.

Although there are several analogue multi‑band compressors around, most seem to be digital these days, principally because the technology allows such huge flexibility, and the band filtering can be designed to minimise the phase‑shifting problems which are inherent in analogue filters. On top of that, digital systems provide more user convienences such as presets, memories, and so on, and the DSP power can be used to offer normalising, look‑ahead transient limiting, multi‑band stereo width control, and a host of other useful signal processing, all for an insignificant extra cost.

However, as you suggest, there is something very nice about well‑designed valve processing (used appropriately), and a valve‑based multi‑band compressor would certainly have its supporters. But it is not quite as simple as buying the component parts and strapping them all together. The phase shifting inherent in the band splitting makes recombining the compressed signal a non‑trivial task, and the filter shapes and slopes have to be very carefully designed. Also, the individual compressors have to have their side‑chains linked in a complex way, so that the compression in each band relates to that of its neighbouring bands. This is to ensure that no one band becomes excessively compressed (or uncompressed), as this will also make recombining the signals very difficult and produce a rather strange and unnatural sound quality.

While I wouldn't want to put you off the idea of experimenting and building your own equipment, the design of a multi‑band compressor is not quite as simple as it might first appear. It also strikes me that a more pragmatic (albeit more expensive) approach might be to use a conventional digital multi‑band compressor followed (or preceded) by a valve preamp stage, to add the valve warmth you are looking for. For an all‑in‑one solution, take a look at the Drawmer DC2476 Masterflow (reviewed SOS October 1999), which is a very competent digital multi‑band compressor with excellent multi‑band valve emulation, allowing you to set the required amount of valve saturation for each band.