Q. Can I output my final mix one channel at a time?

Sound Advice : Mixing

I have recently purchased a Golden Age Project Pre 73 MkII and Comp 54 on the recommendation of someone from the SOS forums, and I am so pleased. I use an RME Babyface and wondered, with my limited hardware, would it be possible to output my final mix one channel at a time through the Comp 54? The reason I ask is that the hardware adds something that no VST seems to be able to do. If someone knows how I could do this it would be great. If it matters, the DAW I am using is Reaper.

Via SOS web site

SOS Technical Editor Hugh Robjohns replies: 

The answer is yes, but it’s not as straightforward as it might appear and you need to be careful.

The basic problem is that when you’re working with a stereo mix the stereo imaging is determined by the subtle level differences of individual instruments in the two channels. A compressor exists to alter the level of whatever you pass through it dynamically, depending on its own level.

Imagine an extreme situation where you have some gentle acoustic guitar in the centre of your mix image, and some occasional heavy percussion panned hard left. If you process those two channels with separate unlinked compressors, the right channel compressor only sees a gentle guitar and does nothing, while the left channel compressor will feel obliged to wind the level back every time the mad drummer breaks out.

While you may like the effect a certain piece of gear (like this Golden Age Project Comp 54) has on your recordings, passing your left and right channels through it separately is not a good idea. The reason for this is that the compressor can only react to what it is fed at any given time. So when the left and right channels are heard together — after being run through the Comp 54 — the sound will be very uneven. You can get around this by setting up an external side-chain input, which will cause the compressor to react to what it gets from the other channel, but with the Comp 54 this is not possible, so another approach altogether might be in order.

Listen to the two compressed channels afterwards in stereo and the result will be a very unsettled guitarist who shuffles rapidly over to the right every time the percussionist breaks out (probably a wise thing to do in the real world, of course, but not very helpful for our stereo mix).

If you process your stereo mix one channel at a time through your single outboard compressor, that’s exactly what will happen. The compressor will only react to whatever it sees in its own channel during in each pass, and when you marry the two compressed recordings together again you will find you have an unstable stereo image. The audibility of this, and how objectionable you find it, will depend on the specific material (the imaging and dynamics of your mix), but the problem will definitely be there.

Stereo compressors avoid this problem by linking the side chains of the two channels, so that whenever one channel decides it has to reduce the gain, the other does too, and by the same amount. In that way it maintains the correct level balance between the two channels and so avoids any stereo image shifts.

You can achieve the same end result if your single outboard compressor has an external side-chain input, but sadly I don’t think the Golden Age Project model does. If it did, what you’d need to do is create a mono version of the stereo mix in your DAW and feed that mono track out to the compressor’s external side-chain input, along with one of the individual stereo mix channels (followed by the other). That way, the compressor will be controlled only by the complete mono mix when processing the separate left and right mix channels, so it will always react in the same way, regardless of what is happening on an individual channel, and there won’t be any image shifting.

That’s no help to you with this setup, of course, but don’t give up yet, as there is another possibility. You could take an entirely different approach, and that’s to compress the mix in a Mid/Side format instead of left-right. It involves a bit more work, obviously, as you’ll need to convert your stereo track from left-right to Mid/Side, then pass each of the new Mid and Side channels separately through the compressor, and then convert the resulting compressed Mid/Side channels back into left-right stereo. Using an M/S plug-in makes the task a lot easier than fiddling around with mixer routing and grouping, and there are several good free ones around.

The advantage of this Mid/Side technique is that, although the Mid and Side signals are being processed separately and independently, the resulting image shifts will be much less obvious. The reason for this is that instead of blatant left-right shifts, they will now be variations in overall image width instead, and that is very much less noticeable to the average listener.

Sorry for the long-winded answer, but I hope that has pointed you in the right direction.

SOS Reviews Editor Matt Houghton adds: I agree with Hugh’s suggestion of M/S compression. I regularly use that when I want to deploy two otherwise unlinkable mono compressors, and there’s no reason why you can’t process the Mid and Side components one at a time. The only issue here will be your inability to preview what you’re doing to a stereo source, so be careful not to overwrite your original audio files! However, I sense that it’s the effect of running through the compressor’s transformers that you’re hoping to achieve. In that case, just set to unity gain and set the threshold so that the unit isn’t compressing, and then run the signal through it. If it is standard L/R compression you want, you could always get another Comp 54, as although they’re mono processors they’re stereo-linkable with a single jack cable.

In Cubase, I find that the best approach to incorporating such outboard devices into my setup is to create an External FX plug-in for each device, and then insert that on each channel and print the result. In Reaper, the equivalent tool is the excellent ReaInsert plug-in. This approach not only makes the process less labour intensive in the long run, but means that you can drag and drop the processor to different points in the channel’s signal chain, should you want to.

Published in SOS February 2012