Q. How do I compress a stereo source?

By Hugh Robjohns

When the TLA 5052 dual valve recording channel is in stereo mode, one set of knobs controls both channels.

I am recording using a spaced-pair miking setup and I want to apply light compression to the signal. Where in the signal chain should I insert the compressor (pre- or post-fade), and will I have to buy a dedicated stereo compressor to do the job properly?

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Technical Editor Hugh Robjohns replies: You will have to use a stereo compressor, or two mono compressors that can be linked together for stereo operation. It is also wise to ensure that all the control parameters on both channels are set the same — that means attack and release time, threshold, ratio and make-up gain settings. In many cases, when a stereo or dual-channel compressor is operating in stereo mode, or when two compressors and stereo-linked, one set of controls becomes redundant while the other controls both channels. However, this is far from standard practice, and any differences between the settings of each channel can produce some very odd and undesirable effects.

When applying compression to a stereo source, it's very important that both channels experience the same amount of gain reduction regardless of which channel signal exceeds the threshold — hence the need for both identical control settings and stereo linking between channels. If you use separate, unlinked compressors for the two channels, then if one compressor reacts to a peak that the other doesn't see, the stereo image will pull towards the uncompressed side, and your listeners will start to feel very sea-sick!

Uniform compression on both channels is particularly important if you're recording using a coincident mic setup, as stereo imaging from a coincident pair is determined by tiny level differences between the two channels. In the case of a spaced-pair miking arrangement — the kind of setup you'll be using — the stereo image is dependent on phase differences as well as level differences, but uneven compression will disrupt the stereo image just the same.

It doesn't matter whether you choose to insert the two compressor channels (one handling each mic of your stereo pair) as a pre-fade or post-fade insert in terms of the stereo linking requirements. However, if you insert it post-fader, then the channel faders effectively become threshold controls and will affect the onset and amount of compression.

Published October 2004