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Q. Can I use a Mono compressor for Stereo compression?

By Hugh Robjohns

The Focusrite ISA430 MkII can be linked to a second unit for stereo compression.The Focusrite ISA430 MkII can be linked to a second unit for stereo compression.

I was wondering if it's possible to use a single mono compressor that can be stereo-linked (the Focusrite ISA430, for example) as a stereo compressor, using the following method. Play a mono mix of the stereo signal through the unit and record the Link Out signal; play the left channel through the unit at the same time as the recorded Link Out signal is fed into the Link In input, and record the output; do the same thing with the right channel; combine the two mono recordings back into stereo. I guess I'd have to be pretty careful about compensating for any delays. Also, the ISA430 (for one) doesn't specify what levels the link signals work at. But might this work?

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Technical Editor Hugh Robjohns replies: In a word, no! There are a number of problems with the process you are proposing.

Firstly, you are assuming that the signal used to link two units together for stereo operation is a normal audio signal. It might be, but equally, it might be a DC-referenced control signal, and the DC reference would be lost if you were recording the signal into a DAW. Similarly, any gain changes that occur anywhere in the recording and replaying of the link buss signal will upset the compression settings.

Next, assuming that you can record and replay the link signal, there is the danger of disturbing the delicate phase relationships between the left and right channels when each is processed separately and re-recorded. This will upset the stereo imaging. Remember that both audio channels are going through two A-D/D-A stages, both subject to random jitter effects controlled by different clocks at different times. Furthermore, the link buss signal is going through another two conversion stages, twice.

Furthermore, there's the delay introduced by the A-D/D-A conversion process to take into account. Remember that the side-chain control signal will have to pass through an A-D stage on recording, and then a D-A stage on replay to control the compressor. The left or right channel passes through a similar pair of D-A and A-D stages. But in order to create the side-chain control signal, the mono sum track used also passes through a D-A stage. Hence, the control signal will be one converter delay out of sync with the original audio, and hence you risk transient compression errors.

When you also factor in the practical difficulty of optimising the compressor settings, plus the huge amount of time and effort this process will take, it appears to be a futile triumph of technology over sense! Why bother trying to benefit from the sonic quality of a unit like the Focusrite ISA430 when you are inherently trashing it by using the process you describe? Given that pretty much everything you produce will need a pass through a compressor sooner or later, why not simply buy or hire a decent stereo compressor?

Published May 2005