Running stereo buses or full mixes through analogue devices like compressors, equalizers, summing boxes etc. is fun. They can add that magical touch to your track – a bit of stardust, the so-called 3D effect, punch, fullness and all sorts of adorable favours. But many times you face a problem even with high-end devices: the stereo image shifts…
I was mastering a track recently which has been run through a very expensive channel strip unit to add some shine to the mix. Sounded gorgeous except for that the mix – most noticeably the bass and the vocals – slightly shifted to the left, which was kind of annoying.
I asked my client to send me the mix without the channel strip processor applied on it. Then I measured the RMS levels for the left and right channels on both versions of the song (you can use Voxengo SPAN for such tasks). It looked like this:
Original (in the box) mix: Left: -18.2 dB, Right: -18.3 dB
Mix run through the channel strip: Left: -19.4 dB, Right: -20.4
You can see that the right channel has become 0.9 dB quieter compared to the relative levels of the original mix. To compensate for this I used the free Stereo Tool VST plugin from Flux where you can adjust the levels of the left and right channels independently (besides many other options). I increased the level of the right channel by 0.9 dB: now the vocals and the bass came back to the center. Problem solved!
Just a final note. Always check the mix with your ears too because RMS values are sometimes misleading. So don’t compensate for the stereo image shift in the mix blindly.