Matching Levels Between Mics Using PhaseScope In Pro Tools

02-01-2014 10:24 AM

To me capturing instruments and especially groups of instruments using stereo mic arrays is one of the simplest but most enjoyable parts of the recording process. With the exception of mid-side arrays it is important for all stereo mic arrays to have their gains carefully matched if the stereo image is not to be tilted to the right or left.
It will never be enough just to plug in in two mics and set the gains on the preamps to the same apparent level. Each microphone and preamp will have its own sensitivity which, while similar, can never be assumed to be identical. The most accurate way to match mic gains is to match them using nulling. To do this arrange both mics side by side and on axis to the sound source. Set the gain of one mic to a level appropriate for the source. Both mics should be panned to the centre. Importantly the second mic should have its polarity inverted. Then, using the first mic as a reference the second mic should have its gain adjusted until the point where the two mics null, at this point there will be the dip in output as the signals cancel, accompanied by a significant dip on the meter showing the summed output of the two mics.
I have to admit that while I have used this procedure in practice I tend to use an alternative which requires less setting up and is more flexible, using the PhaseScope plugin in Pro Tools.
A phase scope displays mono signals as a vertical line, perfectly out of phase signals as a horizontal line and signals present only on the left or right as 45 degree diagonal lines. This plugin can be used to balance gains between stereo mic arrays very accurately and quickly. I have seen many people use a stereo level meter to quickly balance gains but I have found using this PhaseScope method to be more accurate and just as quick.
PhaseScope windows showing matched gains (L), matched gains out of phase (C) and right channel muted (R).
To set your mic gains using this technique just set the gain for one of the mics exactly as you would for a mono source. Then bring up the gain of the second mic to approximately the same level.

Because the overall gain of the array has already been set, it is unnecessary to use the same instruments as will be recorded when matching gains, I often ask someone to stand on the centre line of the array and clap. The important thing is that the PhaseScope is being used to match the gain of the mic whose gain has already been appropriately set for the material being recorded. Using a sound source placed on the centre line the PhaseScope will display a thin, approximately vertical, line. Using fine adjustments it is possible to match the gains until the PhaseScope shows a perfectly vertical line.
PhaseScope showing right channel gain too low (L), too high (C), and identical to left (R).
There are a couple of things to be aware of when using this technique. Not all sounds are supposed to be symmetrical across the stereo field. An example is an acoustic guitar close miked with XY crossed cardioids. The mic facing the body will be louder and this will be reflected in the apparent position shown on meters. The sound used to match the mic gains should be narrow, for example an entire ensemble or a whole drum kit should not be used as it will not show on the PhaseScope as a line but as a cloud of points varying in width.
Strictly speaking relative gains should be properly set prior to recording to allow proper monitoring during recording, in practice a rough balance is often acceptable and fine adjustment of relative gains can be set at the mixing stage. Assuming that the recording has been made on a stereo track, there are a few options for balancing gains but my preferred option is to insert a multi mono trim plugin, unlink the two channels and balance from there.
Unlinking and accessing the different channels in a multi mono plugin
PhaseScope can be used in exactly the same way as it would be if used prior to recording though obviously the trim needs to be before the PhaseScope.
The downside of this this technique is the unexplained omission of PhaseScope from Pro Tools 11. At present there is no AAX version and I’m unaware of any EOL announcement. Considering how much attention metering has received in PT11 it’s a strange oversight on the part of Avid.