Reply To: Phase Correlation Meter?

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Inverting the polarity of a signal effectively rotates the phase by 180 degrees, so I think that’s where the confusion comes in. As phase is technically a function of time, I think “Polarity Inversion” is a more accurate term for what the “Ø” button does on most mixers.

So, back to the start: using the Qu, how to detect that I need to switch polarity? (except from by ear)

You could watch the RTA. If the RTA is following PAFL and you PFL two signals together (“Additive” mode needs to be switched on for this) where they may be some phase cancellation going on, if there is some cancellation you should see a corresponding dip in the RTA. If you then invert the polarity on one of the signals, that dip should return to normal. That said, the RTA is only 31-band, and may not have enough resolution in the frequencies that are being cancelled: your ears are probably more useful.

As for where you need it, it can be useful if you notice phase cancellation when signals from two or more mics in close proximity are electrically summed (eg choir mics that are too close together, snare top and snare bottom etc.) A case can also be made for inverting the polarity on outputs too. Dave Rat has a video explaining how inverting the polarity on a drummer’s wedge can help get a few more dB of GBF before the wedge bleeds into the drum mics. This can be accomplished in software on the GLD on a per-output basis, but I don’t think it’s available on the Qu: physical adapters may be required.