I was reading Mike Senior's Mix Rescue article in SOS November 2011, about Arise's 'Run Run Run', and he used an all-pass filter within Cockos Reaper's ReaEQ plug-in to improve the mono compatibility of the drum overhead mics. I can't find a plug-in that does this in Logic, or an obvious freeware — can you recommend anything?
SOS contributor Mike Senior replies: Well, the first thing to say is that there's nothing particularly 'voodoo' about all-pass filters. The normal filters you get in your EQ (things like peaks and shelves) affect not only the processed sound's frequency response, but also its phase response (ie. the phase relationships between its different frequency components). The only difference with an all-pass filter is that it only affects the phase response, while leaving the frequency response unchanged. By sweeping an all-pass filter around and adjusting its bandwidth, you can change a sound's phase response in a more complex, frequency-dependent manner than is usually possible using things like simple phase rotators or micro-duration delays.
As far as I can remember, Logic doesn't have any built-in variable phase-rotation tools or all-pass filtering facilities at all. Fortunately, there are actually a number of freeware phase-manipulation tools that'll run in Logic. For simple phase rotation, Airwindows' PhaseNudge (www.airwindows.com/phasenudge) and Audiocation's Phase AP1 (www.audiocation.de/plugin) work well. But if you're after something more controllable in the frequency domain, there's nothing to beat Melda's freeware MFreeformPhase (www.meldaproduction.com/MFreeformPhase), which gives you precise control over the phase response via an editable graph of phase against frequency.
Incidentally, if you're suspicious of frequency-specific phase problems in stereo files, check out Voxengo's freeware Correlometer plug-in (voxengo.com/product/correlometer), which provides a spectral phase-correllation display — any dips showing on that display indicate frequency regions where phase mismatch may be impacting on your mono compatibility.