But this technique can also be the source of potential problems especially with stereo sounds because some frequencies will cancel out while others will build-up unnaturally. If you could view a graphical spectrum analysis of the sound (frequency vs. amplitude), you'd see a distinct pattern of steep and regularly spaced peaks and troughs over the entire frequency range. Looking like the teeth of a comb and called the "comb filter effect", it is sometimes heard in theatres, recording studios or living rooms where sound from the loudspeakers reach your ears at different times causing inconsistent and a location-dependent frequency response.

SoundRadix's Auto-Align ($149) is an AU/RTAS plug-in for Mac (a Windows version is coming) that gives you precise control over time alignment when combining two or more recorded tracks from the same source. When inserted on a track in your mix that you designate as the timing reference, it sends sample-accurate timing information over any one of its own eight internal buses to any other "satellite" tracks also running the plug-in, and then uses the DAW's delay-compensation engine to make the correction. It also detects and corrects phase differences and polarity reversal between the source and satellite tracks.

(Since this review was written, SoundRadix has added a switch to the GUI that makes it easier to compare the original and corrected sounds.)

LET'S AUTO-ALIGN

I tested Auto-Align in Pro Tools 9 HD on a Mac Pro 8-core running in 64-bit mode. The plug-in installed quickly and was intuitive to set up and use. I tried it on a bass guitar recording comprising separate tracks for a DI and a miked Aguilar amp. I listened to the two tracks mixed equally without Auto-Align, and they sounded hollow and unusable I inserted the plug on the DI track (my reference) and sent out on bus 7. I also inserted it on the Aguilar track, set it to receive on bus 7, played the song, and clicked the Detect button. After a few seconds, it detected and corrected both time and phase differences with dramatic results: The mix of the two tracks was bigger, deeper, and clearer than just the DI or Aguilar track alone.

There is a polarity reverse switch if you find monaural sources (like this bass) sound better flipped but with the time differences eliminated. I liked the On/Off switch that turns off the delay correction but maintains the phase correction. This is necessary because unfortunately, when simply bypassing on the plug's RTAS GUI, you'll hear its processing latency.

PHASE ALIGNED STEREO PIANO

To keep the left and right channels phase locked, I used the stereo version of Auto-Align on a stereo Ivory piano track. This piano sound was already good and stereophonically wide but I had compromised its width in an effort to build more center channel level. With A-A correcting phase only, I could somewhat maintain the width but got more punch through a phase-accurate center image. The piano sound tightened up and cut the track better without being overbearingly loud.

The plug-in's GUI has a delay display that shows the approximate distance between the two mic sources (tracks), expressed in your choice of samples, milliseconds, or inches/centimeters. The handy Prev and Next buttons allow you to "slide" the timing of the satellite tracks in real time, relative to the reference track--even negative values are allowed. The Way Back Machine lives! To use Auto-Align as an effect, I'd like to see delay time also expressed in musical subdivisions relative to session tempo.

ALIGNING DRUM RECORDINGS

Auto-Align will time-place a distant room mic on a drum kit to match the close mics and minimize comb filtering. On a multi-channel drum kit recording, I used the close snare mic as my reference and sent it to a stereo Auto-Align instance on the overheads and also to a mono instance on the room track.

Correcting the overheads removed a slight delay that smeared the overall kit sound. The room track had a noticeable delay in the kick drum's attack--a doubled attack that softened the overall kick drum sound. The room mic didn't sound bad on the snare so I used the kick drum track for reference and sent its information on another bus to the room track's Auto-Align plug. I ended up with a roomy kit sound with a sharper attack and focus on the kick.

Auto-Align is a wonderful tool to synchronize---time and phase-align multi-track recordings. It does it accurately and automatically and takes the guesswork out of the time-consuming process of manually sliding tracks around the time line.