If you’re in a hurry and need an answer straight away, the short answer is that Auto Align can’t fix phase issues created by timing differences which change over time. Auto Align Post can.

Before Sound Radix released Auto Align a little over 10 years ago, for most engineers, once tracking was complete, making sure that all the mics played nicely together was pretty much a case of checking the polarity switch. Of course experienced tracking engineers understood the importance of mic placement, not just in terms of capturing the target but also how mics will interact with other mics. While optimal mic placement is just as important today as it has always been, since the release of Auto Align, correcting destructive phase relationships between mics has become quick and easy.

From simple scenarios such as combining a dynamic and a ribbon on a guitar amp, to aligning all the mics on a drum kit, Auto Align makes finding the optimal delay necessary to minimise destructive interference between signals quick and simple. It’s easy and effective so why did Sound Radix find in necessary to introduce an alternative version, Auto Align Post? What is the difference between the two?

The Problem Auto Align Solves

The really important thing to understand about the cancellation which can result when offset mics are combined is that they are a product of a specific pitch (or wavelength - same thing) and a specific offset distance between the microphones. If either of these things changes, so do the frequencies which cancel. Why is this?

Sound propagates though air at approximately a foot per millisecond. Time of arrival differences due to the different distances between mics hearing the same source result in sounds cancelling each other’s energy out at multiple frequencies down to a lowest frequency dictated by the distance between the mics. Different notes will give different results with some suffering more cancellation than others.

A difference in distance between two microphones of a foot would cause comb filtering cancellations down into the lower mids. Even a difference between mics of an inche can cause significant issues but by delaying the contribution of the closer of the mics by exactly the right amount, all the destructive interference between the mics can be fixed.

The more mics are being combined together, the more complex this can become. On a drum kit where lots of potential interactions between mics exist it can become necessary to find a ‘best fit’ solution rather than the single correct offset which would exist between just a pair of mics on a single point source like a guitar amp. However the criteria which apply to an audio issue which Auto Align can solve are:

• All mics are hearing common audio - Auto Align doesn’t work on tracks which don’t share any information.

• The difference of time-of-arrival or phase between the signals has to be constant and must not change over time

What Kind Of Situations Is Auto Align Suitable For?

• Where a source is being recorded with multiple microphones

• Where a mic is being used in combination with a DI or line input

• Where bleed between mics is so significant that they can be treated as being multiple captures of the same sound.

So for situations which don’t change over time, Auto Align is ideal

What About Mic/Source Relationships Which Change?

One common issue for which Auto Align doesn’t help is for mics which move relative to each other and the source. The scenario in which this happens probably most frequently is when a lavalier mic on an actor and a boom mic held out of shot my a sound recordist move relative to each other during a scene. Comb filtering is particularly easy to spot when the distances are changing, modulation effects like flanging and chorus are based on this and similar ‘swoosh’ sounds occur under these conditions.

To address precisely this issue Sound Radix created Auto Align Post, a plugin which presents the simplest of interfaces and just fixes the problem. That’s it. An AudioSuite plugin without complex parameters to juggle. Auto Align post is the dynamic time alignment solution for situations where the problem doesn’t stay the same and no single delay value can align the tracks.

Auto Align Post 2

Auto Align Post 2 added a feature which the first version lacked. Although the dynamic time alignment in the first version is accurate, if a high pass filter is used on one mic and not on the other, the phase shift introduced by the high pass filter could result in a cancellation at the bottom end resulting in a lack of low end when the two sources were combined. Auto Align Post 2 corrects for these phase issues.

Perhaps a little confusingly music mixers have discovered Auto Align Post and its reliable results and ease of operation have made it a favourite with some as a solution for correcting static offsets which Auto Align is suitable for!

So while you can’t use Auto Align for dynamic correction, you can use Auto Align Post for fixing static issues. Both are only suitable for correcting issues between mics which are hearing similar audio. If you have phase cancellation issues on unrelated audio you need to check out Sound Radix’s dynamic phase rotation plugin Pi.