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Monday, April 3, 2023

Darrell Thorp using auto align to correct phase

 

 

 

Anytime you have multiple mics recording the same source—whether a single instrument or drum or percussion kit—it creates the potential for phase cancelation and inverted polarities that can adversely affect audio quality during mixdown.

In this excerpt from “Darrell Thorp Mixing the Dee Gees,” Darrell demonstrates how he uses the Sound Radix Auto Align plugin to eliminate those issues on the recording of the late Taylor Hawkins’ drum kit for the song “Tragedy.” (For those unfamiliar, the Dee Gees is the name of Foo Fighters’ Bee Gees tribute band.)

The Problem

On a multitrack drum recording, alignment problems occur because the various kit elements get picked up by different mics, each in a different position relative to the source. That means the waves arrive at slightly different times into each mic and thus get recorded out of alignment.

For example, the graphic below shows a snare track with an overhead track below it. The sound wave you see is a snare hit.

The same snare hit as captured on the snare (above) and overhead mics (below).

Both mics are picking up the same snare hit, but it’s arriving later on the overhead mic than the snare mic because the former is further from the source. The difference between the two in this example is about 162 samples, which, at 48kHz—the sampling rate of the recording—equals about 3.38 milliseconds of delay. It’s not enough to sound like a delay, but it is enough to create phase cancellations, also known as comb filtering, which are particularly noticeable when summed to mono.

Auto Align and similar plugins line up the hits on the various tracks, eliminating those delays. The result is a tighter sound with snappier transients. Auto Align also checks for polarity inversions, which can cause significant phase cancellation.

Auto-Aligning

When you use Auto Align on drums, you put an instance of the plugin on each drum track. You can then individually assign a send and a receive channel for each. All the open instances of Auto Align can communicate with each other. You choose a drum element as a timing reference and designate which of the others you want to sync to it. You can have more than one timing reference in a multitrack drum mix.

Sound Radix Auto-Align.

In the excerpt, Darrell aligns all the drums except the stereo room mics, which he prefers to leave alone, to retain their “loose” feel. He aligns several elements to the snare top channel, including the snare bottom, hi-hat, the mono overhead and the “drum poke mic.” (The drum poke mic is placed next to the beater-head side of the kick and pointed up at the snare.) The snare is a good reference because it’s recorded or bleeds onto every drum track.

Darrell has three overhead mics in this setup, a mono overhead and a stereo pair. He calls the stereo pair OH Hat and OH Floor, based on the side of the kit they’re facing.

He aligns the stereo overheads to the mono overhead. He aligns the tom mics to each other in placement order. The first concert tom gets aligned with OH Floor, and then each successive tom gets aligned to the one next to it.

The Auto Align send and receive channels that Darrell sets for the drum tracks.

Sound Radix offers an article in its support section that describes its recommended method for routing the various drums in Auto Align to get the best results. It’s a similar approach to the technique Darrell uses in the excerpt.

That same article also says, “Because phase correlation is relative, there's really no wrong order way to do it.” So you can experiment with which drums to use as the time reference.

Tweaks

In the video, you’ll see Darrell frequently turn up the Input and Sidechain sliders on the various instances of Auto Align. The sliders are accompanied by meters that show both level and color-coded frequency information.

The Sidechain meter shows the signal level sent from one instance of the plugin and received by another. For Auto Align to function correctly, it’s crucial to ensure the sidechain signal is turned up higher than the noise floor—defined in this case as the bleed from other drum elements on the track. So, for example, you’d probably have to turn the sidechain up high on a kick drum track because it’s likely to have a lot of snare bleed.

Why Alignment Matters

When you time-align drums, it can make them sound tighter and their transients snappier. The difference is often subtle rather than dramatic. Here’s an example.

The drum kit in this example was recorded with separate mics for the snare, kick, rack tom, floor tom, OH L, OH R, Room L and Room R mics. First, you’ll hear it unaligned.

This time, all the tracks were aligned to the OH L mic (a different approach than Darrell used), using a different plugin, Melda Productions MAuto Align.

Notice how the sound tightens up. (If you’re not hearing it, focus on the snare and compare the two again.)

Melda Productions MAuto Align.

Beyond Drums

Although they’re used most often on drums, auto-alignment plugins are also useful on other multi-mic’ed instruments, for example, electric guitar cabinets and stereo mic’ed sources.

Here’s an example of an electric guitar part with a two-mic configuration on the cabinet. A Shure SM57 is close to the amp’s grille, pointing slightly off-axis. The second mic is a beyer-dynamic M160 about 27 inches back and pointed further off-axis.

Here are the two mics summed to mono. Notice how phasey they sound.

This time, they’ve been time-aligned with Sound Radix Auto-Align, which moved this track earlier by 84 samples (1.75ms). The phasey sound is gone, and the track is louder because the phase cancellations have been eliminated.

Written by Puremix Team