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April 20, 2020

Vance Powell bass DI/Amp phase alignment




When recording electric bass, you have several options. You can capture it direct, you can mic an amp, or you can do both simultaneously and combine them in the mix. You could also start with a DI recording and later reamp it, and combine those in the mix.

The latter approach is what Vance Powell did for the band Illiterate Light while recording their song, “Sweet Beast,” in the video “Start to Finish: Vance Powell - Episode 5.” In this excerpt, Powell shows how he aligns the recorded DI and reamped bass tracks, which have a slight timing difference.

From DI to Amp

When you use a reamping device, it takes the line-level signal from the DI track in your DAW, and converts it to high-impedance. The signal appears at its 1/4” output, where you can connect it to an amp that you’ve miked. When you start your DAW’s transport, the audio from the track comes out of the amp as if the player was playing it live in the room, and you record it to its own track.

Reamping allows you to record a copy of a DI track through an amplifier.

The reamped part gives you a tonally different option from the DI part—perhaps more “live” sounding. The only other difference between them is that the reamped track will be slightly delayed.

This occurs because it takes a millisecond or two longer for the signal to go out of your DAW, into your interface, into a reamping box, into the amp and then the mic, and back to your interface and DAW. The delay can be enough to cause comb filtering (which is distortion and cancellation caused by signals out of phase with each other) when it’s played along with the DI track and they’re panned together.

Luckily, there’s more than one way to remedy the problem.

The Little Plug-in That Could

When we pick up the action, Vance has soloed the two bass tracks and inserted UAD Little Labs IPB plug-in on the DI track. The IPB (which stands for “In Between Phase”) is a software emulation of a hardware tool of the same name. It gives you a variety of controls to adjust the position and phase of your track.

Vance uses the Little Labs IPB to tweak the phase differences in the DI and reamped bass tracks

Vance’s method uses two of the IPB’s controls: the Delay Adjust knob, which is a continuous control that delays the audio up to 4ms, and the Phase Invert button, which reverses the polarity of the signal.

He presses the Invert Phase button while listening to the two fuzzed-out bass tracks—hoping to put them even more out of phase with each other—temporarily. Next, he slowly moves the Delay Adjust knob, looking for the setting that results in the lowest signal level. That’s the spot where the comb filtering is at its most pronounced. He then turns off the Invert Phase control, which gives him the most in-phase signal he can get. That’s because it’s now 180 degrees reversed from the point where the most comb-filtering is occurring.

When he compares the sound with the plug-in bypassed and active, you can hear a significant difference. When the plug-in is on, the combined bass sound of the two tracks has more body. With the plug-in bypassed, it’s thinner.

Manual Alignment

If you don’t have an alignment plug-in, don’t worry, you can also line up the two tracks manually. Here’s a step by step:

1. Put the two tracks next to each other in your DAW and zoom out all the way to the sample level (when the waveform turns into a single line). Set your counter to display samples.

Zoomed to sample level.

2. You want to choose a peak from a waveform in one track and compare it with the same one in the other. If you’re manually scrolling the display, move slowly, because it’s easy to lose your place in the timeline when you’re at such a high zoom level. You’ll probably find it most comfortable to use a peak at the beginning of a track or the beginning of a phrase, which has space before it.

You’ll notice that it doesn’t totally line up with the first peak in the reamp track. Insert your cursor at the apex of the waveform on the DI track. You’ll have to eyeball the placement, but at this high a zoom level, it should be sufficiently accurate. Now write down that counter reading in samples.

Put the cursor at the apex of the target wave on the DI track.

3. Drop your cursor at the apex of that same waveform on the reamp track. Look at your counter again. Now open a calculator and subtract the DI track’s counter reading (in samples) from that of the reamped track. The difference is the delay between the two tracks.

Put the cursor at the apex of the target wave on the reamp track.

4. Select the reamp track and shift it earlier by the amount of samples you just calculated. Now, your tracks should be aligned so that you don’t hear any comb-filtering.

Select the reamp track and shift it back by the difference in samples that you calculated.

Note that if you’re shifting a track back in time, you may need to create a little space first, on the left side of it, so there’s room for it to slide backward. Assuming the part doesn’t start right at bar 1, beat 1, you can cut a little off the space at the beginning to create the room you need.

If need be, cut a small portion of the reamp track to make room for it to slide backward.

If the bass comes in right at bar 1, beat 1 of the DAW sequence, you have a couple of choices. One is to select all the regions in your sequence and shift them later by one measure, thus creating space at the front.

Probably the better way to go, however, is to slide the DI track forward instead. While it’s true that the DI track more closely represents the timing of the bass player’s performance, the delay is slight enough that it almost certainly wouldn’t be noticeable. If fact, the alignment using the IPB did essentially the same thing, by delaying the DI track.

Let’s check out an example of this method in action.

Example 1A: A DI and reamped bass part, which are about 200 samples out of alignment.

Example 1B: The same part with the reamp track shifted earlier by 200 samples. Notice how much fatter it sounds.

On the left is the waveform that resulted when the out-of-alignment DI and reamped parts in Example 1A were mixed together. The right shows the same, but from Example 1B, when they were aligned. The gain in 1A is lower, due to phase cancellation.