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Good Timing: Manually Compensating For Outboard Gear Latency

When two signals are combining at the same level but with different timing in a bus or a loudspeaker, the result will be comb filtering upon the combination. To avoid this, the two signals must be corrected to arrive in time with each other. Here's a way to do it.

Example 2

Let’s move along to something that’s a bit trickier, starting with an analog unit in my studio, an Aphex 104 Aurel Exciter. Its claim to fame is that it adds deepness to bass signal without adding gain.

However, it does this through a type of distortion that I don’t want when doing a mix-down, so I decided to add this to the sub output in parallel, which makes it easy to mute its return or leave it up for a little “ear candy.” Additionally, with the matrix mixer in my DSP, I can record just the wet path through it if I’m working with material enhanced by that effect.

If I were to run this effect at a live show, traditionally it would be patched in via an insert point. Figure 4 shows the visual of the path that the sub signal takes on its way through the system. This path creates two additional hops through a DA conversion process, so the wet path is a bit late compared to the dry signal when it combines. This effect is only covering a limited range of frequencies, and when they combine at equal level, there’s actually a drop in combined level.

Figure 4

The method to measure the difference between the two paths is the same as above. Send test signal from the analyzer into the DSP, then through the Aphex unit return into the DSP, and then back into the analyzer. Mute that path and send test signal through the DSP and back into the analyzer.

The approach to correct this is similar to the process of phase aligning a main and sub crossover. The impulse response for the subwoofer frequency range (both paths in this case) can’t be used to find the delay offset. To align both paths, we need to look at the phase trace and match phase degree and slope angle.

This will flatten out the combing and provide maximum summation of the combined signal. It’s technically a type of crossover and is called an “overlap.” The reason it’s a crossover is because its coherent signal arriving at the same level and time offset.

Figure 5 shows the frequency response of both paths. The green trace is the unprocessed subwoofer signal and the red trace is the same signal after passing through the Aphex 104. Note that the Aphex path does indeed give a small boost. The red circles are where the two signals match exactly in level. Note that the range of interest here is about 32 Hz to 100 Hz.

Figure 5

Figure 6 offers the phase traces – as we can see, the Aphex path arrives later. This is indicated by the steeper slope on the red trace. The goal is to get the (green) dry path’s phase to match the red as close as possible.

Figure 6

Finishing It Up

The starting point for me was to choose one of the two frequencies where the level is exactly the same (red circles) – either 50 Hz or 92 Hz. I chose 50 Hz because kick, bass and synth have more impact there. The degree difference in phase divided by 360 divided by the chosen frequency (50) is the formula to get delay time in seconds. The answer times 1,000 provides us with the time difference in milliseconds: 52/360/50 (*1000) equals 2.88 ms.

How much time you spend on getting the alignment as close as possible is up to you. For me, this is just an enhancement I patch in when I want to really feel the room shake so it doesn’t have to be extremely precise. The yellow trace in Figure 7 provides the new phase response with the 2.88 ms delay on the dry path.

Figure 7

Now let’s combine the levels with both paths on at the same time (Figure 8). Bright green is the combo. I turned down the gain a bit on the wet path to only produce about 3 dB of summation when they’re both on, but what’s shown is the maximum that can be attained by phase summing.

Figure 8

The final result is a cool effect. And if you were wondering how I confirmed that the Aphex 104 is a distortion effect unit, take a look at Figure 9.

Figure 9

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