How To: DIY Lookahead Detection

Perform Better Compression with This Studio Trick
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Lookahead detection can be used for many applications, including shaping drum sounds. A compressor plug-in with this capability makes its detector perceive a signal before it actually occurs. That makes it possible for maximum gain reduction to occur before the leading edge of a drum hit, for example, so that the transient peak is fully attenuated. One counterintuitive application is to give abbreviated kick hits powerful sustain. I’ll show you how in this article.

“Great,” you say, “but my compressor doesn’t offer lookahead detection.” No problem: You can make any compressor act as if it does, as long as it has an external sidechain input. Use the following technique, and give your compressor something to look forward to.

COPY AND NUDGE

Make a duplicate copy (AKA mult) of your kick drum track, and nudge the mult forward in time around 2 milliseconds with respect to the original kick drum track (see Figure 1). Instantiate your compressor plug-in on the original track, and route the mult to its external sidechain input. Specifically, route the mult solely to a bus, so that it is removed from your mix bus and won’t be heard, and select the same bus as the compressor’s external sidechain input. With this setup, the kick drum’s compressor will react to each of the mult’s kick drum hits 2 ms before the corresponding hits occur on the original track. Voila, lookahead detection!

Fig. 2. Pro-C screen detail: Note that the pink gain-reduction trace is superimposed over a gray waveform for the original kick drum track, between the compressor’s knee display and its LED-style meters. The trace plunges immediately before the leading edge of the kick waveform and then rebounds. The FabFilter Pro-C plug-in is a breeze to set up using lookahead detection in this manner, because an x-y display in its GUI lets you easily judge the perfect nudge amount for the mult. In the display, a gain-reduction trace is superimposed over the waveform being processed. Check out the bottom-left corner of Pro-C’s GUI in Figure 1. In between the compressor’s knee display and its LED-style meters, a pink gain-reduction trace is superimposed over a gray waveform for the original kick drum track. Note how the trace plunges to its lowest value immediately before the leading edge of the kick’s waveform and then rebounds after the beater slap has expired (see Figure 2).

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What the GUI doesn’t clearly show (because the knee display takes up too much of its real estate) is how the kick drum’s release envelope has been boosted and extended to create powerful sustain. To create this effect, crank Pro-C’s ratio control to ∞:1, select a hard-knee response and lower the threshold to produce very deep compression. Use the fastest attack time possible, and set the release time so that gain reduction reverses just as the kick’s attack expires, thereby restoring level for the shell’s sustain. Boost Pro-C’s Output control to boost the level of the shell’s sustain further. (This will also ostensibly boost the level of the kick’s attack, but since you squashed that to oblivion, the gain boost will have negligible effect on that portion of the signal.)

At this point, the kick drum should sound sick, and not in the slang sense of the word; I mean truly terrible! It will have virtually no punch. We’re going to fix that, using parallel compression. Raise Pro-C’s Dry Mix control to add some of the kick drum’s unprocessed signal to the plug-in’s output. With the right Dry Mix setting, you should be able to achieve the perfect blend between sharp and punchy attack (unprocessed signal) and loud, long sustain (processed signal).

If your compressor doesn’t offer parallel compression, you can print the 100-percent-processed signal to a new track and then use track faders to mix that with the original, unprocessed track to create the blend you want. Just be sure to zoom down to the sample level on your DAW’s waveform display and check that the two tracks are perfectly time-aligned.

BEYOND DRUM SOUNDS

Lookahead detection can be used for myriad other applications. Does your compressor react too slowly to sibilance to effectively de-ess a vocal track? Simply mult the track, then nudge the mult forward in time and filter it to isolate and dramatically hype its high frequencies. Bus the searing mult to the external sidechain input for your compressor placed on the original vocal track. Dial in a high ratio, hard knee and fast attack and release times, and you’re done. Parallel compression is not needed for this particular application.

The great thing is that even the slowest compressor can be transformed into the fastest racehorse imaginable, as long as it’s outfitted with an external sidechain input. Jury-rig your compressor with lookahead detection, and make it look to the future.

Michael Cooper is a recording, mix, mastering and post-production engineer, and a contributing editor for Mix magazine. You can reach Michael at michaelcooper@bendbroadband.com and hear some of his mixes at soundcloud.com/michael-cooper-recording.