What is lookahead on a compressor?

Lookahead in plain English

A compressor has two paths inside it. The audio runs through one, and the other, the detector, measures the level and decides how far to turn the audio down. In an ordinary compressor both paths see each sample at the same moment, so the compressor can't react to a hit until the hit is already coming out. Then the attack time, however short, means the first part of the hit has gone before the gain catches up.

Lookahead moves the audio path back in time. The sound sits in a short delay, say 3 ms, while the detector reads it as it arrives. By the time a hit reaches the output, the detector heard it 3 ms ago and the gain has already been moving for 3 ms. The compressor appears to react before the sound happens, because the sound you hear is a little late.

The basics of threshold, ratio and the rest are in what is compression?, and attack and release covers the timing controls in depth. This page stays on the one control that decides when the attack starts.

Reading the lab

The big screen shows the whole loop. The pale shape is the original and the pink shape is what comes out, lined up with the original the way your DAW would line it up. Under it, the amber lane is the gain reduction, and the amber box marks the hit shown in the close-up.

The close-up is a 160 ms window around one hit. The white line marks the moment the hit arrives, the violet strip before it is how far ahead the compressor can hear, and the solid amber line is the gain. Once you add lookahead, a dotted line appears too: the same compressor with no lookahead, so you can see the whole gain curve slide to the left.

The sweep is the screen to watch. The lab runs the loop through the compressor at every lookahead time from 0 to 20 ms with your other settings and draws two lines. The pink one is overshoot, how far the loudest peak comes out above the level the threshold and ratio say it should. The violet one is how far the zoomed hit has already been turned down when it arrives. A dashed amber line marks where the lookahead equals the attack.

Ratio and release are locked at 8:1 and 120 ms, which leaves the threshold, the attack and the lookahead free. The panel on the right explains what your settings are doing with numbers measured from the loop, and A/B switches the compressor off at matched loudness.

Catching the front of a hit

A drum hit goes from silence to its loudest point in a millisecond or two. That spike, the transient, is the part a compressor has the hardest time with, because it's over almost as soon as the detector notices it.

In the first step of the lab the attack is 1 ms, which is fast, and there's no lookahead. When the loudest hit on the drum loop lands, the gain has only come down 2.7 dB, and the peak gets 14.5 dB past the level the 8:1 ratio should hold it to. The hit sounds compressed after its first few milliseconds, and the click at the front is untouched.

The obvious fix is a faster attack, and that works up to a point. Push the attack down towards zero and the gain starts moving as fast as the waveform itself. A kick drum's low end might take 20 ms per cycle, and gain that rises and falls within one of those cycles bends the shape of the wave, which you hear as distortion. Lookahead lets you keep a sensible attack, say 1 ms, and start it early instead.

Give the same compressor 1 ms of lookahead and the gain is 10.3 dB down when the hit arrives. The attack hasn't changed at all; it has simply started sooner.

How much lookahead do you need?

On many compressors, the lab's included, the attack time is how long the gain takes to get about two thirds of the way to the full gain reduction. It keeps easing towards the target after that, and after three attack times it's about 95% of the way there.

That gives you a starting rule: lookahead between one and three times the attack. Less than that and most of the attack still happens after the hit has started. More than that and the gain has already finished coming down, so the extra time only moves the gain change earlier.

The drum loop with a 1 ms attack shows the same thing in numbers. The overshoot on the loudest peak is 14.5 dB with no lookahead, 10.3 dB with 1 ms, 5.8 dB with 3 ms, and 4.5 dB with the full 20 ms. Going from 3 ms to 20 ms makes the delay nearly seven times longer and takes off another 1.3 dB.

The last few decibels don't go away however far ahead you look. Between the big peaks of a waveform the level dips, the compressor starts to let go, and the next peak arrives before a 1 ms attack has fully clamped down again. Getting rid of that needs a limiter, which holds the gain at the lowest value needed anywhere in its lookahead window, or a clipper after the compressor to shave off what's left.

Lookahead with a slow attack

A slow attack is how a compressor adds punch. With a 20 ms attack and no lookahead, the front of each hit gets through while the body behind it is turned down, so the transient stands further above the rest. On the drum loop the peak goes 22.6 dB past the ratio's line, and here that's the sound you're after.

Add 20 ms of lookahead to that and the slow attack starts 20 ms early, which means it's well under way by the time the hit arrives. The zoomed hit comes in 9.8 dB down instead of 1.6 dB, and the punch from the slow attack is mostly gone. Lookahead and a slow attack pull in opposite directions, so when you're compressing for punch, switch lookahead off. The fast vs slow attack lesson covers when to choose each.

Too much lookahead

The gain change starts as early as the lookahead lets it. With 20 ms of lookahead and a 1 ms attack, the lab's drum loop has the gain coming down almost 20 ms before each hit. Whatever plays in that gap gets turned down along with the hit: the tail of the note before, or a hi-hat just ahead of a snare.

On the drum loop you won't hear it, because the loudest hits come out of near silence and there's nothing in front of them to turn down. On a sound where something plays right up to the next hit, it's a small dip just before each one, and on sustained parts with sudden loud notes it can sound as if the track pulls back slightly before the note.

The release moves earlier by the same amount, since the detector hears the sound drop away before you do. The gain starts recovering while the end of a loud note is still coming out. With a few milliseconds of lookahead that's too small to matter.

Lookahead, latency and delay compensation

A compressor with lookahead holds the audio back, so the track comes out late by exactly the lookahead time. In samples that's the lookahead in seconds times the sample rate: 5 ms is 220 samples at 44.1 kHz and 240 at 48 kHz, and the lab's maximum of 20 ms is 882 samples at 44.1 kHz.

The plug-in tells your DAW how late it is, and the DAW delays every other track by the same amount so they all stay in time. This is called plug-in delay compensation, and Ableton Live, Logic Pro, FL Studio, Cubase and Pro Tools all do it automatically. On playback you'll never know it's there.

Where the delay does matter is recording. If you're singing or playing through a track that has lookahead on it, you hear yourself late. Sound travels about 34 cm in a millisecond, so 10 ms of extra delay is like standing 3.4 m further from your speakers, which is enough to feel when you're playing. Logic's Low Latency Mode bypasses plug-ins that add more delay than a limit you set, which is one way round it. The other is to switch lookahead off while you record and back on to mix.

It can also catch you out when you blend. If you split a track yourself and compress one copy with lookahead, and something in your routing isn't compensated, the two copies end up a few milliseconds apart. Mixed together, they cancel at some frequencies and add at others, which is comb filtering, and it sounds thin and hollow. A compressor's own mix knob normally lines the two up inside the plug-in, so this only happens with routing you build yourself.

Lookahead in real plug-ins

Lookahead is a digital feature, because it needs the audio stored in a delay. Hardware compressors like the Universal Audio 1176 and the Teletronix LA-2A have no lookahead, and plug-ins modelled on them usually leave it out. Among stock and general-purpose compressors it varies: Ableton Live's Compressor has a switch with 0, 1 and 10 ms, and FabFilter Pro-C 2 has a continuous control from 0 to 20 ms.

Nearly every mastering limiter uses lookahead, often a fixed few milliseconds you never see, because a limiter has to catch every peak at its ceiling without distorting it. Gates use it the other way round: a noise gate with lookahead opens just before a hit, so even a gentle attack keeps the whole transient.

How to set lookahead on a compressor

1. Decide what the compressor is forIf you want the front of each hit caught, lookahead helps. If you want it left alone for punch, keep lookahead off and stop here.
2. Set the threshold and ratio firstGet the gain reduction meter moving on the peaks you care about, with lookahead off.
3. Set a fast but sensible attackAround 0.5 to 1 ms on drums, a few milliseconds on vocals and bass.
4. Add lookahead equal to the attackListen to the click at the front of the hits and watch the output meter. The peaks should drop.
5. Try two or three times the attackStop at the point where more lookahead no longer changes the front of the hit.
6. Compare at matched loudnessSwitch the compressor off and on with the levels matched, and check the sound before each hit hasn't started to dip.

Common lookahead mistakes

Using it when you wanted punch

If a drum sounds flat and smaller after compression with a slow attack, check whether lookahead is on. It moves the attack earlier and catches the transient you were trying to keep.

Turning it all the way up to be safe

Past about three times the attack, more lookahead catches almost nothing more. What it does add is delay, and a gain change that starts so early it turns down whatever plays before the hit.

Leaving it on while recording

A few milliseconds of lookahead on a vocal or guitar bus is easy to miss until the performer says the timing feels odd. Switch it off, or use your DAW's low-latency monitoring mode, while you track.

Expecting it to stop every peak

On a compressor, lookahead only gets the gain moving in time; it can't make the attack instant, so a few decibels still get through. If nothing may pass a set level, use a limiter.

Train your ears

The quiz under the lab hides the controls and asks whether lookahead is on, with every round played at the loudness of the original. The first four rounds use 5 ms, which you can hear as the click at the front of each drum hit disappearing. The last four use 2 ms with a 2 ms attack, where the difference is smaller. Your best score is saved in your browser.

Questions people ask

Keep going