Dynamic Range Compression: What Is The Lookahead Control?
If you've read my other articles on dynamic range compression, you'll know how important the process is in audio production. In addition to the typical threshold, ratio, attack and release time and makeup gain parameters, a compressor may very well benefit from a lookahead feature.
What is compressor lookahead? Lookahead is a control that allows a compressor to “see” the input signal before it is processed in order to handle transient information better and maintain a smooth attack. Lookahead is achieved by duplicating and delaying the incoming signal, using the first signal as a sidechain.
In this article, we'll deepen our understanding of compressor lookahead and how it is used to improve the performance of the compressors that offer such control.
A Brief Discussion On Audio Dynamic Range Compression
First, let's quickly discuss what dynamic range compression is. With a basic understanding of compression, we can garner a better understanding of lookahead.
What is dynamic range compression? Dynamic range compression is the process of reducing the dynamic range of an audio signal (the difference in amplitude between the highest and lowest points). Compression does so by attenuating the signal amplitude above a set threshold point.
So compressors will effectively attenuate the output of the loudest parts of a signal while allowing the quieter parts to pass at full volume. This effectively compresses/reduces the dynamic range of the signal in question.
The “loud” and “quiet” parts of an audio signal are determined by a threshold parameter set in the compressor. Anything above the threshold gets attenuated by a defined ratio (input level to output level), and anything below the threshold is not attenuated.
To visualize compression more easily, I like to think of an automatic volume fader. As the input signal exceeds a set threshold, the fader will automatically reduce the volume. As the input signal drops below the set threshold, the automatic fader will return to its original position.
Compression has a wide variety of applications in audio mixing/production. These applications include, but are not limited to, the following:
- Maintaining a more consistent level across the entirety of the audio signal/track
- Preventing overloading/clipping
- Sidechaining elements together
- Enhancing sustain
- Enhancing transients
- Adding “movement” to a signal
- Adding depth to a mix
- Uncovering nuanced information in an audio signal
- De-essing
- “Gluing” a mix together (making it more cohesive)
Related Article On Compression
To learn more about compression, check out my Complete Guide To Audio Compression & Compressors.
What Is Compressor Lookahead?
As mentioned in the opening paragraphs, compressor lookahead is a control that allows the compressor to “see” the input signal in advance of it affecting the signal.
With lookahead, a compressor will be able to react to the input signal earlier than it would otherwise be able to. This effectively allows the compressor to act upon very fast transients that may pass through even the fastest of attack times.
By relieving the compressor of complete reliance on its attack time, lookahead can allow the compressor to react more appropriately to incoming changes in signal amplitude.
This, in turn, produces a more natural-sounding compression without the downward pump the sometimes happens with less-than-ideal attack times. The sound is likened to a more sonically-neutral slow attack, but the compressor can handle the fast transients.
In other words, lookahead will allow for instantaneous reaction from the compressor.
The lookahead function of a compressor works with delay and sidechaining.
When lookahead is engaged, the input signal is effectively duplicated. The dry signal (the one that is not delayed) is fed to the compressor's sidechain, while the wet/delayed signal (often between 1 ms to 10 ms delay times) is fed to the compressor input.
The sidechain information triggers the compressor. As the sidechain signal exceeds the threshold, the compressor engages.
The compressor's attenuation/effect, however, affects the delayed input signal.
So then, the compressor can effectively “look ahead” of the delayed input signal, reacting according to the sidechain and acting more instantaneously on the delayed signal.
Note that, with a delay time of 1 to 10 milliseconds, the output signal won't be perceived as being out of time. There may, however, be issues with the phase between the compressor's output signal and other signals in the mix.
These phase issues can be corrected in most digital audio workstations via delay compensation to avoid phase or sync issues.
DIY Lookahead
Many compressors will not have a lookahead feature, and that's alright. We can achieve lookahead manually if we want the benefit of more instantaneous compression.
DIY lookahead can be manually produced by following this procedure:
- Duplicate a track.
- Send the original through a 100% wet single-repeat delay between 1 to 10 milliseconds (as a starting point).
- Send the delayed signal through a compressor.
- Assign the duplicated track to the sidechain input of the compressor.
- Ensure the duplicated track is not being sent to the main output or master bus.
By doing so, you'll have the compressor acting upon the “delayed” track according to the input of the “lookahead” track. This effectively achieves the same outcome as the compressor lookahead control.
Note that you'll have to adjust the delay time to achieve the best/desired results.
Call To Action!
Find a compressor or limiter with a lookahead control. Start with no lookahead, which is the easiest to understand, and dial in the other parameters to achieve an appropriate amount of compression (3 to 6 dB of gain reduction on transient peaks is a good starting point). Pay special attention to the attack and release times and how they cause the compressor to process the signal.
Proceed to increase the lookahead and listen critically to how the sound of the compression changes. Make a note of how the sound changes.
Repeat the process on different tracks and even with different compressors and limiters to develop a better understanding of how the lookahead control, when available, affects the sound of your compressors.
Have any thoughts, questions or concerns? I invite you to add them to the comment section at the bottom of the page! I'd love to hear your insights and inquiries and will do my best to add to the conversation. Thanks!
Related Questions
What are the main controls of a compressor? The main controls/parameters of a dynamic range compressor are as follows (with links to in-depth articles):
What is audio data compression? Audio data compression is the process of encoding digital audio information into fewer bits than the original signal/file, thereby compressing/reducing the file size. Data compression can be either lossless (eliminating redundant info) or lossy (eliminating unnecessary or “less-important” info).
Popular lossless audio compression formats include:
- FLAC (Free Lossless Audio Codec)
- ALAC (Apple Lossless Audio Codec)
- APE (Monkey’s Audio)
- OFR (OptimFROG)
- WV (WavPak)
- TTA (True Audio)
- WMAL (Windows Media Audio Lossless)
- Dolby TrueHD
- MLP (Meridian Lossless Packing)
- MPEG-4 ALS (Audio Lossless Coding)
- MPEG-4 SLS (Scalable Lossless Coding)
- RealAudio Lossless
Popular lossy audio compression formats include:
- Dolby Digital
- Dolby Digital Plus
- DTS Coherent
- MPEG-1
- MPEG-2
- MPEG-4
- MPEG-H
- Vorbis
- WMA (Windows Media Audio)
Related Article
To learn more about data compression in audio, check out my article How Does Digital Audio Data Compression Work?
