It sounds kind of geeky and high tech, like one of those techniques that you’ve read about but haven’t really felt the need to try — possibly because you aren’t completely sure what it is or when you might need to use it. Parallel compression isn’t difficult, and it’s useful both as a problem solver and as a method of reducing the total dynamic range of a track while retaining some of the dynamics from the original performance.
The concept behind parallel compression is that by combining a clean, unprocessed signal with a highly compressed signal, the compressed signal will have the effect of increasing low-volume information while the clean (uncompressed) signal will allow transient signals through at higher volume levels. The goal is to preserve transients on the loudest sections of a song, while bringing up the lower-level signals that might otherwise get lost.
How It Works
The name of the technique — parallel compression — is the biggest hint. Simply create two identical signal paths running in parallel, and put a limiter on one of them. If you compress, or limit, the peaks on a given track (or a bus with multiple tracks) and then increase the volume of that track or bus until the quietest signals are where you want them to be, you’ve most likely destroyed the loudest transients. With parallel compression, high-level and low-level signals are treated separately. The trick is to use a compressor with a high compression ratio — 20:1 or even 40:1 ratios can work. When using this much gain reduction on a track, choose a compressor that remains relatively clean, even at high levels of gain reduction.
When you duplicate a track (or a bus), the two tracks playing together will be 6dB louder than if you only play one track; naturally, this will make the quiet parts and the loud parts equally louder. Insert a compressor/limiter on one of those tracks, with the threshold set so that the quietest parts of the track are unaffected, and with the ratio set to a level greater than 20:1. When the loudest part of the track is played, the compressed signal will be up to 20dB quieter than the uncompressed track (depending on the threshold setting), which means that within the context of a mix, you won’t hear the compressed track. Therefore, all the dynamics occurring in those loud sections will be unaffected. Be sure that you have delay compensation turned on in your DAW; otherwise, the combined tracks may have some ugly phasing issues due to latency caused by the compressor plug-in.
Setting Up Buses
Let’s back up a step: if you don’t make a practice of using buses in your mixes, you may not realize the benefits. By sending all your drum tracks to a drum bus before they go to the main outputs of your DAW, you can add EQ or compression to the entire kit, and you can control the level of the drums in relation to the rest of the tracks. The same is true with guitar tracks; once you have the perfect balance among all the electric guitars, sending all the electric tracks to a single stereo bus allows you to maintain that balance for the guitars while adjusting the overall level of all the electric guitars in relation to the mix as a whole. Have a mix with lots of background vocals? Treat them the same way — set up a background vocal bus to add processing across all those vocal tracks with one plug-in, which saves system resources and ultimately time.
In Use
If you’re accustomed to using buses, parallel compression is simple to set up on any tracks that are bused. Take, for example, drums: if the drums are mixed and bused together to a stereo aux return, all you need to do is duplicate that return, then add your compressor of choice to the duplicate return that you just created. Set the threshold of that compressor so that the quietest parts of the drum track are unaffected, and use a compression ratio of at least 20:1. Set the output faders of both channels to unity gain. The compressed track will be out of the way during the loudest points in the uncompressed track. That’s the basic idea of parallel compression.
The Takeaway
Parallel compression is an interesting technique for the knowledgeable mix or mastering engineer. It can be used on acoustic guitars, both lead and background vocals, and piano — anywhere that the total dynamic range of the instrument exceeds the proper boundaries for a mix.
