The Complete Guide to Understanding Compression

Today, many DAW users tend to operate compressors without enough background knowledge - without knowing what compressors "actually" do. Compression can be a blessing or a curse. It can make a poor performance sound tight or ruin the dynamics of a good performance. Moreover, it can polish the master output or make it sound tiny and crushed. Let's dive a bit deeper into it.  

Brief History

In the early days of radio, producers had to adjust levels manually. The limiter was born to disrupt this archaic practice - a device that automatically attenuates sudden peaks above a threshold to prevent clipping. To protect equipment, recording engineers adopted this idea and developed leveling amplifiers. Since limiting is a form of compression, one could say it initiated compression techniques.  

The Definition

In audio, compression reduces the dynamic range of an audio signal by squashing the loudest peaks in the signal and boosting the lowest peaks by a certain ratio. To put it simply, a compressor is a dynamic range processor. The parameters given on a compressor are typical:

• Threshold: determines how loud the input signal has to be to activate the compressor
• Ratio: determines the amount of compression applied to peaks that pass the threshold. For example, 2:1 means the output will have half the dynamics of the input.
• Attack: determines how quickly the compression reacts to the input signal after it passes the threshold.
• Release: determines how quickly the levels will return to normal after the signal dips below the threshold.
• Output gain: this allows you to boost the overall output level after compression. It is usually used to optimize the signal after its adjusted range.

There are some exceptions and additions when it comes to different gear.  

Common Types of Compressors

VCA: Voltage-controlled amplifier compressors use an integrated circuit to allow for very precise control. They are less colored and do not suffer from the side effects of distortion that other compressors do. This makes VCAs ideal for a lot of tasks.

Opto-Compression Opto (short for optical) refers to the light-sensitive circuits that control compression amounts in this piece of gear. Optical compressors have a unique slow-acting attack, while the release has an almost two-staged quality. The device begins releasing quickly but is ultimately slow in returning the signal to its uncompressed level.

FET: Field Effect Compressors use transistors to emulate a valve sound with high reliability at the cost of a higher signal-to-noise ratio. They are popular on vocals and great for drum compression.  

Techniques

As we have been in a digital era for quite some time, most audio processing equipment (including compressors) have digital functionality. Let's look at some digitally emulated techniques that proved helpful throughout this period.  

Serial Compression (Insert Effect)

Serial compression, unlike parallel compression, does not require duplicate tracks. It simply feeds the entire track output into the compressor's input so that 100% of the sound is affected. For example, take a vocal you want to process and insert a compressor onto its channel strip. Set the threshold pretty high so that you do not lose the character of their voice. Then, add another compressor to address the overall level. Give that one a slower attack/release, a lower threshold, and some heat on the gain. Experiment with the order of the two compressors to achieve different coloration.  

Parallel Compression (Return Effect)

Let's say we have a drum loop we want to process. We could insert a compressor on the channel and compress the entire signal like before or use another elegant parallel compression technique. In this configuration, the dry signal is mixed with the compressed signal, preserving the genuine character of the loop while still impacting the listener with compression. This way, you can control the mix of dry and wet sounds. Many VST compressors today have a dry/wet control, which lets you balance processed and original signals. Remember, when a device has a variable dry/wet parameter set to anywhere other than 0 or 100, that in itself means it is a parallel effect, even when inserted serially.  

Group Compression

This is a great way to "glue" the elements together. By busing multiple tracks to a channel with a compressor, you are forcing them to move and change together, thus creating the illusion of a tighter performance. Group compression is excellent for mending sloppy ensemble performances and making multiple drum samples sound like one person is playing them.  

Brick Wall Compression

A set-up in which the compression ratio is infinity to 1. No matter how loud the input is or how damaging the distortion is, the output will be flattened. This effect was more desirable in the 90s and early 2000s than now, but it is still evident in music from artists like Daft Punk and Justice.  

Multi-band Compression

A multi-band compressor is nothing more than a compressor that divides the spectrum into 4 (typically) bands and allows you to compress the bands independently. It is like an EQ and a compressor in one. You could use it on a synth track where some frequency ranges are peaking (or too low) in the overall mix. This way, you can locate that specific range and apply compression while preserving the dynamics of the rest of the spectrum.  

Side-chain Compression (Ducking)

Side-chain is a feature built into most compressors these days. It allows another track in the mix to trigger or "key" compression. Initially, this technique was used to automate level balance. For example, instead of recording volume automation on a track that needs to get out of the way of a vocal, you can use a side-chained compressor with the vocal as the "key." This way, when the vocal passes the threshold, the accompanying track will duck for it automatically. This technique is often used in commercials to automate the balance between narration and background music. Today, this effect is almost exclusively used creatively. It imposes that pumping, gasping effect you've probably heard in just about every pop production since 2010. The kick drum squashes interfering instruments each time it hits them and pulls them back up like a ball on a chain.