COMPRESSION
Date: 25-02-2023
Today’s lesson was all about compression.
Compression reduces the overall dynamic range of a piece of audio by detecting when it exceeds a specified level, and then attenuating it by a specified amount. Put simply, it narrows the difference between the loudest and softest parts of a track so that it’s more consistent in level.
We looked at the key aspects and variables of compression:
- Treshold
The threshold is the level at which the compression effect is engaged. For example, if the threshold level is set at – 10 dB, only signal peaks that extend above that level will be compressed
2. Ratio
Ratio specifies the amount of compression applied to the signal. This setting is expressed in decibels. For example, a ratio of 2:1 indicates that a signal exceeding the threshold by 2 dB will be attenuated down by 1 dB. A signal exceeding the threshold by 8 dB will be attenuated down by 4 dB, etc. A ratio of 1:1 represents “unity gain” — in other words, no attenuation.
A ratio of around 3:1 is considered moderate compression. 5:1 would be considered medium compression. 8:1 starts getting into strong compression, and 20:1 thru ∞:1 (infinity to one) would be considered “limiting” — an effect that can be used to ensure that a signal essentially never exceeds the set threshold.
3. Attack & release
Attack-time: This refers to the time it takes for the signal to become fully compressed after exceeding the threshold. Faster attack times are usually between 20 and 800 microseconds, while slower times generally range from 10 to 100 milliseconds. Some compressors express this as slopes in dB per second, rather than time.
Release-time: You can think of release as the opposite of attack. It’s the time it takes for the signal to go from the compressed state back to the original non-compressed state.
Release times will be considerably longer than attack times, generally ranging from 40‑60 ms to 2‑5 seconds. Like attack times, these can also be expressed as slopes in dB per second.
4. Knee
The knee refers to how the compressor transitions between non-compressed and compressed states. Most compressors allow you to choose either a “soft” or a “hard” knee. While some compressors allow you to select positions between the two types of knees.
We explored the different types of compression:
1. Tube
Tube compressors have been around since the 1950s, and are known for imparting warmth, color, and harmonics thanks to their tube gain stage. They tend to have slower attack and release times, resulting in a more “vintage” sound that can be difficult to achieve with other compressor types.
2. Optical
Optical compressors employ a light element and an optical cell. As the amplitude of the signal increases, the light element emits more light, causing the optical cell to attenuate the amplitude of the output signal.
3. FET
FET or “Field Effect Transistor” compressors emulate tube sound, but with transistor circuits. They are fast, clean, and reliable — perfect for punching up drums, vocals, bass, guitars, and more.
4. VCA
Fast and punchy VCA compressors run the gamut. From the Rolls-Royce compression of the SSL G Bus or E, to the hot-rod attitude of the legendary dbx 160, this category of solid-state compressors can add unrelenting character to drums, guitars, and the mix bus.
