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Compressors and Compression for a Better Mix
In simple terms, a compressor is an automatic volume control. Loud sounds over a certain threshold are reduced in level; quiet sounds are not reduced. In this way it reduces the dynamic range of an audio signal. This may be done for aesthetic reasons, to deal with technical limitations of audio equipment, or to improve audibility of audio in noisy environments.
In a noisy environment background noise can overpower quiet sounds. A comfortable listening level for loud sounds makes the quiet sounds inaudible: A comfortable listening level for quiet sounds makes the loud sounds too loud.To make both the soft and loud parts of a sound audible at the same time, compression is used. Compression reduces the level of the loud sounds, but not the quiet sounds, thus, the level can be raised to a point where the quiet sounds are audible, but without the loud sounds being too loud. Contrast this with the complementary process of an expander, which increases the dynamic range of a signal.
With the advent of the CD and digital music, record companies, mixing engineers and mastering engineers have been gradually increasing the overall volume of commercial albums, originally they would just push the volume up so that the single loudest point was at full volume, but more recently by using higher degrees of compression and limiting during mixing and mastering. Compression algorithms have been engineered specifically to accomplish the task of maximizing audio level in the digital stream. Hard limiting or hard clipping can result, affecting the tone and timbre of the music in a way that one critic describes as "dogshit". The effort to increase loudness has been referred to as the "loudness wars".
Most television commercials are compressed heavily (typically to a dynamic range of no more than 3dB) in order to achieve near-maximum perceived loudness while staying within permissible limits.
TC's Compressor DeEsser is a great tool for users who are new to compression because of the quality and ease of use.
Note: Equalization and other effects apply before reverb then compression and limiting!
Compressor Settings
Most compressors have the following controls, though they may be labeled slightly differently:
Threshold. This is the level at which gain reduction begins to happen. Usually measured in dB. Lower threshold values increase the amount of compression, as less signal is required for gain reduction to occur. A threshold of -20db would initiate compression any time the signal went above it.
Ratio. This is the ratio of change between input level and output level once the threshold is reached. For example, a ratio of 4:1 means that an input level increase of 4 db would only result in an output level increase of 1 db. Example: if you take the -20db threshold and take your input signal that reached -12db with no compression (8db above the threshold), if you compress it at 4:1 (8/4=2) the signal would only reach -18db (-20+2=-18).
Attack. The amount of time it takes for gain reduction to take place once the threshold is reached. The ratio is not applied instantaneously but over a period of time (the attack time) Usually measured in microseconds or milliseconds. Use longer attack times when you want more of the transient information to pass through without being reduced (for example, allowing the initial attack of a snare drum).
Release. The amount of time it takes for gain to return to normal when the signal drops below the threshold. Usually measured in microseconds or milliseconds.
Makeup Gain. Brings the level of the whole signal back up to a decent level after it has been reduced by the compressor. This also has the effect making quiet parts (that are not being compressed) louder.
Of course, some compressors have more or fewer controls. Indeed, the classic LA-2A has only an input gain (essentially controls the threshold), peak reduction, and a nob for makeup gain, and a switch for choosing between "Compress" and "Limit". Attack time, release time, and ratio are fixed. What Is the "Right" Amount of Compression?
Compression is highly dependent on source material, and as such, there is no preset amount of compression that will work for any given material. Some compressors do have presets for certain types of audio, and these can be a good starting point for the inexperienced,, but remember that you will still have to adjust the input/threshold for it to work properly because every recording is done with different headroom and dynamics. Generally speaking you will get better results by learning to use compression, and understanding how the controls affect the audio signal. Experiment and listen! General instrument specific guidelines Vocals
fast attack and release, ratio depends on the recording and vocal style. usually a soft knee.
fast attack usually somewhat fast release.
bit slower attack and slower release, so you leave or accentuate the transient of the hit. of course if you want to smooth out the bass and bury your bass in the track by getting rid of the attack, have a fast attack on the compressor.
Super fast attack and release, with usually a high threshold,
Compressing drums is an art to say the least. The different amounts and styles of compression can completely and utterly change the way the drums sound. Whether you're going for those HUGE snare crack's or a more subtle tightness, knowing how to compress can save you from a weak sounding mix. The attack and how big you make the transient peak is the most identifiable part of a hit. On the compressor if you have too fast an attack the transient peak will be cut and your drum won't hit hard, but if you have a slower attack that initiates the compressor right after the transient peak it'll accentuate the hit. The compression after initiating and bringing down part of the sustain, the signal falls below the threshold and slowly releases bringing up the decay making the drum last longer and sound larger and more full. This is really a very general overview, you must experiment and listen to find the desired sound.
BOTTOM LINE
Listen! and go extreme before backing down to a good sound, it's the only way to really hear it. don't piddle around .5 db change of threshold here, go extreme and go way lower or way higher, or crank or lower the ratio and listen to the difference.
TIP: set the compressor to it's fastest release and slowest attack. set the ratio around 4:1 and set the threshold to about 7-10db reduction. slowly roll the attack back until it starts to "muffle" the signal. adjust it so that it's just short of that. Then start making the release longer until it starts to decrease the signals perceived volume, set it just short of that. now adjust the threshold and makeup gain to taste. |
