Compressor
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[edit] Introduction
- In a single sentence: A compressor is an automatic fader control, adjusting the level (volume) of an audio signal automatically.
A compressor is a device that is used to control the level of the signal that is passed through it. This was first done by using a CV to control the amplification of the signal so if the volume of the signal goes over a certain threshold then the compressor seeks to reduce the volume by a certain ratio that determines how many dBs will be needed to increase the output of the compressor by 1dB over the threshold.
Example: If a compressor is set on a ratio of 8:1 and -10dB threshold, then the signal has to increase by 8 dB to increase the output of the compressor to -9dB. If the level of the signal has relatively small fluctuations, then the output of the compressor will never be more then -10dB.
Compression has many uses, including:
- You can squeeze forward a frequency in a signal that ain't as loud as other parts of the sample so they can be emphasized.
- You can reduce peaks in a signal so that the overall loudness can be increased without unwanted clipping.
- You can increase the thickness of the sound because when the signal is compressed, it's given a wider range in the frequency spectrum so it appears to be all equally 'close' to you.
just to name a few..
[edit] Using Compressor(s)
The most useful way to train yourself to hear and use a compressor is to start by setting the compressor first so the threshold is very low, 80% or so to the left if you will, the ratio as high as possible, attack to zero and the release as long as you can. Then you should be able to hear a drastic compression. Then move the attack so you can hear a small fade, so that the signal isn't compressed until some milliseconds after the signal punches in. Play with the attack and release settings to see what works better, or worse. Adjust the threshold and makeup gain stages as well.
Some people start every track by placing a compressor with default settings on every channel, just to keep the input to the mixer reasonably even, but that isn't necessary. But there is one thing that is dangerous and that is to run too much bass through it, in that sense if you are compressing a bass sound, then it's always better to place the compressor before you place an eq that is to be used to add more volume to the lower frequencies.
For a bass sound at 170bpm I've given the attack about 20-28 ms. Then when that nice subtle fade is apparent the release is set, or for how long will that part of the signal be compressed, this can vary much between sounds as it gives a different character to the sound, but by setting the release so it stops right before the next note kicks in then the compressor gives a rhythmic feel to the sound and in the same moment increases the dynamic of sound, at 170bpm this is often around 150ms. But some compressors have an adaptive release so the release gets in more sync with what is happening in the tune. The next step is to set the ratio of the compression, when decreasing the ratio you will hear that this 'effect' will slowly fade away and the volume will increase, stop when this effect has almost faded away. Then you set the threshold as much as you can without loosing the compression. If the sound is compressed too much then there is a risk of loosing the 'edge' that the sound has.
Setting compressors in a good way is something that comes with a lot of practice so just keep on tweaking. Experiment!!
[edit] Multiband compressors
A normal compressor effects the entire frequency band (all input, 20hz - 20Khz).
A multiband compressor splits the audio into 2 or 3 or 5 or more frequency bands (with filters) and compresses each part individually.
[edit] Side-chaining with compressors
Side-chaining uses the dynamic level of another input to control the compression level of the signal. Not all compressors implement this feature, and those that do may implement it in different ways. RTFM!
Side-chaining is often used to control the Bass and Kick drum relationship. The Kick drum and Bass often occupy the same frequency range and as a result cause a clash in the mix. The results of this include headroom reduction and an overall muddy mix.
Side-chaining eliminates this problem by allowing the Bass to duck away from the Kick drum momentarily. The Kick drum signal is fed into the compressor on the Bass track, so when the Kick drum activates, the Bass tracks compressor lowers the volume level for a very short time. The reaction of the compressor to this input can be adjusted in the same way as normal compression.
Artistically, side-chaining is used to create a throbbing, pumping sound in Techno and Drum and Bass, as the bassline sound pulses in and out with the Kick drum.
[edit] List of frequently used software compressors
- PSP Mastercomp
- PSP VintageWarmer (also multiband and tube overdrive)
- Cakewalk VC-64 (vintage channel strip, with descent compressor)
[edit] List of frequently used hardware compressors
- dbx 166
- dbx 266
- TLAudio fatman 1
- TLAudio fatman 2
- TLAudio C1
