Disclaimer: The discussion of hardware compression can lead to a fight over ITB (digital) vs OTB (analog) mixing. I’m not going to fight that fight. I use both kinds of mixing. OTB mixing gives me a demonstrably different mix, which I happen to think is better and more appropriate for some mixes. I also mix ITB sometimes. YMMV.
Here is a list of the outboard compressors currently in use at Euphonic Studio. See photograph…
1. Aphex 651 (x2)
2. Aphex 661 (x4)
3. Urei 7110 (x2)
4. Teac patch bay
5. Symetrix 501 (x2)
6. JBL M712
7. Behringer MDX2100
8. DBX 160XT (x2)
9. Warm Audio WA76
These connect to the board inserts with 1/4″ insert cables. Muliple copies of compressors are wired to be used in stereo for stereo groups on the mixer.
When I start a mix, I have learned which compressors give me the compression and character I want for different types of tracks. Generally, I use the WA76 for lead vocals although the DBX 160s also work well for vox. I like the Aphex comps for guitars, piano, and other string instruments in that general range. I frequently use the Urei 7110 or a DBX 160 for bass. The Symetrix 501s work great on the drum buss.
Those are just starting points. The actual track and the final mix may dictate a different compressor.
ALSO… not shown in this photo (because it lives in the effects rack, see that page) is the 2 buss compressor, which is currently a Chameleon 7720.
The concept of compression is easy to understand. In practice it gets slightly more complicated, and it takes some experience to get that piece of it right. It’s important to get a handle on this if you are mixing, and you should remember that squashing your mix flat with too much compression is a formula for an over-loud mix that may very well sound lame.
Compressors are a critical factor in creating the sound of modern recordings. Here is the 10,000 foot view of how they work and why we need them.
It’s easy to visualize someone singing or playing an instrument and the volume changes that naturally occur as they work their way through a song. For example, most singers naturally get louder as the pitch of the melody gets higher because it takes more energy to sing high notes than low notes. A person playing an acoustic guitar might be pretty consistent with their strumming but unless they are a robot there will be variations in the volume of the instrument. And let’s not forget that dynamics is one of the musicians’ most powerful tools for expression. These changes in volume are desirable in one context, that is, we want the recording to have some dynamic range. If the entire song is heavily compressed to one volume or a very narrow dynamic range, (which has been an unfortunate trend in recordings) it squashes the life out of the song.
As your mix takes shape, the dynamics of individual tracks become more apparent and more important. That strumming guitar part may have a perfect place where it sits in the mix but due to variations in the way it was played, if we adjust it so it’s perfect for the loudest parts, the quiet parts are too quiet. You might think that the solution would be to ride the volume knob for the guitar while you’re mixing; in essence this is what compression does, but it does it very quickly (which no human could possibly do) and automatically according to the way you set up the compressor controls.
Most compressors have 4 basic controls although this is not a rule. They are Threshold, Attack, Release, and Ratio.
Threshold is the setting that determines at what level the compressor starts to compress. At a low setting, the compressor will be more active, and a higher setting will require that the signal get louder before the compressor kicks in. Hint: it’s easy to set this too low, and if your compressor is always on you may need to raise the threshold.
Attack controls the window of time that the compressor will wait before it starts compressing. This is an important setting because lots of instrument sounds have a tone variance that occurs when the note is sounded and varying the attack time can allow us to emphasize the part of the tone that we want to hear. Imagine picking a single string on a guitar. Right when the pick leaves the string the sound of the plastic pick leaving the string can be heard… a sort of click. But a very short time later that sound has completely disappeared and now you are hearing the sound of the string as it decays in volume from loud to soft. As the string energy decays, the harmonic content of the sound changes due to the physics of the way strings work and the specific sound of the instrument. By varying the attack time, we can change the sound of the instrument by choosing where the compressor starts pulling the volume down.
Release is the setting that adjusts how long the compressor stays active after the input signal drops below the threshold setting. By adjusting the release time (assuming the other parameters are correct) you can make the compressor “pump” with the rhythm. This can be a “feature” or it might sound terrible, depending on the application. Again, YMMV.
Ratio is the the amount of amplification applied to any given level and is usually expressed in the form of
For example, a ratio of 2:1 means that for each two units increase in level, there will be a gain of one units of amplification. What this does, in effect, is to set the angle of the response line. A flatter line means less amplification.
Other compressor goodies….
Some hardware compressors and most software plugins have some additional controls.
Gates set the output of the compressor to zero below a threshold that you set. These can be handy to stop hum and other noises from getting amplified by the compressor.
The Knee setting means that the ratio of the compressor changes depending upon how much signal is going into it. This is used to clamp down the top part of the input range and is handy to prevent over-recording. On some hardware compressors the knee is a preset that you turn on or set to hard or soft, where on plugins it tends to be more adjustable.
A Limiter function can put a hard ceiling on the output of the compressor. By definition, a ratio setting of 10:1 or higher sets your compressor to limiting mode. Some compressors have a separate control for setting a hard limit.
Makeup Gain or Output is used to control the overall level of the track after the compression is done. Frequently the compressor is used to control loud peaks in the track, and this makes the track quieter. You can raise/lower the compressor’s output to whatever level is appropriate for the track.