Demystifying Compressors

By • July 5, 2011

In the past compressors did not play a big role in orchestral music, however with the advent of hybrid scores and scoring mixers coming out of the record world more and more, they can now play a much larger role. I have come to love the character that compression can add to recordings.

While what I am going to discuss applies to software compressors as well as hardware compressors, it is true in the former to a lesser degree. Most of the engineers I spoke with felt that despite celebrity endorsements, as good as some software compressors may be today, they do not yet truly rival a really good hardware compressor. But in the modern DAW-based compositional world where composers are reluctant to commit processing to a part and where the cost of owning several good hardware ones can be prohibitive, most are going to use at least some software compressors.

Whether a compressor is Hardware or software, there are a number of different types of compressors. The most common are: VCA (Voltage Controlled Amplifier), FET (Field Effect Transistor), Opto (Opto Isolators) and Vari-Gain (discrete circuitry other than VCAs, FETs, or Opto Isolators.)

As usual with all things audio, it is impossible to determine the usage of these, in regards to when to choose one type or the other. When I asked these engineers how they go about choosing one type or another the answer was invariably some form of, “I rely on my ears and my experience.”

When pressed however, they did give me some general guidelines.

One of the most commonly used Opto compressors is the Teletronix LA2a, a tube-driven “leveling amplifier”. It is considered a very natural way to control dynamics. In the hardware, the tube pre-amplification and post compression tube amplification give it a unique sound that is very pleasing. It does not react to the original attack as fast as some others so most of the transient information is preserved, giving it a nice, round sound on a bass or vocals, for instance. Obviously, a software version has no tubes so the effect will be less than a real one but it still is a good place to start for that kind of sound. I like it for more gentle and “natural” sounding compression.

The 1176, a “peak limiter”, is perhaps the most widely used FET compressor. Because its attack time can respond quite fast, independent of peak duration, it is frequently used when going for a brighter and crisper sound, on snares or high-pitched male rock vocals for example. Commonly, when you want faster, more aggressive compression than might sound good with an Opto compressor, you would choose this kind of compressor.

VCAs are the most common, least expensive type of compressor and perhaps the most versatile. These were created as an alternative to more expensive and difficult to design tube compressors. Indeed, the DBX 160A is probably been in more studios than any other. They give you the greatest range of control over level and dynamics. While they can run the gamut from gentle to super aggressive, personally aggressive sound is what I want one of these for.

Vari-Gain compressors are kind of their own beast. One example is the Manley Vari-Mu, which is a popular one, a sort of a kissing cousin to the venerable Fairchild 670 “Mu/Tube” of Beatles’ fame. They have a very “creamy” sound. My limited experience with these, as a composer primarily rather than an engineer, is what they work well on they work very well on indeed, and what they do not not, they do not.

There are also lots of console emulations of the built-in compressors found on Neve, SSL, Trident, Harrison, and Helios boards, etc. The different versions by different companies however, vary sonically to my ears a great deal and each tends to have its own fans. Frequently, they are used on busses and 2-busses to give a finished, pre-mastered sound, or simply for more volume and punch.

Finally, in the software world there are a number of compressors that do not conform to any of these that are prized by many engineers, especially ProTools based engineers, including the compressor built into ProTools and the Waves C series. They tend to compress without adding really noticeable character, in other words not changing the tonality of the sound. They feature extremely variable attack and release times, filtering, and sidechain options that do not exist in some hardware units or faithful emulations

I want to give a tip of the hat to Barry Rudolph of Mix Magazine, whose article helped me understand the following in a way I had not previously. My explanation here will be more basic so you might well want to check out his later for a deeper understanding.

Hardware or software once again, all compressors have four basic parameters in common: threshold level, ratio, attack time, and release time.

The threshold is the level of volume the compressor must see before it kicks in; a “sensitivity” if you will, and it is expressed as a specific dB level. It works with the “knee”, which is when the compressor starts to apply gain reduction A “hard” knee is sudden and dramatic while a “soft” knee is more subtle.

The ratio is the compressor’s answer to, “when I kick in, how much do you want me to kick in?” For example, if set to a 10:1 ratio, this means that for an increase of 10 dB of signal, will result in the compressor output being raised only by 1 dB.

The attack time is how fast it takes the compressor to start compressing when the input signal has reached the set threshold level and is set in a range from as little as 10 uS (picoseconds) to more than 300 mS (milliseconds). The faster the attack, the less of the sound of the transients is preserved.

The release time is the time the compressor requires to return to the pre-compressed level. Slower times give that “pumping” effect which may or may not be what you want.

Once all these are set, you may find the overall level to now be too loud or too soft. Most compressors have a final “makeup” gain to compensate accordingly, after the compression is done.

Compression is a valuable tool for composers to create their own sound and I encourage you to experiment with all the character and sonic versatility it can bring to your compositions.

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