Studio Log

Comparing Compressors

Monday June 11th, 2007

To my ear, the proper use of compressors has a greater overall impact on the quality of a mix down than any other processing device. But unlike Equalization, Delay, Reverb or any of the other sexier and more obvious sound processing effects, compression is a subtler and more difficult effect for most people to wrap their ears around.

A compressor compresses dynamic range. Dynamic range is the difference in volume between loud and soft parts of a performance. Allow me to make a grotesque and exaggerated comparison.

If you watch a William Shatner monologue on a classic episode of Star Trek he can go from low reflective mumble to stark belicose shouting faster than one of us mere mortals can say “my ship, ... my crew.” That’s an example of LOTS of dynamic range.

If you watch Jack Webb deliver a monologue on a classic episode of Dragnet he manages to shame the bad guys with a relentless onslaught of verbiage that doesn’t vary a whit in pitch or volume for what seems like 5 minutes at a time. There’s no sign that the man breathes during his empassioned soliloquies. That’s a lack of dynamic range.

When you listen to Stairway to Heaven by Led Zeppelin it starts out low and quiet gets progressively louder and more dramatic, and eventually comes back to finish low and quiet. That’s dynamic range.

When you listen to a more modern production like Animals by Nickleback, the song grabs you by the throat right at the outset and does not let up until it’s over. That’s a lack of dynamic range.

Lack of dynamic range doesn’t mean loud though. A 14 year old mumbling in his mashed potatoes that he’s bored and everything is stupid is also demonstrating a lack of dyamic range.

In musical situations dynamic range is something that an engineer wants to control. You typically wouldn’t want a crucial note on a solo or a lyric in the chorus to drown out every other detail nor do you want it to be buried like a mumble. So the compressor is a crucial tool for establishing clarity and sharpening detail in a complex multi-track mix.

There are plenty of articles that explain the operating controls of most compressors so that is not my focus here. I want to point out that it is important to consider how different brands of compressors actually sound different from each other. Whether they’re rack mounted boxes of electronics or digital software simulations of the real thing, there is a whole world of factors that go into choosing which box you use for which situation. It’s a little bit science, a little bit art and a little bit alchemy.

Let’s look at a few of the common ones. I’m going to describe three compressors that I have had the opportunity to use both as hardware and as software plug-ins. They are all commonly available and two of them are even replicated by more than one software designer. Each of of these units has a unique tonality that results from their particular blend of electronic components and the attack and release characteristics of the compression circuit. For the purposes of this article, the sofware versions are close enough to the real thing to serve as acceptable substitutions.

The Focusrite d2 is a software version of the Focusrite Red series compressor. Most versions of ProTools come with the d2 as part of a plug-in bundle. The best way to describe it is “smooth.” In fact, if you run a track through a d2 it’s bound to come out creamy smooth. The real life box runs for about $3500 and it sounds like it when you run a bass, a piano or a vocal through it. I have run sampled piano tracks through a d2 that began to take on the timbre and stately tonality of a concert piano. Needless to say, this kind of effect can make the job of EQ’ing a track much easier.

The Teletroniks LA-2A is big chrome faced box of tubes and large capacitors that generates a lot of heat when plugged in and turned on. Not surprisingly it adds a lot of real warmth to a track, especially vocals. A track that sounds brittle and slightly edgy becomes closer, warmer and more personal when run through one of these babies.

The Urei 1176 is described in some literature as the “swiss army knife” of compressors and that’s not a bad description. Depending on how you tweak it can provide a warmth similar to the LA-2A. But what the 1176 is really good at is punch. An otherwise muddy bass or kick drum track can start to find some pointiness and an identity with an 1176.

With plug-ins in ProTools you are quickly spoiled. When I worked in a very well equipped million dollar studio I had compressors on every channel of the console and a few vintage classics in the rack that I could access via the patch bay. I would have to think my choices through ahead of time and I usually saved the two coveted Teletronics LA-2A’s for the lead vocal track and maybe the funky jazz bass track. Sure the Piano could benefit from it too but that was the breaks. With digital simulations I can use a virtual wall full of the classic tube compressor on every track if I deem that the song calls for it.

But I usually mix it up. Different compressors are better at different things. And while I didn’t consciously seek to obtain 11 different kinds of compression and limiting plug-ins, I find that I’m using them all and that I could probably use 2 or 3 more to really complete the set.

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