Big Rob is a good friend, talented engineer and I'd like to thank him for this installment's topic.
Talk to a musician about compression and they'll usually get a glazed look in their eye. Unless they are technically savvy, most musicians know what a compressor is capable of, but not really how to use it to their advantage. They know they can get an instrument louder in the mix, but that's it. Using compression properly is what separates the chaff from the wheat.
Case in point - A guitar-playing friend asked me to listen to a mix and wanted my opinion on how to improve it. Most noticeably, was the lack of dynamics and abundance of noise. Looking more closely, I discovered he had a compressor on every track and each were reducing the gain by 10dB (decibels) or more! His gain structure (another blog topic for another day) was completely out of whack. I explained to him that his overuse of compression is wreaking havoc with his gain structure and therefore, his mix. The most efficient method would be to remove all the compressors and start over.
There are times when overuse of compression can create a desireable effect. The Who experimented with compression on cymbals to create locomotive sounds. Most often, however, "pumping" and "breathing" is the side effect of too much compression.
We all hear compression everyday. When you listen to the radio, the audio is passed through several compressors before it is broadcast. Listening to any commercially released music has passed through at least one compressor, if not several. Heck, even our middle ear has a compressor. That's what makes VU or RMS meters more relevant. But, that's yet another discussion altogether.
The compressor was designed, primarily, as a level control device. Back in the days of vinyl, a Mahler orchestra, with a dynamic range of 136dB would need to fit onto a record with a total dynamic range of 68dB. Radio stations have an even smaller dynamic range to fit the same material. That's why the first compressers were called "leveling amplifiers." This is known as "downward compression" (compressing down audio peaks) and is the most common useage.
I like to think of compressors in terms of plumbing. The water level is the signal coming into the unit. The compressor circuit is like a valve. Basically, compressors "compress" a signal by a defined ratio, once it has reached a certain threshold. The input:output ratio is fairly simple. A ratio of 4:1 indicates that for every 4dB of input, only 1dB is let out. A ratio of 10:1 or higher is known as "limiting."
Most compressors have a few simple controls. Optical compressors, such as Universal Audio's LA2A, are the most simple. Having only two controls - input and output gain. Other compressor designs allow for more flexibility. The Urei 1176 had four buttons for different ratios, and attack and release controls along with the input and output controls. The "attack" control delays the time before the compressor kicks in and the "release" control delays the amount of time the compressor releases after it has gone below the threshold. Digital compressors can have a function called "look-ahead." Because analog compressors have to react to a signal, they tend to distort easily. The look-ahead feature allows the compressor to see an audio peak before it comes and can prepare for it. The result is more gain before distortion. Different units have even more controls. There just isn't space here to discuss them all.
In the early days of recording, engineers like Geoff Emerick (whose book "Here, There, and Everywhere is currently on my nightstand), used a compresser on John Lennon's acoustic guitar to level out his playing. If you've ever heard an acoustic guitar picked in person, the level can be inconsistent. There could be a note here and there that pops out. Compression helps even out the performance. Bass guitars benefit greatly from compression. Geoff also began using compression on drum mics to prevent his console from distorting.
On that note (pun intended), here is a fun method for using compression with drums that is really popular. It's called New York compression or parallel compression. Basically, send your drum mix
to two parallel outputs. Across one output, insert a compressor with a low ratio, low threshold, a fast attack and slow release. This is a method of upward compression (compressing the quiet parts up) and is very transparent. It's also a good method for getting a drums to sound more agressive when the drummer has had a few too many drinks or has poor technique.
A handy method for compressing vocals is called "serial compression." As the name indicates, there are more than one compressor, in serial, on a given channel. Typically two, one is used to gain control of the overall level. The second, is to take out loud peaks the first let through. Sterling Winfield (Hell Yeah, Pantera) uses a variation on this technique. Using two Tubetech CL1B's, he'll feed a vocal through both. One has an optical compressor type setting and the other a slow attack and release. The result is a musical and controlled vocal performance.
In mastering, limiters are used to compress the peaks down and increase the overall output. One side effect of this practice is that instruments tend to sound smaller as they become more compressed. Often, intruments can be practically "crushed" out of a mix. One limiter I've become fond of is the Sonnox Limiter. It has feature no other digital limiter on the market has - an attack control. By slowing down the attack on the limiter, I can hear the drums get bigger. The guitars get more rhythmic. In fact, the entire mix improves.
With compression, the sky is the limit. This seemingly innocuous and common utility can play a major role in the quality of our music. As I've said before in other postings, there are no hard and fast rules when it comes to audio. My best advice is to not look at the numbers on the faceplate and use your ears. Play with the settings until it sounds "right."
Rock. Roll. Repeat.