Are You Addicted to Compression?
When was the last time you had your compression fix? Was it on vocals? Drum bus perhaps? Regardless of what kind of user you are, I’m willing to bet the addiction runs deep, and you use frequently. For those of you that are new to the game, or need a refresher, this article might be the fix you’re looking for. Let’s take a dive into the world of dynamic range compression and explore types and some of the more common uses.
Dynamic range compression can go one of two ways; downwards or upwards. Downwards audio compression tames the amplitude peaks, while upward compression boosts the amplitude of lower level signal. Most compressors react to an audio signal once a threshold is crossed. When the amplitude is greater or less than than the threshold, the compressor will either reduce the amplitude over the threshold (downward) or increase the amplitude under the threshold (upward)*. For the sake of this article, let’s focus on downward compression and how it’s used a tool, and also as an effect.
*I used to teach this stuff back in the day, and I would literally watch student’s eyes roll back into their heads. I’ll try to keep the tech talk and math to a minimum.
Some of you probably already have a grasp on the myriad flavors of compression that exist today. Optical, VCA, FET, and variable mu are the four main types; and all incorporate their own special kind of mojo that may, or many not, be suitable for varying program material. Let’s start off with what might be considered to be the most unique of the lot - the optical compressor.
Possibly the easiest to wrap one’s head around, optical compressors use light and a photocell to provide a result that is musically appealing and easy to operate. As signal enters the input stage, a light illuminates based off of the amplitude, or power of said signal. The greater the amplitude, the greater the intensity of the light, which becomes the catalyst for how much compression is applied. As you can see from the image of the Teletronix LA2A compressor above, operation is also relatively simple. The two knob design incorporates an output gain stage (make up gain) and a peak reduction knob, which works in tandem with the photocell. Some units also have the ability to limit, which increases the compression ratio.
Optical compressors tend to be incredibly smooth on attack and the release will vary depending on how long the circuit is engaged. Common uses for optical compressors are: bass guitar, vocals, strings, and some drum applications. The key to really understanding the sound of this compressor is in experimentation. Place an optical compressor on every instrument imaginable! Learn how this beast feels and sounds, and then determine what works within the context of a song. Many engineers will strap an optical compressor across an insert solely for the sonic characteristics of the box, and may even possibly forgo compression altogether.
Probably the most common and most flexible, VCA (Voltage-Controlled Amplifier) compressors are used for just about any and every kind of program material. The SSL Bus Plus (see above), is based off the classic SSL console bus found in the master section, which for years was only available in the large-format analog consoles by SSL. One can also find VCA-style compressors from: DBX, Neve, API, and many other manufacturers.
Remember our trusty optical compressor above, and its relative ease of operation? Well, VCA compressors tend to be a bit more surgical and flexible. Most have the ability to control the attack, ratio, release, threshold, make up gain, and sometimes more. Faster attack times mean unruly, fast transients can be better controlled. The components utilized in VCA compressors also allow for a smaller footprint. Used in mastering, tracking, and everything in-between, it’s not unusual to see a multitude of VCA style compressors in commercial and project studios. This type of compressor is also commonly used to provide the mythical “glue” to stereo mixes or sub-groups.
Once again, experiment with this type compressor. You will most likely find that one flavor is suitable for a certain type of instrument or sound, while another might fail at that. Get used to using your ears too. Don’t rely on the meter(s) to provide the information you’re looking for. If you can’t hear how the compressor is reacting to the signal, then what’s the point?
FET compressors (Field Effect Transistor) are similar in scope to many VCA style compressors in their flexibility. However, VCA compressors tend to be less colored and more transparent (great for mastering), while FET compressors add color and harmonics (great for adding additional character) to signal. And also like the VCA style compressor, FETs offer greater control over compression parameters. Attack, ratio, release, threshold, and make up gain are usually adjustable, and sometimes more.
Uses vary for FET compressors, and tend to be very popular with percussive elements. They are also incredibly popular on vocals, but the coloration might steer some away from this use. Sometime they are also used in tandem with other style compressors to tame unruly transients. Maybe a vocal take is just a bit too radical to be tamed by an optical compressor? Then place a FET compressor before it to grab those radical transients! This can also be applied to other scenarios where one type of compressor doesn’t effectively handle the workload.
Like the optical compressor, the variable mu flavor is incredibly musical, and also known for its sonic characteristics and warmth. Loaded with tubes, it’s the actual tubes that produce the gain reduction by manipulating the bias of the tubes. This also accounts for the “soft knee” or slower attack once the signal crosses the threshold.
Remember the image at the top of the article? The Fairchild 670 is one of the most coveted stereo compressors of all time and also comes in a mono channel. Weighing in at over 60 pounds employing 20 tubes and 14 transformers, this legendary box has been heard on countless recordings and masters. A keen eye will notice Fairchild compressors in the multi-part Beatles documentary “Get Back”, as they were used heavily in Abbey Road and in their own studio.
Variable mu compressors are commonly used in the recording process as well as the mastering process. While using these in the mastering process might seem contradictory to the use of more transparent VCA style compressors, it’s exactly the sound and character that many are looking for in the final master. Sometimes they find their way before a cutting lathe when producing vinyl lacquers to help tame transients and also add a greater degree of control to the mid and side program material. Once again, it takes time and experimentation to really understand and hear how these devices work.
All the compressors mentioned above can be found in both hardware and plug in form. What are the main differences between the two? For most it will be cost and space. Hardware compressors can cost anywhere from a couple hundred dollars to $40,000! Space also becomes an issue when your racks fill up and you soon realize you only have a small handful of options. The plug in format is not as cost prohibitive and your favorite compressor can be used as many times as your computer’s or host’s processor will allow. Plug ins also afford the user instant recall in your DAW’s session - this is not possible with hardware. Are there sonic differences? Yes. Will these differences have an affect on your work? Yes, but it’s negligible. Most commercial studios today work in some kind of hybrid environment, utilizing both hardware and software options. Use your ears and run with what works for you!
