The Basics Of Compression
A compressor is a device that turns down the volume of an input source when that volume exceeds a certain level. A compressor listens to the volume of an incoming audio signal and reduces that volume to a certain level that you set. That level you set is called the threshold. When a signal crosses the threshold it is reduced by something called the ratio. A ratio will be displayed as a given number to one.
When you set a threshold you are setting the maximum volume the signal can go. Let’s say you set a threshold for 0 dB. Every time the signal crosses that point the compressor kicks in. Now let’s say you set a ratio for 3.5 to one. That means that every time the signal crosses the threshold by 3.5 dB then the audio is reduced by 1 dB.
Every compressor has two basic elements. The first is called the detection circuit. The detection circuit is the portion of the compressor that listens to the audio and tells the compressor how much volume the audio has. The second element is the gain reduction circuit. The gain reduction circuit tells the compressor how to apply the ratio and other settings. It tells the compressor how much volume to cut out of the signal. Compressors work in milliseconds and work much faster than you could if you just use your hands to adjust the signal using a fader.
A brief history of compression
Compressors have been around almost since the beginning of audio recording. The compressors they had back then are much different from the software compressors we have today. So let’s look at the history of Compression and talk about how it all got started.
The first audio compressors were actually used in radio. They were designed to be a protection circuit. This is when AM radio was the most popular format. Compressors were used as an automatic gain control to prevent over modulation of the AM radio signal. If the AM signal became over modulated it went into other frequencies and it was not picked up by the radio at all. Before compressors someone manually rode the audio fader and adapted to the changes in levels. Back then they did not want the signal to peak and overload the transmitter. They did not want the signal to drift into other frequencies. This was not a big deal with FM which came later, but back in the days of AM the modulations were much closer together. The first audio compressors only had a few controls. The compression effect was only meant to be a protection circuit. The engineer still rode the fader. That was the point of most compressors and limiters of the 30’s and 40’s. They were mainly used as a protection circuit.
Around the 50’s and 60’s they were beginning to design compressors to take the work of the engineer. There was a big television boom that brought about an increase in compressor technology. A lot of compressors at that time were designed with the master channel in mind. They had to be reliable and at a very high quality. All of the compression circuits were designed using tubes, field effect transistors, and transformers. These were analog designs. All compression was performed using very complicated electronic circuits.
In the late 60’s and early 70’s we saw more advancements in compression technology. We began to see rack mounted compressors. The actual function of the compressors changed and they were begining to be used for more musical applications. Having an input, ratio, attack, and released was considered a huge improvement from that era. In the late 60’s, 70’s, and on into the 80’s we started to see compression technology included in the channel strips of recording consoles. So compression went from being a protection effect to being integrated into audio as a tool.Then in the 90’s we went to digital compressors. There was a sudden shift in compressor technology. Hardware manufacturers could make a digital compressor with midi that could fit in your rack. Software manufactures could make all types of compressors that could adapt to the different needs of the engineer. Today in the era of digital compressors we have many more options. Compression today can also be used for multiple reasons and they are often used as an effect.
The Basic Controls of a Compressor
Let’s take a moment and discuss the common controls found on today’s compressors. Some of these controls may not be found on all compressors but this will give you a general overview.
Threshold: This sets the limit or volume the audio can go before compression kicks in.
Ratio: This sets the amount of loudness reduction the compressor will do after it has kicked in.
Attack: This determines how fast the compression will kick in after the signal crosses the threshold.
Release: This feature will determine how soon the compressor releases the signal once it has engaged the audio signal. Some compressors may have an auto release function that can be turned on.
Knee: Refers to the amount of compression that kicks in before the threshold. The more compression kicks in before the threshold is crossed the softer the Knee. This feature tends to make the compressor sound smoother as it kicks in. No compression before the threshold is considered hard knee compression.
Gain Reduction Meter: This meter will tell you how much the signal is being reduced by the compressor.
Make Up Gain: This feature will increase the portions of the signal that does not hit threshold levels. It makes the softer parts of the signal louder
These days there are many types of compressors to fill many types of roles in the recording process. Compression is used as an effect. It is used in mastering as well as mixing. Compression can be used to keep a meter from peaking. There are multi-band compressors that combine compression with EQ. Multi-band compressors allow you to compress any given frequency range that you set.
You will find that when using a compressor as an effect each one sounds a little different. For this reason old analog compressors are simulated using algorithms. These algorithms are a mathematical equation that runs on a computer which simulates a compressor’s circuit. Software vendors make all kinds of compressors that simulate older analog hardware. The older hardware is known for its unique sound and colorization of the audio signal. Each of these classic circuits will sound different. These classic compressors sound great when used as an effect.
A limiter is a type of compression. A limiter will control maximum volume. A limiter will not allow any volume over the threshold. This is not always the case with a compressor. Limiters have very short attack and release times as well. Limiters are also referred to as brick wall limiters because they won’t allow anything passed the threshold.
Sometimes you can find some compressors that have a built-in noise gate. A noise gate will cut the audio signal when it falls below a given DB range that you set. This is good for eliminating background noise that remains constant throughout the audio signal. Noise gates may have their own attack and release controls. They will also have their own threshold. It works in the reverse way that the compressor threshold works. When an audio signal falls below a set threshold the entire signal is cut.