A compressor is an effect used to narrow the dynamic range (the difference in volume between loud and soft) by compressing the sound.

The three major parameters of a compressor are threshold (the volume at which the compressor operates), ratio (the rate of compression), and output gain (volume increase).

The threshold determines the target volume, and any sound that is louder than that is reduced in volume, according to the rate of compression set by the ratio. The final volume can be adjusted as necessary using the output gain.

Compressing the louder sounds in order to reduce the difference in volume between loud and soft can make the music easier to mix and more comfortable to listen to.

Performances by instruments and vocals do not always maintain the same volume. Adjusting the volume so that the audience can comfortably listen is an important element in mixing.

By using a compressor in just the right amount on vocals, which have a wide dynamic range, the sound will not overload even if the vocalist belts out a note, making it possible to combine the vocals at an appropriate volume. By using a compressor on a bass guitar, its sound can become clean and smooth, making it easier to put the entire mix together. Using a compressor on a snare drum or other instruments with a sharp attack can bring greater impact to the sound.

As a compressor works to reduce louder sounds, it can be useful in protecting equipment from sudden and sharp spikes in volume.

Seasoned engineers can use compressors to revitalize tracks and make them sound more exciting by adding liveliness to guitar parts, adding punch to bass lines, bringing a tighter sound to snare drums, and making vocals sound clearer. That said, if compressors are not used correctly, the sound may loose its dynamic punch.

Yamaha's 1-knob comp features just one knob to express that kind of control used by seasoned engineers. This single-knob compressor allows you to achieve the optimal compression effect from vocals to instruments, making your music sound authentic without the need to remember difficult function names and operating principles, and without the need to hook up another external compression device.

When using a Yamaha MG20XU or similar mixing console (which has several built-in 1-knob comps), it's a good idea to use the single-knob compression function when mixing live music.