This effect emulates the sound of various acoustic environments. It is a similar sound to an echo, but is distinct in that only the ambiance of the sound persists, rather than a repetition of the original sound. Common control functions include "Time," "Room Size" and "Depth." The time control governs how long the ambiance persists. The "Room Size" determines the resonance of the reverb. The depth control determines what portion of the original ambiance persists. Rackmount reverb units typically have a digital control interface that allow you to make precise adjustments to the various parameters.
In most audio applications, distortion is an unwanted consequence of a very strong signal overloading the recipient device. However, when delivered in a controlled manner, distortion creates a distinctive "crunch" that enhances guitar tone. Rackmount distortion units offer a wider scope for customization, including the option to control the harmonics, contour, equalization and "fuzz." Sounds achievable with a rackmount distortion range from a warm, mellow overdrive to a screaming, heavy metal distortion.
Modulation effects manipulate the frequency of the original guitar sound with a variety of sonic results, including chorus, phase, flange and wah effects. Modulation stompboxes typically offer only one type of effect, but rackmount effects have sufficient processing power to offer a range of modulation effects options. With a rackmount modulation unit, you control the rate, depth, intensity, ratio and level of each modulation to create anything from a subtle pulsing sound to an intense swirl or "fluttering" sound. The guitar sound in the introduction to "Come As You Are" by Nirvana features a prominent example of intense modulation.
By sampling and storing the original sound, a digital delay unit can repeat that sound at a variety of time intervals, volume levels and intensity. The Edge from U2 uses a rackmount TC Electronics digital delay unit to create his signature "slap-back" sound. Floor-based delay units are typically less sophisticated and complex than rackmount effects.
This is a dynamic effect that controls the volume level of the guitar. By cutting loud sounds and boosting quiet sounds, compression stabilizes the output level and keeps it within a predefined range. This stops unwanted peaks and dips in the performance. You adjust the "threshold" parameters to determine the exact volume at which the compressor starts to cut and boost. The "Ratio" parameter determines the intensity of the volume adjustment.