Direct input, or direct injection, devices capture direct instrument signals. These signals are beneficial to audio engineers because they are unaffected by a microphone's frequency response or an amplifier's unique tone characteristics, which gives the engineer a higher degree of control over equalization and tone characteristics at the mixing stage. Direct input guitar signals are especially useful. Unlike traditional guitar recording techniques, wherein microphones capture guitar performances that cannot be altered once they have been recorded, direct signals can be modified during post-production. This allows engineers to focus on the quality of the performance, rather than the quality of the audio.
Direct input (DI) boxes are the primary means by which direct signals are converted to usable audio. Electric guitars produce high-impedance, unbalanced signals that cannot drive the low-impedance, balanced inputs on most mixing consoles. Direct input boxes convert electric guitar signals to the appropriate impedance level for mixing consoles, and balance the output to reduce interference from other electrical sources. Guitars are connected to DI boxes via instrument cable, and DI boxes are connected to mixing consoles via XLR cable, where the signal is then treated as a balanced microphone input.
Some direct input devices contain built-in amplifier modeling technology that allows users to generate usable guitar tones without the use of an amplifier. The Digitech GNX4, Line 6 POD xt Live, and BOSS GT-8 are three popular stomp box-style amp modeling and effects units that are popular in both live and studio settings. Although these units can be used in front of amplifiers as effects rigs, their direct input capabilities allow users to plug directly into the venue's PA system, which eliminates the need for amps. In addition, these units allow for silent recording in studio settings.
It is common for guitarists to spend hours capturing the perfect performance, only to grow dissatisfied with their amplifier tone and head back into the studio. In modern recording settings, this guessing game is avoided by capturing DI signals, then applying amp modeling software during the mixing stage to simulate traditional amplifier tones. This method is popular because it allows musicians to perfect performances without worrying about latent dissatisfaction as a result of poorly chosen amplifier settings. It is also attractive to musicians on a budget, who cannot afford the expensive vintage amplifier collections simulated by such technology.