DI unit
A DI unit (direct input or direct inject) is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high output impedance unbalanced output signal to a low-impedance, microphone level, balanced input, usually via an XLR connector and XLR cable.
DI units are also referred to as a DI box, direct box, or simply DI, with each letter pronounced, as in "Dee Eye." The term is variously claimed to stand for direct input, direct injection (a British term), direct induction or direct interface.
"Traditional guitar amps and speakers are fairly lo-fi devices that squash dynamic range, introduce distortion and narrow the frequency range. In contrast, a properly interfaced direct guitar signal will sound super clean, dynamic, punchy and far brighter." - Guitar.com Magazine
DIs are frequently used to connect an electric guitar, electric bass or electronic musical keyboard to a mixing console's microphone input jack. Its signal comes "direct" from the source instrument without passing through the air as sound waves, and thus is isolated from other sounds and avoids effects of microphone or room acoustics. The DI performs level matching, balancing, and either active buffering or passive impedance matching or impedance bridging. DI units are typically metal boxes with input and output jacks and, for more expensive units, “ground lift” and attenuator switches.
DI boxes are extensively used with professional and semi-professional PA systems, professional sound reinforcement systems and in sound recording studios. Manufacturers produce a wide range of units, from inexpensive, basic, passive units to expensive, sophisticated, active units. DI boxes may provide numerous features and user-controllable options (e.g., a user-selectable 0dB, 20dB or 40dB pad and/or a "ground lift" switch). They may come in different types of enclosures, usually a metal chassis that helps to protect against electrical interference. Some bass amplifiers have built-in DI units, so that the bass amp's output signal can be connected directly to a mixing board in a sound reinforcement/live show or recording context.