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DI unit
A DI unit, DI box, direct box, or simply DI (variously claimed to stand for ''direct input'', ''direct injection'', ''direct induction'' or ''direct interface''), is a device typically used in recording studios to connect a high-impedance, line level, unbalanced output signal to a low-impedance microphone level balanced input, usually via XLR connector. DIs are frequently used to connect an electric guitar or electric bass to a mixing console's microphone input. The DI performs level matching, balancing, and either active buffering or passive impedance matching/impedance bridging to minimize noise, distortion, and ground loops.
DI boxes are extensively used with professional and semi-professional PA systems and in sound recording studios.
==History==
Passive direct boxes first appeared in the United States in the middle 1960s, most notably in Detroit at radio stations and recording studios like "Motown", "United Sound Systems", "Golden World Records", Tera Shirma Studios and the Metro-Audio Capstan Roller remote recording truck. These DIs were custom made by engineers like Ed Wolfrum with his "Wolfbox" and by concert sound companies to solve certain problems associated with amplifying electronic musical instruments, especially electric guitars. These boxes typically contained an audio transformer (like the Triad A11J through 1974) with a turns ratio from approximately 8:1 to 12:1, and thus an impedance ratio of around 144:1. With this kind of transformer, the output voltage of the instrument is stepped down to a range compatible with the typical mixing console's microphone preamp. The typical console preamp input impedance of 1,500 ohms would appear to the electronic instrument as a high input impedance of 216,000 ohms.〔 Section 2.2.3: Line to Microphone Input or 'Direct Box'.〕
The passive direct box was suitable for most electronic musical instruments but it negatively colored the sound of ones with weaker output signals, such as Fender Rhodes pianos and Fender Precision Basses with single-coil pickups. To accommodate these instruments, active direct boxes were designed containing powered electronic circuitry which increased the input impedance from about 200,000 to above 1,000,000 ohms. In 1975, a 48-volt phantom powered active direct box was designed for Leon Russell's recording studio, its circuitry published in ''dB'', the sound engineering magazine. The sound company Tycobrahe, known for supporting large rock festivals such as California Jam, offered an active direct box for sale in 1977, a model capable of +9 dBm line level output, with a built-in attenuator to compensate for various input levels.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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