Use a DI when you want to connect two devices and you have any of these issues:
- impedance mismatch
- line level mismatch
- differences in wiring or connectors (e.g. Balanced XLR to Unbalanced 1/4" Tip-Sleeve)
- noise - especially "hum" (Ground Loop)
A DI unit or DI box is an electronic device designed for connecting a piece of equipment with an electronic audio output to a standard microphone or line level input. It performs both level and impedance matching to minimise both noise and distortion. DI is variously claimed to stand for direct input, direct injection or direct interface. DI units are extensively used with professional and semi-professional PA systems and in sound recording studios. -- Wikipedia
You will also see the term DI used to refer to devices used to modify the tone as well as other properties of a signal. This is often in the context of Acoustic Guitar and Electric Bass. In the picture above the first two are passive DIs used for solving problems. The others are all sold as DIs that also shape the sound.
More info about DI's
This is an excerpt - follow the link above for the full story.
What's in a D.I. Box?
D.I. Boxes are constructed using one of two common techniques.
- The first type uses electronic circuitry and are known as active D.I. Boxes. They require either Phantom Power or a battery supply.
- The second type uses an audio transformer and are known as either transformer or passive D.I. Boxes. They require no power supply.
A D.I. box is required to perform three separate basic tasks:-
1. Impedance Conversion. 2. Unbalanced to balanced conversion. 3. Earth isolation.
The medium or high impedance of a signal source is converted to a low impedance suitable for feeding down a long multicore to a mixing desk’s microphone input. A low impedance enables long cable runs, with very little quality loss, and also low susceptibility to external electrical interference which can cause hum and buzzes.
A D.I. box should provide a high input impedance for connection to a signal source, and a low output impedance for connection to the microphone input of a mixing desk.
Unbalanced To Balanced Conversion
The unbalanced (2 conductor) wiring of a signal source is converted to the balanced (3 conductor) wiring of a mixing desk’s microphone input. A balanced cable provides good rejection of electrical interference, while an unbalanced cable does not. Active D.I. Boxes are potentially capable of providing excellent unbalanced to balanced conversion, but due to cost restrictions, most are poor performers in this area. D.I. Boxes that incorporate transformers provide excellent unbalanced to balanced conversion.
A D.I. Box provides isolation between the earth wiring of a signal source (e.g. musical instrument) and the sound system to which it is being connected. This prevents earth loops from occuring. An earth loop occurs when a device, such as a keyboard, is connected to the mains earth via more than one path. The first path is via the instrument’s own power cable to the mains earth. The second path is via the interconnecting audio cable to the sound system, then via the sound system’s power cable to the main’s earth. Any resultant circulating earth current is amplified and is heard as a hum or buzz. These unwanted earth currents are usually induced from nearby power and lighting cables.