A DI, or direct input, is used to connect an instrument or audio source directly to a mixing desk, audio interface, or PA system without using a microphone. It converts the high-impedance, unbalanced signal from instruments like guitars, basses, or keyboards into a low-impedance, balanced signal suitable for long cable runs and professional audio equipment.

DI boxes are commonly used in live and studio environments to provide a clean, noise-free signal with consistent level and tone. They help reduce hum and interference, and are essential when recording or amplifying instruments directly, especially bass guitars, acoustic instruments, and modern digital rigs.