DI box

A direct injection box takes an unbalanced, high-impedance output — a bass guitar, a keyboard, a laptop headphone socket — and converts it to a balanced, low-impedance signal of the kind a microphone cable is designed to carry.

The reason this matters is distance. An unbalanced cable picks up interference along its length and has no way to reject it, so a guitar lead run forty feet to a stage box arrives with hum and whatever the dimmers are doing. A balanced line carries the signal twice, in opposite polarity, and the receiving end subtracts one from the other — interference picked up equally by both is cancelled. That is why the audio stage box is full of three-pin connectors and why a laptop plugged straight into a long cable usually buzzes.

A DI also carries a ground lift switch, which disconnects the shield at one end to break a loop between two pieces of equipment fed from different power. That switch solves a large proportion of hums on a stage in about two seconds, and knowing to reach for it is most of the skill.

Active boxes need power and handle weak sources better; passive boxes need none and tolerate loud ones. A laptop playing walk-in music is a DI, and a show that forgets one ends up with a cable taped across a ballroom floor.

Backline rental — owned inventory, our own crew, and a site survey before anything is specified.

• Gain structure Setting the level at each stage of the signal chain so the system runs well above its noise and well below its ceiling — the thing that is wrong when a mix sounds thin, noisy or brittle.
• Front of house (FOH) The mix position out in the room where the engineer sits, and the audio the audience actually hears — not a department, and not a place a venue should treat as spare seating.