Which of the following is not a common input type for a digital mixer?

In the context of digital mixing, a common input type refers to the various sources from which audio signals can be received by a digital mixer. Microphones, instruments (such as guitars or keyboards), and line inputs are all standard audio sources that connect directly to a mixer to capture live sound or process audio signals.

Microphones are essential for capturing vocal sound, instruments provide more tonal variety and are crucial for music mixing, and line inputs are typically used for connecting devices like synthesizers or audio interfaces that output level signals suitable for mixing.

USB, while a widely used format for digital connectivity, is not a traditional input type for a typical digital mixer in the same sense as a microphone, instrument, or line connection. While some modern mixers may offer USB connectivity for features like direct recording to a computer or playback from a computer, it does not serve as a primary audio input source like the others mentioned. Instead, USB primarily functions as a method for data transfer and signal routing rather than a typical audio input. Hence, identifying USB as not being a common input type aligns with the context of standard audio mixing practices.