Is an insert jack for input or output?

It’s both! The entire signal from the corresponding channel, subgroup, or mix bus is sent through this jack. The tip of this jack is an output, which is commonly used to send signal out to a serial processor (compressor, equalizer, sonic maximizer, etc.); the ring is an input which allows signal to return from the processor; and the sleeve is a ground connection. Because the insert jack is an input and an output jack, it creates a loop, which is formed when a signal processor is inserted between the output (tip) and the input (ring).
The insert jack can also be used as two different types of direct outs.
If a tip-sleeve 1/4″ cable is plugged all the way in to the second click, the signal is sent out from the tip (output) of the insert jack and does not return to the ring (input), which breaks the loop and does not allow signal to flow through the rest of the channel. This is referred to as a direct out with signal interruption. if a tip-sleeve 1/4″ cable is plugged in only to the first click, the signal is sent out the cable and continues through the channel, since the insert loop has not been broken. This is referred to as a direct without signal interruption.

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