Visual Clues

Why does every stage we’ve ever shot begin with an audible start signal? How hard could it be it integrate some kind of connecter into a CED (or other) timer that would allow for some shooter-initiated action to start the timer? Humans are not bats, we rely on sight, not sound to get around in our environment. Despite this, every stage begins with “Are you ready? Standby… BEEP!”

What if a stage began with the shooter reacting to a visual signal, such as a random popper falling from a tripwire controlled by the RO or something similar? Where in the rulebook does it say that the start signal always has to be audible?