this may be obvious to everyone else, but took me a while... stereo!
[there are two mirrors to drive (or, equivalently, a point in 2D to target with the laser) and i couldn't see how to do that with a single audio signal.]
[there are two mirrors to drive (or, equivalently, a point in 2D to target with the laser) and i couldn't see how to do that with a single audio signal.]