On 03/04/2011 15:56, RussellMc wrote:
> If the second spot is not at an identical resonance node on the
> string, which will usually be the case, then the moments of reversal
> of motion of the spot will occur at different times for the two LEDs.

I think it's probably just to make sure of this, like the old turntable=20
strobes (for speed tuning) which generally had more than one line of=20
dots which were slightly offset. This would be similar for the string,=20
one point being slightly offset from the other which should make it=20
easier to detect relative movement.

--=20
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