I am studying The Art of Electronics and am confused about this particular part about a signal rectifier. In the first picture above, the first cap and resistor function differentiate the signal, and the signal passes the diode if the voltage is > .6V. The second circuit improves this by allowing any signal of above 0 V to pass. I am confused about two things. First, why is R2 necessary? It seems like we could just measure the voltage coming out of the LED without the resistor there... Also, I do not understand how circuit 2 makes it so any signal > 0 V will pass. Can someone explain these two things?