i get what u mean and had Define OSC 20 and the HC oscillator

i dont get what u mean by the point below.
How i measure CH1 and CH2? what u mean by "miss CH2"

2) as your channels follow each other, you must know you'll measure CH1, miss CH2, miss CH1, measure CH2, measure CH1, miss CH2, miss CH1 , measure CH2, etc, etc ...