Any electrical signal can be interpreted as analog or digital. When measuring a voltage, if you have an analog meter, the deflection of the needle is proportional to the voltage. If you have a digital meter, it gives you a decimal approximation of the numeric value of the voltage. Just because the voltage can be anywhere from 0 to 5 doesnt mean the signal is analog. If the voltage is greater than a certain amount its interpreted as 1, less than that is interpreted as 0. This system allows a small change in voltage due to signal noise less of a problem, a 1 can fluctuate between around 3V and 5V without any loss of data. This is why pictures are higher quality, less effective signal noise. An analog TV will convert the voltage value directly to a picture displayed on the screen, through linear circuitry such as op amps, meaning that if the signal fluctuates, the picture fluctuates. I've heard of trinary computers before, which use 3 different voltage ranges instead of 2 for logic signals. These are faster, and generally use less bandwidth to send a given number, but are more prone to signal noise. Its also important to note that the voltages in computer memory also fluctuate (this is why you lose the data in RAM when you switch off), so this needs to be taken into account, and the memory needs to be refreshed more often. What youre suggesting with quantum computers (not a field I know much about) seems to be that each digit can be transmitted as an analog signal, ie. an infinite number of possible values. If there is no signal noise in a quantum computer, this is possible.