# Analog signal vs. alternating current

I am seeing that both analog signals and alternating current are represented by the same graph:

As far as I think I know, an alternating current travels back and forth with gradual changes in voltage but an analog signal over say, a telephone line is sent via DC current.

Is it the DC current that is changing in voltage when an analog signal is being represented this way?

• current doesn't "change in voltage" ... try clarifying what you're really asking.
– user16324
Commented May 15, 2016 at 23:19

I think you are confusing two different concepts.

DC means the current only travels IN ONE direction.

This means the polarity of the signal never changes.

AC means the current alternates the direction so the polarity of the signal changes with time.

Digital (when referring to signals) means the signal can only have one of two "states" or "values" (on or off). Whereas analog signals can have infinite different "states" or "values".

Here a few graphs explaining the differences:

• An analog signal can be a very slowly varying voltage, such as the output signal from a thermometer, where the voltage represents current room temperature, where the value may vary slightly over the course of a day. Commented May 16, 2016 at 0:06
• This is one way of defining "AC" and "DC". But in my experience it's much more common and more useful to define "DC" as meaning constant with time, while any signal (or component of a signal) that varies with time is considered AC. Commented May 16, 2016 at 0:12