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

enter image description here

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?

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

2 Answers 2


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:

enter image description here

  • 1
    \$\begingroup\$ 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. \$\endgroup\$ Commented May 16, 2016 at 0:06
  • 6
    \$\begingroup\$ 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. \$\endgroup\$
    – The Photon
    Commented May 16, 2016 at 0:12

I like marco-a's answer and The Photon's comment to that answer.

I would like to add to it, based on what I've understood so far, while trying to carefully avoid English-language-semantics-related ambiguities -

As per Ohm's law (V = IR), current is directly proportional to voltage. This means that a change in magnitude/quantity of voltage/potential difference could potentially have an impact on the magnitude/quantity of current depending on whether the resistance of the material stays constant or not. This is true regardless of whether this voltage has polarity (alternate between negative and positive, which in turn, makes the current change direction) or not.

Note that the direction of current can be changed by changing the polarity of the potential difference/voltage across the two ends of the wire.

Hopefully, this helps you understand that the change/variation in a signal's voltage magnitude, whether that change/variation is continuous (=> analog) or discrete (constant for a specific amount of time => digital), is independent of the direction of current.


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