Why do signal lines use AC current?

@Connor to transfer any signal, it is inevitable that a signal (or AC) current will be produced. The current may be incidental if the signal voltage is the important part of the signal. On the other hand the AC current may be the wholly important part of the signal and the voltage is incidental.

Why is it inevitable? Apologies, just to check the terminology, does AC here mean Alternating current? If it does, why are power rails direct current?

It's inevitable because you cannot transport a voltage signal without producing a little bit of current and vice versa. Power rails are DC although in reality they are constant voltage and nobody ever uses the the term "direct voltage". It's just one of those silly things that you get used to.

Sorry to be persistent, but what does the term AC mean in this context? Does it mean alternating current? What's the difference between DC and constant voltage?

@Connor signals are quite often AC in that they have an average value of zero and are positive and negative in a cyclic way. AC strictly means alternating current but we also describe alternating voltages as "AC voltages" and, I'm sure this can confuse things. Same for DC; we say a battery produces a DC voltage but, what we mean is that the voltage produced remains always positive (and is usually stable in the case of a battery).

I think calling a signal line AC confuses things more than it clarifies them.

@KenShirriff are you implying I should dumb down my answer in some way or, was your comment aimed at the OP? If aimed at me then please use the @AndyAka prefix or the same for the OP. What alternative to "AC" would you suggest?

@Connor: Those sound like excellent questions to ask as a top-level question ( meta.stackexchange.com/questions/232296/… ). Meanwhile, electronics.stackexchange.com/questions/173754/… may give some context.

@user1937198 read what I said --> *It might be a pure AC voltage/current or an AC voltage/current superimposed with a DC level*<-- I'm more interested in my answer being strictly correct than I am in dumbing it down to please folks who might not, at first, comprehend what I'm saying. No compromise here I'm afraid to say.

@Andyaka Yes, theoretically by Fourier anything can be thought of as a collection of sine waves, but thats not necessarily the most useful abstraction when trying to explain the difference between a TTL square wave signal encoding of a random bit pattern, vs a 60 Hz clean mains power supply. Whats important is that there is a variation in the voltage (or more rarely current), and that variation has meaning.

@Andyaka By your definition, pretty much all mains power would count as signal rails and not power rails.

@user1937198 who said anything about sinewaves <-- why are you complicating things? If you are not happy about what I have said maybe you should make a downvote and move on to make your own answer.

@Andyaka Because you said alternating current. Its the alternating that I have an issue. If I transmit continuous 1s in TTL logic, where is the alternation?

Think about what I said. If you don't understand that bit then maybe ask a new formal question.

To quote wikipedia: 'Alternating current (AC) is an electric current which periodically reverses direction' @Andyaka Where is the periodicity in a truely random bit pattern? Or do you have an alternative definition of Alternating Current?

@Andyaka With the way this answer is phrased right now, its not clear what is a core definition of a signal line, or if some of the statements are just examples of what a signal line could look like.

AC voltage superimposed on a DC level <— do you understand that bit? Can you see that a TTL signal is equivalent to an AC signal superimposed on a DC level. If you can’t see that then ask a new question.

I know what you mean, and I think so do most of the people commenting with suggestions on how to make it clearer for people that don't already know what a "signal line" vs. "power rail" is. It seems to me that you're assuming a lot of background understanding of many things, like exactly what you mean by "AC" in this context (varying voltage); in other contexts it means a periodically repeating voltage, often sinusoidal, often as part of a power supply, as @user1937198 pointed out. Also, the DC average of a TTL signal depends on what information it's carrying, if it's not periodic.

I know the feeling of trying to answer a question that seems to be about one thing, but people who don't know the answer to it often are missing huge amounts of other knowledge, too. I run into the same thing on Stack Overflow in tags where I'm an expert. Writing an answer that's technically correct but relies on understanding everything else isn't as helpful to most future readers as it could be. It's more work to find ways to cover other background knowledge without bogging down or cluttering up the real answer. I sometimes start with a one-sentence summary and then explain what it means.

I don't think it's helpful to think of this as "dumbing down" the answer, that's not what people are asking for, or at least not the only way to bridge the gap between your level and the beginners who don't know what a "power rail" is. You don't need to say anything that's technically incorrect, just define your terms. (Sometimes I write answers with a simplification of the "lies to students" variety, with a footnote pointing out this isn't always true, to keep that clutter out of the meat of the explanation.)

This question does already have a good answer that looks more beginner-friendly to me, so I don't think there's any actual need to spend a lot of your time reworking this answer. I just wanted to talk about writing answers to "confused beginner" questions in general, and try to put the comments from other users in a different light, since you seemed to react very negatively to the suggestions that I think made some sense.

@PeterCordes irrespective of your opinions I'm not changing my answer one bit so, I suggest you move on to pastures new like I've been trying to hint to the other guy.

@Andyaka Like I said, there's already a good answer so it doesn't matter much whether you change yours or not for this question. My goal in commenting was to talk about writing answers that can maybe be more helpful to total beginners without being worse or less technical, moreso than to continue to argue over this answer specifically. I'd already moved on from that.