So, despite all the people chiming in say that any real electric signal can’t be pure unvarying DC or pure constant-frequency AC, I don’t think anyone has answered the spirit of the OP. Obviously any real information-carrying signal must vary over time in some way, and therefore can’t be either ‘pure’ AC or DC. Nearly as obviously, any real voltage signal can be decomposed into an alternating signal that ranges above and below zero volts and a constant bias voltage.
But the question still remains “In current conventional, standard, audio systems (RCA jacks and standard speakers), is the signal and current of constant polarity or alternating (in other words, is the bias greater than the alternating portion of the signal)?”
It appears that for telephones, the answer is ‘constant polarity’, as there’s a DC bias current that’s larger than the alternating signal.
I’m guessing that for standard RCA-jack audio signals, there’s no DC bias, and the actual current changes directions, but I don’t think anyone has stated this definitely.
As an aside are there any systems where there’s a bias (intentional) that is smaller than the alternating signal? In other words, where current can alternate, but is more often/higher in one direction than another?
Thanks for all your answers so far, I can see that an audio signal may be either in differing circumstances, but Quercus is right, no one has really answered my question. Quercus pharsed it much more elegantly than me.
An unpowered microphone (“electret”) puts out an alternating signal with zero bias.
Some professional microphones use a bias voltage between the pre-amp & the mike.
A modern digital radio microphone will zero any bias before digitizing the signal for radio transmission.
A power amp output to drive an unpowered speaker will use zero bias.
A power amp output to drive a powered speaker may have bias or not. Generally not, at least not on purpose.
Typical line-level interconnects, e.g. RCA cables between components in home audio systems common since the 1970s do not use bias. Again due to vagaries in the circuitry a little bias may be present due to the two components power supplies not being identical. That unintentional and undesired bias will be filtered immediately on input.
Traditional unpowered phonograph cartridges do not use bias.
While the terms AC and DC can mean different things and have no hard and fast definition, let me say a few things.
Pure DC does not imply constant voltage or current, only that the voltage or current do not at any point in time become negative. As long as the value does not change sign we have DC.
Pure AC does not in any way imply a sine function or any other particular function. AC only implies the absence of DC so that the integral of the signal over time is zero.
In general terms audio signals fed to an amplifier or coming out of an amplifier are pure AC and contain no DC bias but in particular cases this may not be the case and for specific reasons it may contain DC bias which would be filtered out later.
Inside linear amplifiers the opposite is true and the AC audio signal is superimposed on a DC signal which is then amplified and then the DC is stripped off and the AC is output.
DC stands for direct *current *(not direct voltage), and AC stands for alternating *current *(not alternating voltage).
If you have a wire (or any circuit component, for that matter) and the current never changes direction, then by definition it is DC. The current can go up, down, even to zero. But as long as it never changes direction (i.e. goes negative), then it is by definition DC. Obviously, current flows in both directions in an AC system.
As a side note, you know the 120 VAC receptacle on your wall? It is more correct to call it an alternating voltage (AV) receptacle than an AC receptacle, since the former is always true, and the latter is usually - but not always - true.
Having said that, EEs are sloppy when it comes to the terms AC and DC. When we talk about an audio signal that is represented by a voltage, for example, we will say it’s an “AC” signal, when in reality it’s an AV signal. We also like to break signals up into their AC and DC components. Take a class-A transistor amplifier, for example. Strictly speaking, the overall current through the emitter is DC at all times, since its direction never changes. EEs will say, “There is a DC current through the transistor for bias purposes, and a small AC current riding on the DC that represents audio.”
So to address the OP, intelligent signals (e.g. audio) are usually represented by a time-varying voltage (often around 1 V rms) or time-varrying current. Overall, it may or may not be direct current (DC). Overall, it may or may not be alternating current (AC). Overall, it may or may not be direct voltage (DV). Overall, it may or may not be alternating voltage (AV). It all depends if a constant (non time-varying) voltage or current is added to the audio signal inside the electronic circuit.
See, I wouldn’t call that either wrong or sloppy. Rather, I’d say that’s the way the terms have come to be defined within the industry. As you note, from an engineering standpoint, it’s useful to think of a time varying DC signal as being composed of an AC signal plus a constant DC component.
“Pitiful” is putting it a little harshly — if correctly.
True, the Apple II’s built-in audio was quite limited: 1-bit square-waves that had to be driven by the main CPU. And when the CPU was busy producing sound, it couldn’t be doing much of anything else. That said, there are still some cute tricks people have made the machine to do over the years, usually through the magic of pulse-width modulation:
[ul]
[li]There was a commercial program called Electric Duet that could play music in two simultaneous voices. Each voice could use any of several “instruments”, not that these instruments were anything you’d confuse with a violin or piano. They were just simple tones.[/li]
[li]There was another commercial program named S.A.M. that did human voice synthesis, if crudely.[/li]
[li]At least one person has written software that reads an arbitrary 8-bit waveform in memory and plays it through the speaker, although the effective resolution of the output is more like 5 bits instead of 8. It’s still pretty amazing to hear, given the limits of platform and the sort of sound you get accustomed to hearing.[/li][/ul]
Sorry, this is all a bit off-topic.
“Alternating current. A periodic current the average value of which over a period is zero. Note: unless distinctly specified otherwise, the term alternating current refers to a current that reverses at regularly recurring intervals of time and that has alternately positive and negative values.”
“Direct current. A unidirectional current in which the changes in value are either zero or so small that they may be neglected. Note: as ordinarily used, the term designates a practically nonpulsating current. A given current would be considered a direct current in some applications, but would not necessarily be so considered in other applications.”
-IEEE Standard Dictionary of Electrical and Electronics Terms
FWIW here’s another attempt to answer the OP usefully:
Usually electrical signals for audio systems have an average value of zero and have currents and voltages that often change sign, and generally they would better be called AC than DC. However, there are some cases where there is a constant bias in the signal such that its voltage and/or current polarity does not change. Typically this is done to serve some special function like providing power to a microphone containing its own small amplifier. Telephones have distinctive features that can use biases. Generally when thinking about electronic audio signals people set aside the issue of DC bias except when thinking about whatever special function it serves. As an aside, one often sees “coupling capacitors” used in circuits to pass a rapidly fluctuating signal along while blocking a DC bias, and this may be done in many places, in audio circuitry as well as radio and other kinds of circuits.
Since sound is a radiating fluctuation in air pressure, typically by an amount that is tiny relative to the absolute air pressure, one could say that the air pressure is almost constant and does not alternate in sign, and that there is a superficial similarity between this situation and electrical audio signals having a constant bias. But the similarity is only superficial, and the reasons electrical audio signals might be given a bias are unrelated to the idea of air’s relatively large and constant pressure relative to sound pressure.
Finally, the question depends as much on the customary uses of terms like AC and DC as it does on the technical specifics of what is happening. The terms “AC” and “DC” are used more in describing sources of electrical power than they are in describing signals, and this is particularly true of “AC”. Similarly, the terms “RF” or “radio frequency” and “digital” are often used to describe signals, and they denote the purpose of the signal or establish a mindset for considering a circuit. For example, the power cord on a PC carries AC power, and the RX and TX wires on a PC’s COM port carry digital information. In both cases there is some voltage and some current, and both are changing sign. But when thinking about the line cord, one is interested in the current and voltage available and not at all interested in when the peak of the waveform arrives, whereas when thinking about the COM port the opposite is true.
OK, here is the short answer: The audio signal is always AC but it may be superimposed on some DC for specific reasons. In the case of POTS telephone the audio AC is sent along the same wire which carries the DC which supplies the phone and it is for this reason that the sum of both is of constant polarity. If the power current were sent through a different wire the audio could be pure AC but this is not a good arrangement as it is better to use one wire for both.
>Why must the average be zero?
>As long as the current is sometimes reversing direction, then it is (by definition) AC.
Crafter_Man, I’m just quoting the IEEE’s dictionary. I think these words ARE the definition, the official definition, of AC. Who better than the IEEE to define what AC means? Where better than in their dictionary to find said definition?
I understand what you’re saying. Perhaps there are two definitions of AC, then:
AC is what’s left over after you subtract the DC component from the overall signal.
When looking at the overall signal, if the current ever reverses direction (i.e. goes negative), then it is an AC signal.
You are using #1. I am using #2.
Look at the top waveform in this pic. Let’s assume these represent current and not voltage. Most EEs would say, “It is an AC signal with a DC component.” (Definition #1.) But a purist might argue that the top waveform is really a DC signal, since the current never reverses direction. (Definition #2.)
So which definition is correct? I dunno. I would normally say “it is an AC signal with a DC component.” But at the same time I would have to acknowledge that, in the very strictest sense, it is a DC signal, since it never reverses direction.
Regardless of what the IEEE’s dictionary says, I believe this to be the “official” definition of DC. (Definition #2 from my previous post.) But the same article goes on to mention “Various definitions,” and describes how DC can also be defined to be a component in an overall signal (Definition #1).
Well, I am not going to argue about definitions and semantics. The only important thing is that we all understand the concepts and that the words are clear in each case. But arguing over whether something is DC or AC is like arguing whether Obama is socialist or not: It can never be settled to the satisfaction of all because someone will just use his own chosen definition. Labels are only important as a means to understanding, they are not important per se.
I think the last half dozen posts have been accurate and insightful regarding the technical points, and that they also help make clear that the usages of the terms AC and DC can be somewhat varied.
King Solomon Hill apparently feels helped - another victory for the Board.
> […] the Wiki article on DC […] Regardless of what the IEEE’s dictionary says, I believe this to be the “official” definition of DC.
Wow - how bold, how independent, CrafterMan - even for you! This seems to me like an unusually clear cut case where there is one well established official authority for the entire field, who has made their position perfectly clear. And you’re going with the unvetted volunteer contributions to Wikipedia instead? I think I might go along with you if you said you believe this to be the working or popular or effective definition, but “official” seems to me to leave no doubt. Maybe we should look up “official”… Wait - is this it? did you actually write the Wikipedia article???
>Well, I am not going to argue about definitions and semantics.
Good 'nuff. But I sometimes like to. Definitions and semantics are important too. I think the OP is more interested in what direction(s) the current flows than in the definitions per se, though subtleties in the definitions at least helped create some confusion. If labels are important as a means to understanding, their definitions are important per se. The labels themselves are then just spellings and pronunciations.
Anyway, thanks for an interesting discussion, so far…
No… I am not saying it must be the official definition because Wiki states it. I would never use Wiki as a citable source for anything. I am simply saying that Wiki just happens to have the right definition.
I think it can be safely stated that DC - in the very strictest sense - means the current flows in one direction only. Hence the Wiki article is correct. None-the-less, EEs will refer to a small, time-varying signal that “rides” on a large DC bias as “AC,” even though in the very strictest sense this is not correct. But as stated by Sailor, it does not cause a problem, as we all understand what it means.
