Ac

[eraos]

This may be a dumb question, but I'm still going to ask.

If, in AC, the current is alternating, how does it get anything done? How does an audio signal get from point A to point B if the current is constantly reversing direction?

Is it like sound propagation?

 

[wheelema]

Woah. Heavy.

 

[eraos]

Oh, and I also read (from Wikipedia)

The particles [electrons] themselves often move quite slowly, while the electric field that drives them propagates at close to the speed of light.

That must have something to do with it .. ?

 

[boingoman]

Is it like sound propagation?

Yes. The whole audio world is based on the fact that AC and sound are similar in certain ways.

For instance, in a small sound system, a microphone converts sound waves from your mouth to an AC voltage. The voltage swings back and forth from + to - as the sound pressure changes from + to -. The frequency and amplitude of the sound determine the frequency and amplitude of the AC signal.

At the other end, the AC voltage from the amp causes the speaker cone to move back and forth as it changes direction, creating sound waves at the same frequency and amplitude of the signal voltage.

ps- Know what an analogy is? Making comparisons of things that are similar yet dissimilar. For example: Your skeleton is like the frame of a house. They both provide support. One is bone, the other is wood. Same, but different.

In the same way, AC and sound pressure are analogous. They both have frequency, amplitude, and can be positive or negative (have polarity). But one is sound, the other is electricity. Sound is positive and negative air pressure moving past something. AC is positive or negative voltage relative to a reference point. Similar but different.

Also key is realizing that AC doesn't switch polarity instantly. The voltage swings from positive through zero to negative and back again as it changes directions. Hence the pictures of AC sine waves you see. Sound pressure works the same way.

Early engineers used to say the AC signal voltage from a mic was an electrical analog of sound. Exactly the same, yet different. That's where the word analog in audio comes from. These days the term is mostly used to distinguish gear that only uses signal voltages from gear that converts those signals to digital information, but the word was in audio way before digital stuff came around.

 

[cpl_crud]

Another thing to realise is that the information carried by a cable isn't moving "up and down" the conductors, it's actually moving, as a wave, between the cold and hot conductors. As it passes, it creates a disturbance that moves the electrons in the condutcor, which then propagate the wave further down the conductor...

The whole theory behind EM wave propogation is amazing (well, to me it is at least).

In answer to the "how does anything get done" question, remebering that an AC source is (roughly) a sine wave, you can take power out of that. The socket on your wall may read 110V (or 240V here- whatever), but that is the Mean voltage- in reality the maximum voltage of a 240V source is around 380V...

But, with any sine wave, you can take away "bits" of its power. The simplest example is a light bulb. Logic says that, with an alternating current, the lightbulb should flash on at off at 100Hz (2 x 50Hz). However, you've got to remember that the light is created by the temprature of the element at any given time. As the current through the element goes to 0 A, the element still has some heat in it, which it starts to dissipate. However, before it can cool too much, the current is now back on, albeit going in the other direction. This current still heats up the element, and the light bulb still glows. Because of the speed of the "switching", the element doesn't get a chance to cool enough the visibly alter the light output.

You can do similar things with electronic components- a component wit ha reactance (like resistance but for AC) will take some of the energy out of the wave. In addition, the reactance of some components (like an inductor and capcitor) will alter the phase of the incident wave.
This leads to a number of really cool effects, like phase-locking, and frequency-based filters.
If you want to know more about that type of stuff, look up RLC circuts.

Finally, remember that even though an AC signal does go from both high +ve to high -ve potential, it's never really both at teh same time, so you can always take power out of it to do something instantaneously.

 

[boingoman]

If, in AC, the current is alternating, how does it get anything done? How does an audio signal get from point A to point B if the current is constantly reversing direction?

I wanted to come back and try to answer this for you without drifting off into never-never land.

The AC voltage that carries a signal doesn't go anywhere. It travels in a loop or circuit, and passes the signal along as it goes.

Let's say you could hook a mic directly to a speaker. Now there is a wire loop that goes from one pin of the mic to the other. They are directly connected by the voice coil of the speaker. Another word for loop is circuit.

As you speak into the mic, an AC voltage and current are generated in that loop. It doesn't go anywhere but through that loop. But the speaker cone will move as the current passes through it. Which way the speaker moves depends on which way the current flows.

So the signal information has gone from your mouth, through the wire, and out of the speaker. The AC signal voltage is just a carrier on a closed loop.

In the same way, signals pass from mic to pre. Pluggin a mic into a pre forms a loop from one pin of the mic to the other through the pre.

Instead of moving a speaker, the AC from a mic causes the preamp to output an amplifed version of the AC signal it reads at it's inputs. Plugging the pre output into a mixer forms another AC loop, from one pin of the pre out, through the mixer input, to the other pin on the pre. There is another circuit in the mixer waiting for the frequency and amplitude info from the mic pre AC output. It "reads" the info in the AC signal from the pre.

A signal chain is a series of closed loops, each reacting to what happens in the one before it. Separate, but interconnected, allowing the signal to be passed from one to the other.

 

[eraos]

Thanks for the responses.

This current still heats up the element, and the light bulb still glows. Because of the speed of the "switching", the element doesn't get a chance to cool enough the visibly alter the light output.

Yeah, that makes sense to me, but that's also why this concept doesn't make sense for me. I don't see how that means audio will go from one point to another.

The AC voltage that carries a signal doesn't go anywhere. It travels in a loop or circuit, and passes the signal along as it goes.

So, first let me get something straight. You're saying that audio signal is different from AC. You're saying the AC is only a carrier of the audio signal?

Having a solid definition of what I'm talking about my help me understand what's going on.

 

[cpl_crud]

So, first let me get something straight. You're saying that audio signal is different from AC. You're saying the AC is only a carrier of the audio signal?

The audio signal creates the AC current in the cables, through it's oscilation.

Once turned into an electrical signal, the "sound" becomes an Electromagnetic (EM) wave.
As this wave passes along a cable (which is acting as a waveguide) the oscillating magnetic feild causes the electrons in the conductors to move backwards and forwards- which is your AC current. The motion of these electrons causes the wave to move along, between the two conductors.

They're kind of symbotic like that.

It's a bit hard to get your head around without knowing the basics of EM propagation and boundary conditions, but it's a topic that I find particularly interesting. Then again I'm a physics nerd.

 

[boingoman]

You're saying the AC is only a carrier of the audio signal?

Not like a mailman carrying a letter from one place to another. I'd say copper wire is the carrier of the signal. Signals are information. The information is AC voltage that corresponds to audio. It's easy to confuse. The AC signal is kind of like "One if by land, two if by sea". The fact of a lamp signalled an invasion. The number of lamps told what kind.

The AC signal voltage says "message arriving" in a form your gear can understand. The characteristics of that voltage determine the content of the message.

For instance, if you play a 100hz tone into a mic, it will output a 100hz AC voltage. If you play a 500hz tone into it, it puts out a 500hz AC signal.

So it's the information that is important. Gear passes info using variable AC. Mics turn audio into variable AC. Speakers turn AC into audio.

And sometimes, copper wire isn't the carrier. This is part of what I was getting at with the loop concept.

You know about transformers? A transformer is two pieces of wire wrapped around each other. One is the input, one is the output. They don't touch, they are insulated from each other. Yet if you put an AC voltage through the input wire, voltage with the same frequency comes from the output.

In a mic transformer, the input wire is hooked to the + and - of the mic capsule. The output wire is hooked to the + and - of the XLR connector.

So the mic diaphragm and connector are not physically connected in any way, yet the information passes. What happens is this: When a voltage passes through a wire, an electromagnetic field forms around it. When you talk into a mic, the capsule generates a signal voltage. It passes through the transformer input wire, making a field. This field varies with the signal voltage, which varies with your voice. If you put another piece of wire next to the first, this field will induce an exact copy of the signal voltage in this second wire. This is connected to the XLR connector. This principle is called induction. A voltage on the primary (input) induces a like voltage on the secondary (output).

Then the mic is connected to a pre, and the whole process happens again. Different AC loop, same info passed along.


So the information, in the form of an AC signal voltage, has been passed on to another piece of gear, even though the original AC signal is bouncing back and forth along it's little loop in the mic.

In this case, the medium of information transfer was an electromagetic induction field in a transformer.

 

[eraos]

Again, thanks for the replies. Give me a bit of time to digest.

 

[Dani Pace]

Look around in almost anything electrical (radios, amps, stereos, whatever) and you will see an assortment of parts, some of these are diodes. Diodes are like "one way valves," they allow current to flow in one direction and keep it from flowing back. These little guys are what keeps the loop flowing in the same direction.

 

[cpl_crud]

Look around in almost anything electrical (radios, amps, stereos, whatever) and you will see an assortment of parts, some of these are diodes. Diodes are like "one way valves," they allow current to flow in one direction and keep it from flowing back. These little guys are what keeps the loop flowing in the same direction.

How have diodes got anything to do with audio signal, unless you are trying to build a class B amp?

Putting a diode in an audio chain will "delete" half of the information.
Sure, you can make bridges to convert AC into DC, but the information stored in a DC signal is 1 bit- much, much less than most "audiophiles" would consider decent quality...