# Re: ALSA sound recording frustration

```lee wrote:
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```On Thu, Nov 27, 2008 at 09:11:31PM -0500, Napoleon wrote:
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```lee wrote:
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```On Thu, Nov 27, 2008 at 07:06:42PM +0200, Rob de Graaf wrote:
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```An input which measures voltages (here the microphone input) has
infinite large impedance, hence no current will flow.
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```How can there be voltage (or voltage measured) without current
flowing?

http://en.wikipedia.org/wiki/Volt:

"The volt is defined as the potential difference across a conductor
when a current of one ampere dissipates one watt of power."
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The wikipedia article is correct as to the definition of a volt in mathematical terms. However, voltage is the electrical equivalent of water pressure. You can have water pressure, even if there is no water flowing (i.e. a full water tower with no water flowing out).
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But how do know that there is voltage when you cannot measure it or
otherwise make evident that there is? As far as I understand it, you
cannot do that without current flowing. You can do it for water
pressure without water flowing, but I don't see how you could do it
for voltage without flow.

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Actually, you can't do it for water without some water flowing, either. In order to measure the water pressure, you have to transfer some of the energy from the water to the measuring device - hence a flow, albeit a small one.
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The same is true for voltage. You can't *measure* it without some current flowing. But just because you aren't measuring it does not mean the voltage does not exist. Just as the water pressure still exists, even when you are not measuring it.
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An ideal voltmeter will have infinite impedance, so no current would flow. But while nothing is ideal, the best voltmeters have impedances of 100M ohms per volt and higher. So while there is a current flowing, the amount is so negligible as to be effectively not there. Just like you can effectively ignore the water flow when measuring the pressure.
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You still have a voltage across a battery, even when no current is flowing. And you still have voltage at your outlet, even though no current is flowing.
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How do you know?

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The question is immaterial. Whether you know or not is not related to the presence (or absence) of voltage.
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Without getting into the more technical stuff, for correct operation impedances need to match. Any mismatch lower performance. A small mismatch probably won't be noticed but a larger one will.
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Yes, what you're explaining is pretty much the point: It's not a good
idea to connect incompatible things like a headphone outlet to a
microphone inlet without appropriate adaptation. It might work or not,
it might damage something or not ...

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