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Just something I've always wondered—most sound cards have both a line in and a microphone, and I've never been able to tell the difference between anything recorded with them. Lately I've seen cards without the line in, so is there actually a difference? If so, specifically what should they be used for?

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For migration to SD please Tim –  Rory Alsop Jan 27 '14 at 16:14

3 Answers 3

up vote 16 down vote accepted

Line in will usually be stereo and about 10Kohm impedance.

Mic in is mono and about 600-1Kohm impedance and expecting SIGNIFICANTLY lower levels than the line, as it has a preamp to pick up the very low levels present in a microphone.

Sometimes microphone in have XLR connector to allow Phantom Power

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Mic inputs are more like 2K, 2.5K, 3K, while the mic output is 150-300 ohm. The rule of thumb is that the load should be at least 10x the source. –  endolith Dec 20 '10 at 20:47

A microphone input is typically a very low level signal, and is mono. A line in will be expecting a much higher input level, and will usually be stereo.

The sound card should have an additional pre-amp stage for the mic to bring it up to line levels.

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If you have a mic plugged in, and some other input in Line In, your computer's operating system will (or should) use the mic when you're using Skype or something. That way you don't have to do any configuration to have the mic work properly.

On good cards, the mic line will have lower impedance and should be mono. If you try to use it for something else the sound will be distorted.

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Standard PC sound cards typically only have one input. Sounds from the mic/line/CD/etc. inputs are mixed together. The OS doesn't have anything to do with selecting which inputs are chosen. Skype or other software handles this. It isn't recording from a particular jack either... it just adjusts the mixer correctly, so that the mic isn't muted or something. –  Brad Dec 26 '10 at 19:08
I've never owned a sound card, onboard or otherwise, that mixed the inputs. And there's plenty of software that uses the device selected by the OS for "Recording" rather than managing things itself. –  Matthew Read Dec 26 '10 at 20:25

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