Stereo Mic Input Versus Line Input

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Stereo Mic Input Versus Line Input

Permanent link to this post Posted by vencabot » Mon Jan 25, 2010 10:10 pm

Hey there, all. I was just experimenting with the inputs of my sound-card today, and I realised that I've owned this PC for a couple of years and never knew that the two microphone inputs could record in stereo. I plugged the headphone port into a mic port, messed with the levels (turned off the mic-boost, obviously), and it records identically to line-in.

What, then, is the difference between a stereo mic input and a line input? Why are they labeled differently? I did a little bit of Googling, and all that anyone says is that mic inputs are usually mono and that they might send a bit of power to the microphone. In that case, if I, say, plugged my CD player into my microphone port, could it potentially damage my CD player because of the power that the port is sending back? Was it unwise to plug my PC's headphone output into it?

It's a mystery! Can anyone fill me in?

-David Hernandez
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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by kozikowski » Mon Jan 25, 2010 10:28 pm

http://www.kozco.com/tech/audioconnecto ... ctors.html

Mic-In is always mono, but the software that manages it almost always puts that one sound on both left and right if you're building a stereo show. It can seem like your show is in stereo, but it's not. It's in two-track mono.

The battery supplied on the ring connection is only 5 volts and it's heavily buffered, so it shouldn't do any damage if you plug the wrong thing in. You're most likely to get into trouble with sound levels. The Mic-In is expecting a tiny, delicate, mono microphone signal and the Line-In is expecting a powerful, massive, stereo sound signal. Usually when you cross them, the show is heavily distorted and overloaded. The words crunchy, harsh, and crackly get used a lot. There is no way to rescue that show. Digital overload is immediately obvious and permanent.

If you did get clear sound, you have the celebrity sound card that doesn't throw up if you cross the cables. Most sound cards will not do that, and yes, you do have to get the right connector to do the job.

Some sound cards can switch between Mono Mic-In and Stereo Line-In in software. These sound cards do not have two connectors.

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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by steve » Mon Jan 25, 2010 10:33 pm

vencabot wrote:I did a little bit of Googling, and all that anyone says is that mic inputs are usually mono and that they might send a bit of power to the microphone.
That's correct. There is usually a 5v supply on one of the contacts. The current should be limited to no more than a couple of milliamp so it is unlikely to do damage but it's safest to avoid plugging other types of devices in there.

Other than the microphone input has an option of 20dB boost and the 5v supply there may be no difference. Exactly how manufacturers implement microphone inputs is their design decision, but it's usually based on (low) cost rather than any performance considerations.

On high quality music sound cards it's a different story and there will be considerable differences between Mic and Line inputs. On these you should definitely avoid putting a high level signal into the mic input. The microphone input will be a balanced 600 Ohm input with high gain circuitry, whereas the Line input will be a low gain input, usually unbalanced, with an input impedance of around 20k Ohms.
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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by vencabot » Mon Jan 25, 2010 10:40 pm

Thank you for your response, Koz.

Although both my front and back "mic" inputs are labeled as such (microphone), they do take a stereo input. As I explained, I plugged the headphone port of my sound card into both mic ports in sequence and recorded the output: what I didn't make clear enough, perhaps, is that I was playing a tune that begins with an overt left-to-right pan of a vocal sample, and the recording to the "mic" input reflected this pan appropriately across the left and right channels that it was recording.

At first, I thought for sure that forcing Audacity to record stereo from the mic port would only result in my mono microphone's output being mirrored across both channels (the way that it used to on my old laptop). When this turned out to not be the case (the left channel got no signal), I plugged in the mentioned stereo output (headphones-out), and, sure enough, the mic records in stereo.

Thank you for the warning regarding the levels, but I did anticipate that problem by lowering the levels accordingly (disabling the mic-boost entirely and adjusting the capture amplification). Like I said, the recording that Audacity captured was very identical to the same thing recorded via line-in.

Stevethefiddle, you've posted while I was typing this response, haha. Thank you for your response as well; it's very enlightening.

Both of you have said that the power sent back through the mic port is probably not enough to cause any damage, but that it should probably be avoided, and so I will avoid it -- after all, I do have a line-in port as well. It just seems weird that these microphone ports would record in stereo... I've heard of stereo microphones, but I assumed that those would be recorded over line-in for that reason. Hm.

Thanks again.

-David
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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by kozikowski » Mon Jan 25, 2010 11:13 pm

The two services have been hopelessly scrambled by the different manufacturers most of whom want desperately for the difference to just go away. Many posters begin their question with: "I plugged my mixer into my computer Mic Line-In."

I bet that if you plugged a "real" computer microphone into your mic-in connector, it wouldn't work. Further, I bet there is no 5v battery on the ring connector.

I didn't write about this, but the difference between commercial microphone level and line level hovers around a thousand to one. Scaling up to a silly analogy, that's the difference between the shiny glass High Tension power lines above the trees feeding your whole neighborhood and a flashlight battery. 1.5 volts versus 1500 volts. Not easy to get both of those on one wire without damaging something.

Microphone manufacturers have been pushing higher and higher sound levels and conveniently hiding the sound quality which always suffers when they do that. I would kill to have the 20dB Microphone Boost on all our sound cards. That's the only way some of our training and demonstration equipment works.

I only talked about the high volume and overload end of the problem. The other end of things is in trouble, too. If you plug a "real" microphone in, chances are the volume level of the microphone and the noise level of the sound card will be the same thing. "Microphone Test ffffffffffffffffffff Hello, testing ffffffffffffffff."

You can actually recover a little bit from that, so that's not as bad as overload which is permanently fatal, but you can't get a quality show from either one.

Not easy this microphone thing.

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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by steve » Mon Jan 25, 2010 11:21 pm

Stereo mics (usually) use mic level inputs just the same as ordinary mono microphones. They will often use the same kind of mini-jack as an ordinary mono computer microphone (with tip, ring and sleeve connections). Whereas a normal mono computer microphone uses the tip and the sleeve for the microphone and the ring for the 5v, the stereo mics use the ring for both one of the mics (there are 2 microphones in a stereo mic) and also for the 5v.

It is highly unusual that a computer microphone input works in stereo (these sort of microphones are usually used with portable recorders, pocket video cameras and suchlike). It may be interesting to try and plug a cheap stereo microphone with your sound card and see if it will actually record in stereo from it. A friend recently bought a cheap Chinese copy of a Sony stereo mic from ebay for the equivalent of about $5 (including postage). At that price it may be worth testing just for the fun of it.
The microphone he bought was certainly not up to the standard of a real Sony microphone, either in build quality or sound, but for $5 the sound is surprising not terrible (though the build quality is extremely flimsy). These microphones are very readily available - if you're interested, look up "ECM-DS70P"
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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by steve » Mon Jan 25, 2010 11:22 pm

kozikowski wrote:"Microphone Test ffffffffffffffffffff Hello, testing ffffffffffffffff."

Ha ha, that's just what my laptop microphone sounds like :D
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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by vencabot » Mon Jan 25, 2010 11:32 pm

Koz,

Thanks again for the information. I use an ElectroVoice stage microphone for my PC mic; it's a pretty nice, classical (dare I say, "real") microphone, and it works fine with my microphone ports. That being said, yes, the amplitude of the output is extremely low as should be expected from an unamplified mic, but the sound-card's microphone boost (which can be turned off or set to three different increments of amplification) brings the amplitude up to even clipping levels if the general capture volume is maxed out along with the boost. My only complaint about the inputs on this sound-card is that they've all got a bit of an offset that becomes increasingly obvious as the signal is amplified, but it's very manageable, and I can't expect an integrated sound-card to be perfect, I suppose. The background noise on the front mic is practically non-existent, although it is noticeable on the back mic -- nothing that I'd be particularly ashamed to record over, though. I do wonder if the ports are powered at all, as many mic ports apparently are... I mean, if they weren't, they'd essentially just be line-in but with some extra, optional boost, seems like.

Stevethefiddle,

Thank you for describing the anatomy of a mini-plug; I see that I've had it all wrong all of these years. I'm no expert with audio gear (obviously), but I've always noticed that mono plugs have one ring and stereo plugs have two. As such, I sort of always imagined that the actual tip and sleeve of the plug did nothing, really, but rather the output signal was read through the rings -- thus, one ring for mono and two for stereo. So, despite outputting stereo, stereo mics still only have one ring? Also, if a mono signal is actually sent through the tip and sleeve of the plug and the ring is for power, what is the second ring for on stereo (line) plugs?

I can't believe that I've had it so wrong... I mean, before today, I never would have even imagined that power would be sent FROM an input port and BACK through an audio cable to the attached device...

-David
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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by steve » Mon Jan 25, 2010 11:59 pm

vencabot wrote:thus, one ring for mono and two for stereo.

Just to be confusing, some stereo mics have 2 rings, in which case it's tip + one ring for signal, 2nd ring for 5v, sleeve for common (earth).
Some stereo mics have just one ring as seen here (and the arrangement is as described previously):
Image

Computer microphones invariably have one ring. Tip=signal, ring=5v, sleeve=common.

vencabot wrote:never would have even imagined that power would be sent FROM an input port and BACK through an audio cable to the attached device...

That's why they call it "phantom" power (it's invisible). Computer microphones are usually a cheap form of "Condenser" microphone (technically called an "electret" microphone). They include a tiny amplifier which needs to be powered, hence the 5v. True condenser microphones require more voltage, typically 48v which is usually supplied in a similar manner - by sending the (phantom) voltage up the microphone cable.
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Re: Stereo Mic Input Versus Line Input

Permanent link to this post Posted by vencabot » Tue Jan 26, 2010 12:09 am

Steve,

I've always been aware that USB cords can both transmit data and send power back to the device, but I didn't know that microphone ports have batteries and that audio cords could work in a similar way. Wild.

So, again, what is the second ring on a stereo plug for? Maybe I should just look it up on Wikipedia... or maybe one of the rings is still for power, just in case. I mean, I guess there's no reason why I couldn't use one of my "line cables" to hook up a mic. So, maybe... it's the tip for one signal, the first ring for power, and the second ring for the second signal. But, then again, you said that, on some stereo mics, the second ring is for power, and so standard ports would probably have to support that, too. So, maybe... it can take a signal and send power on both rings? Man...

Yeah, I'm gonna look it up, haha.

-David

EDIT: Learning a lot about the subject on Wikipedia. I don't mean to sound like a total novice, and please don't waste any more of your time answering my silly questions when I'm sure that there are plenty of excellent resources for this sort of thing online. Thank you for all of your help.

EDIT: By the way, if I sound like I have no idea what I'm talking about in some of my sentences in previous posts, it's because I've actually been referring to the black, insulated rings as the "rings." I honestly thought that this is where the conduction happened, despite the fact that the rest of the plug is the shiny, metallic... yeah. So, I've never used a "computer microphone," but I guess that I would have been confused by it because its tip would have looked like what I typically understand to be a "stereo" plug. This is because, unlike my stage mic, the plug has two black, insulated rings, which split the plug into three conductors: the tip for the output signal, the middle ring for power, and the rest of the plug (sleeve) for ground. As such, computer mic ports would have to be prepared for three-conductor plugs, just like line-in ports, although they'd handle these plugs differently. Apparently, MY mic ports must be able to use the ring for power and/or as another input signal, meaning that they can record in stereo, so, aside from sending power (can line-in ports typically do this?) and having a dedicated mic amplifier, they're no different from line-in: after all, if I plugged my mic into a line-in and amplified it to a huge extent, I could record from it.

I'm still a bit weirded out by the prospect that my mic ports MUST be able to send phantom power in order to be compatible with typical computer microphones, but they can also use that middle ring to record from, apparently.
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