Sudden massive "DC offset" on mic input.

[Zbigniev Hamson]

First of all, I'm not sure if this is a hardware issue or a software issue, but my guess at the moment is that it's hardware. Anyway...

I have a pretty basic, barebones setup. Just a cheap laptop that's about 3 years old with a single 3.5mm input that doubles as a mic and line in input (I'd never given a moment's thought about how it actually does this until today btw). Into this I plug my guitar effects pedal (on line in) for electric guitars, or my microphone (on mic) for vocals or acoustic guitars. It's simple, but it's worked for me so far.

But as of last night I'm finding that if I try and record with the microphone the signal snaps right up to full. There's no actual sound, it just looks like a huge DC offset. I took the following snapshot from Audacity to show what I mean. You can see the effect when I move the volume input slider from totally off to minimal signal - a great whopping instant signal...



I don't think this is a problem with the microphone, as the laptop has an internal mic it uses if nothing is plugged into the socket and I am getting exactly the same problem when using this. And it's not an Audacity problem either as I can't get any sound using Skype or anything else. So it seems as though something has gone wrong with the hardware.

Line in still works fine and in fact I can record from the mic if I select Line in, it's just obviously very quiet. I admit I don't quite know how the laptop uses the single 3.5mm input for both mic and line in, or what swapping this input in the Audacity software actually does to the signal. All I can think is that my sound card has become unearthed or buggered in some other way. But then if I CAN record vocals with it set to line in (at a low level) then that suggests it IS still working and that it might be a software/driver issue. I really don't know.

I should point out that after about half an hour of fiddling last nigh it did spontaneously start working fine again. Then after 10 seconds it went back to this situation. Intermittent connection?

[richiebee]

Check in the Control Panel and make sure that the "recording" source hasn't changed. Maybe its not set to record from the microphone input at all, and what you're picking up is a signal from something else.

[Zbigniev Hamson]

Recording source is still "Realtek AC97 Audio", that's the only option I get from the drop down menu.

I do remember now that I did delete a bunch of audio codecs recently to try and sort out another problem, but I don't see how an audio codec would be involved anyway when trying to record straight to an uncompressed wav. I'm having no other audio issues anyway.

[richiebee]

Don't you get configuring options in the control panel? Drop down tells you the device (Realtek AC97), then underneath, you get a config button or something similar. That allows you to choose the input port and often the input level. I can't tell you exactly, because I don't have Realtek drivers installed at all.

Audio doesn't just become a wav. It still needs a converter even though its an uncompressed format. I don't know how this works, but if it were me, I'd try and get back your codecs first.

[Zbigniev Hamson]

I've looked in Control Panel > Sounds and Audio Devices. I can get a couple of options for playback but not recording.

I've accessed the recording options on Volume Control, ticked microphone. I get a couple of advanced options on there, namely Mic Boost (just on or off, no slider) and Mic2 Select (on or off, don't even know what this is) - neither of those do anything.

I've installed the latest driver, it's made no difference.

[brandondrury]

What about Line in? Does it work or do you still get the same error?

Brandon

[Zbigniev Hamson]

Line in works fine still (or seemingly fine). I can record the mic with the input set to "line in", but the level is of course very very low. I can amplify it afterwards, but I'm sure I'll be losing quality doing it this way.

I got all the default codecs back and it's made no difference, so I don't think that was the problem.

[brandondrury]

I wonder if the jack itself could be causing this. 1/8" jacks are notorious for being unreliable.

Brandon

[Zbigniev Hamson]

Well, as I said, I'm pretty sure it's not the jack because it's the same jack for line in and mic, and line in is working fine. Plus I get exactly the same problem with the internal microphone too, and that's not connected to the jack at all.

[magno]

Have you tried uninstalling the sound card and reinstalling?