Waveform offset when mic recording

On one machine, when I record from a PC mic, the waveform has a negative (downward) offset, but on another machine, it doesn't. This is merely annoying since I can fix the offset with the CE2k amplification function. But... is there any way to fix this on the way in? Would this be a purely hardware issue requiring a different sound card to fix it? I'm guessing that the recording is getting at least some of the condenser mic bias (phantom power) in on the recording.

On one machine, when I record from a PC mic, the waveform has a negative (downward) offset, but on another machine, it doesn't. This is merely annoying since I can fix the offset with the CE2k amplification function. But... is there any way to fix this on the way in? Would this be a purely hardware issue requiring a different sound card to fix it? I'm guessing that the recording is getting at least some of the condenser mic bias (phantom power) in on the recording.

You're right - you are guessing...

Speech generally produces an assymetrical waveform, and it's normally positive-going. It's produced simply because your breath is travelling on a bed of air, which is moving forward, and that's all that is causing this - if causes an offset of the microphone's diaphragm, which is reflected in the output of the mic. If you have a recording that is negative-going, this is because you have the polarity of the mic reversed - a forward motion is universally regarded as positive.

Most of the time it doesn't matter in the slightest, but if you are 'correcting' it using the DC offset option, the chances are that you are actually adding a baseline offset rather than removing one!

Do you get an offset when you record very low-level sounds (set mic gain and record only backgound noise)?  If a low-level waveform records more or less centered, but higher-level sounds are assymectrical, I would not try to "correct" it.  It's likely the nature of the material.  If your low-level recording is offset by the same amount as high-level audio, then you have a DC offset issue somewhere in your signal chain.

No, the silent baseline is also offset.

How can I put the wrong polarity on a PC desk mic plugged into a PC? If it was plugged in reversed polarity, the amplifier chip in the condenser mic would be reverse biased and possibly burn out, wouldn't it? On another computer this mic never had that offset problem.

No, the silent baseline is also offset.

How can I put the wrong polarity on a PC desk mic plugged into a PC? If it was plugged in reversed polarity, the amplifier chip in the condenser mic would be reverse biased and possibly burn out, wouldn't it? On another computer this mic never had that offset problem.

If the baseline is offset, then this is a genuine DC offset that can be legitimately corrected, and this is due to an awful mic preamp, and not the mic. Mic preamps in PCs are invariably awful - they aren't designed in any way for serious recording, and are generally meant only to be used for things like internet telephony, and possibly voice recognition (although they aren't too good for that either...)

The signal polarity of a mic has nothing to do with the impedance converter power at all - it's not a 'polarising' voltage in one of these mics - they use pre-polarised electret capsules. The supply to the mic is to run the FET impedance converter that sits immediately behind the diaphragm itself. In a proper balanced feed, you can feed phantom power to a mic and have the signal polarity either way around, but only one way is deemed correct, as I described above. It's perfectly possible for the mic preamp signal polarity to be designed incorrectly, but it doesn't really matter, because signal polarity can always be reversed in CE/CEP/AA.

Oh, and if you reverse the supply feed to the impedance converter, it simply won't work, rather than burn out - the FET won't be driven into a conductive state at all.

I have noticed DC offset problems also.  It was visible on recorded "silence" which was actually quite noisy on a SoundBlaster card.  When I upgraded to a professional 24 bit 96 kHz card, the problem went away. 

I have also noticed that some synthesizers create asymetrical waveforms.  To me, this is a design flaw since the bias is a waste of headroom and doesn't reproduce as well. 

Some VST reverb plug ins also create bias.  I think this is also a design flaw, with the exception of when the reverb time is so minute that it is echoing the instantaneous polarity of the original waveform. 

I think that microphone placement, and mic quality affects this phenomenon. 

Some of this bias can be fixed a little bit with a steep high pass filter at around 40 Hz. 

I think that microphone placement, and mic quality affects this phenomenon. 

If you put a microphone in a position where there is destructive interference as a significant path between the source and mic position, then yes it will alter the effect - but it will also alter the signal you record. The quality of the mic should make no difference - unless it is so poor as to not capture the source properly.

As for the amount of headroom lost, I really don't think that it's that much of an issue, myself. Chances are you will lose most of the effects of this in a mix anyway. The normal way to get rid of the effect with minimal difference to the signal is to use an all-pass filter with a phase-shift - this alters the phase relationships of the signal harmonics, which is what gives rise to the effect in the first place, from a technical POV.

The normal way to get rid of the effect with minimal difference to the signal is to use an all-pass filter with a phase-shift - this alters the phase relationships of the signal harmonics, which is what gives rise to the effect in the first place, from a technical POV.

I suppose in principle this could also produce the effect...

If you work with sensible headroom it's not an issue anyway.

Paul

I suppose in principle this could also produce the effect...

Don't see why not - there's always a certain amount of juggling to do if you remove offset this way, regardless. I've tried it once or twice, but I wouldn't normally bother to correct it for speech, because other dynamics processing would destroy it anyway, and I simply wouldn't do it to a singer or a wind instrument.

I think that microphone placement, and mic quality affects this phenomenon. 

If you put a microphone in a position where there is destructive interference as a significant path between the source and mic position, then yes it will alter the effect - but it will also alter the signal you record. The quality of the mic should make no difference - unless it is so poor as to not capture the source properly.

As for the amount of headroom lost, I really don't think that it's that much of an issue, myself. Chances are you will lose most of the effects of this in a mix anyway. The normal way to get rid of the effect with minimal difference to the signal is to use an all-pass filter with a phase-shift - this alters the phase relationships of the signal harmonics, which is what gives rise to the effect in the first place, from a technical POV.

THANKS for the filter tip.  I do think that mic quality matters in terms of polarity bias.  If the mic diaphragm doesn't vibrate equally in both directions you could get bias.  Also, if the mic is placed somewhere that it gets a huge puff of air instead of just vibrations you could get bias. 

When I look at my asymmetrical synth waveforms recorded at full scale, I can manually readjust them to me symetrical and I find that doing it give me back 3-6 dB of headroom back.  This headroom could have been used during recording for a hotter signal if the waveform wasn't defective.  The DC bias also eats up headroom when mixed with other instruments.  The situation gets a lot worse when you have multiple layers of instruments all with the same polarity of DC bias.  Suddenly the whole mix is happening above or below the zero point and lots of decibels are lost.  I think its a pain in the ass and isn't hidden in the mix.  Loudspeakers are designed for alternating current and not DC, so I think there's also the possibility of it resulting in slight distortion on speakers too. 

If the mic diaphragm doesn't vibrate equally in both directions you could get bias.

No, you would get distortion.

This headroom could have been used during recording for a hotter signal if the waveform wasn't defective.

The waveform isn't "defective" - that's how the waveform is.  You don't need a hotter signal at this stage - leave that to final mastering if you want to contribute to the dreadful loudness race.

Paul

If the mic diaphragm doesn't vibrate equally in both directions you could get bias.

No, you would get distortion.

This headroom could have been used during recording for a hotter signal if the waveform wasn't defective.

The waveform isn't "defective" - that's how the waveform is.  You don't need a hotter signal at this stage - leave that to final mastering if you want to contribute to the dreadful loudness race.

Paul

Perhaps you aren't understanding what I'm talking about at all.  You might call it distortion, I call it an asymetrical waveform.  But for the type of waveforms I'm talking about, if you make them symetrical, they sound the same only without the bias and therefore more headroom.  More headroom means I can record a hotter signal and therefore the sound is more audible in the mix.  It has nothing to do with mastering.  To me the waveform is defective.  As I said before, cummulative layering of asymetrical waves of the same polarity wastes/eats up dynamic range fast.  I'm talking about a synthesized waveform.  It's just as easy for synth designers to make symetrical waveforms, so to me the asymetrical waveforms are a design flaw. 

As far as mics go, if the diaphragm is pinned too hard in one direction and not allowed to freely vibrate, you can get bias.  This can be from bad mic design, or from a strong puff/blast of air/wind or from both.  Just think about it and visualise it in your head. 

As far as mics go, if the diaphragm is pinned too hard in one direction and not allowed to freely vibrate, you can get bias.  This can be from bad mic design, or from a strong puff/blast of air/wind or from both.  Just think about it and visualise it in your head. 

You simply don't understand how microphones work, do you? Or the nature of vibration, come to that...

Perhaps you aren't understanding what I'm talking about at all.  You might call it distortion, I call it an asymetrical waveform.

I think Paul understood perfectly, and if he calls this effect "distortion", that's because it is.

There's another discussion of asymmetry in this thread:

http://audiomastersforum.net/amforum/index.php?topic=5798.0

where I provided a method of "fixing" the "problem". In fact it's the "all-pass filter" method mentioned by Steve. Note that although I showed how it could be "fixed" and why you might want to, I never do this myself, for similar reasons to those Steve & Paul alluded to.

I know how mics work and I know about vibrations.  Thanks for the tip on how to correct the issue with the phase thing. 
I also discovered that certain settings of the dynamics processor even out the waveform for some reason.  If you guys don't like to correct it, that's ok with me, but personally, I'll take the 3-6 dB of gain/headroom back.