Topic: What happened to my waveform?

[tailschao]

On both channels, the part of the waveform below the line is larger than above the line (Technical Terminology I think not ) What is this called? What is likely to have caused it?

It doesn't seem to have affected the sound at all (In fact, I'd say it sounds absolutely fantastic), but then I don't really know what I'm listening for. I've never had anything like this before, which isn't saying much as it is only my third taping attempt ever, but still...

It's not that important, but just to satisfy my curiosity?
Thanks

[Charlie Miller]

Looks like a DC offset problem. You can fix it with just about any audio mastering software.

[china_rider]

I have never had this happen but have always wondered what causes a DC offset problem?

[Dede2002]

On both channels, the part of the waveform below the line is larger than above the line (Technical Terminology I think not ) What is this called? What is likely to have caused it?

It doesn't seem to have affected the sound at all (In fact, I'd say it sounds absolutely fantastic), but then I don't really know what I'm listening for. I've never had anything like this before, which isn't saying much as it is only my third taping attempt ever, but still...

It's not that important, but just to satisfy my curiosity?
Thanks

Same thing happened to me twice. Same band, same venue, same recorder, same BB, different mics. No problems with sound quality. And nothing happened after DC Offset filter was used. Looks the same

[dmonkey]

Ditto. I've seen this frequently on my R-44 recordings. I've tried DC offset correction and the waveform looks the same after the filter runs. Everything sounds fine, just looks a little unbalanced on the screen. I'm puzzled, but not too worried because everything sounds fine. Oh well.

[JasonSobel]

were there horns in the band.

[SmokinJoe]

It's not really a DC offset issue, because in the quiet spots the sound is nicely balanced around the centerline.

My guess is that you are on the very edge of brickwalling, but it's not quite bad enough that it sounds like crap.  Perhaps it's just a case of a battery starting to run low, and in the quiet spots you don't need as much bias voltage.  At the beginning of the track the wave form is balanced.... soon after the voltage starts to drop.  Notice right by the "1" in STE-001, there is clipping both positive and negative.  In the middle of the track, just negative clipping.  If it were brickwalling because the mics couldn't take the SPL, then you wouldn't have that one spot near the 1 which went to full amplitude, or maybe you dropped the gain after that.... 

[tailschao]

The batteries could well have been running low, I was using a ST-9100 as preamp (Along with STC-11's and a Zoom H2), which as far as I know doesn't have any way of telling specifically how much battery is left - but you already know that, of course (after reading your sig ).

I really need to buy a couple of 9V rechageables and make sure they are always fully charged before a show. Or at least buy band new non-rechargables regularly.

Thanks again

[su6oxone]

The batteries could well have been running low, I was using a ST-9100 as preamp (Along with STC-11's and a Zoom H2), which as far as I know doesn't have any way of telling specifically how much battery is left - but you already know that, of course (after reading your sig ).

I really need to buy a couple of 9V rechageables and make sure they are always fully charged before a show. Or at least buy band new non-rechargables regularly.

Thanks again

I got this same odd waveform back when I taped the North Mississippi Allstars using AT853 with the 9100, but never got it when using the same equipment at other shows.  But, like others have said, my recording sounded great so who knows... 

[Dede2002]

Batteries running low? I don't think so.
It happened twice and I had a fresh set of batteries on both concerts.
It's really odd.
Thanks for the responses 

[DSatz]

Hi--it might be a combination of two things:

(a) Live sound isn't inherently symmetrical in the way some people seem to expect. Think of what happens when a drum is struck: There's a sudden positive increase in pressure, followed by restoration and some ringing--but nothing else reaches the level of the initial attack. Thus the net "area under the curve" will be greater on the positive side than on the negative side.

(b) Some microphones, some cables, some preamps and some recorders invert the polarity of whatever they record. There's a widespread notion that "absolute phase" isn't audible; some equipment manufacturers and recording engineers aren't careful about it. And polarity inversion can easily occur when you connect balanced signals to the unbalanced inputs of some piece of equipment.

If you're recording sounds of type (a) on equipment of type (b), you can very well get waveforms like the ones you're seeing, even when there isn't much DC offset in the long term.

Inversion of signal polarity is definitely audible to some people with some sounds on some playback equipment. It's clearly not as important as some other characteristics of recordings (e.g. "did you remember to turn the recorder on?" is more important in my book), but it's not a total non-issue, either.

--best regards

P.S.: Looking again at that waveform, I'd guess that it's in inverse polarity overall--but I'm also struck by the near-constant limit of the excursions in one direction. So maybe something else really is going on here other than (or in addition to) inherent asymmetry of the "live" sound pressure levels.

tailschao, do you have access to a tone generator? You might connect your preamp and recorder just like they were, and with their gain settings just as they were. Feed some microphone-level test signals into the preamp at various levels, record the signals, and look at the resulting waveforms. If they're not fully symmetrical, something is distorting. It would be good to rule that out if you can.

As an example, the attached jpeg image shows the result of asymmetric "soft clipping" of a sinusoidal signal. This corresponds to a significant amount of distortion that includes odd-order harmonics, which are generally more audibly disturbing than even-order harmonics of the same amplitude.

[Dede2002]

DSatz,

Well, this is a thread started by Tailschao, but I'd like to say thank you.I had the same thing happened to me twice, never after, never before: only with the same band at the same venue. My waveforms also presents the same "near constant limit of the excursions in one direction", to use your words ( very well put, as usual).
It's really odd.
Thanks a lot one more time.

[tailschao]

As Dede said, thanks a lot DSatz

I don't have access to a tone generator unfortunately, but what about if I played this file through my computer, plug the computer headphone out into the preamp and recorder, and compare the resulting waveform? Would that work? If yes, how important is it that I have the exact same gain settings? I don't know exactly what I was at, it was dark, I was handheld video recording as well so I adjusted the preamp by touch. I know it was not much, less than 11 O'clock, but that's as specific as I know. I don't have the recorder with me at the moment so I can't try it, but I'll get it back on Tuesday.

Would it be worth posting a sample of the recording to see if you/someone can hear it distorting? Oh, by the way Jason, no, there's no horns in the band. It was a tribute band to Queen, so just drums, one electric guitar, one bass guitar, one keyboard, and vocals.