Topic: What the? Strange Waveform?

[BayTaynt3d]

Alright, I've had time for some tests at home. I, of course, could not achieve the same SPLs as the concert the other night, but this problem was entirely NOT reproduceable in a controlled environment. My test tone was centered perfectly around 0 (no DC offset at all) and the waveforms were perfectly symmetrical on the plus and minus side (no phase shift), and the waves were perfectly smooth (no harmonic distortion). Also, at this point, I've mastered the first opening set and both sets of the main act, and the only time this problem ever occured was when the TRUMPET (I said trombone before, but it was the trumpet) was "BLASTING." In fact, it was when the trumpet was soloing and blasting directly into the mic when this happened.

So, I'm pretty sure my gear is fine, which is kind of what I thought. And this exercise was useful for me because I learned a few things about phase shift, harmonic distortion, and DC offset. But, I'm still curious as hell at what exactly caused it? Was it the horn overloading/clipping the FOH's mic? Was the phase inverted on that mic/track at the FOH SBD somehow? Was the FOH's mic shot or defective? Or could this have been caused by a horn that is being "blasted"? Very interesting questions...

Thanks for all of your help, and I'll continue to watch this to see if it happens again, and whether it only happens with horns or only in that venue or whatever...

-- Taint

[BayTaynt3d]

Been on a little surfing rampage... And I'm beginning to think this has something to do with the natural harmonics and waveform shape that is associated with a brass instrument being BLASTED and MUTED at the same time (you know, a mute is that plunger they hold in front of the bell of the instrument). I'm not a 100% sure it was muted, but I think it might have been. And in a way, doesn't that make sense. Think about the polarity issues of making a matrix when you point your mics out at the audience and mix them in with the SBD with mics pointing back at the band. In that case, you're usually better off flipping the polarity. Well, when you mute a trumpet, aren't you forcing some of the sound back, around, and out of the bell (colliding with more sound coming out)? Alright, I'm probably making shit up at this point, but here's some more stuff I found...

Look at the example of a trumpet waveform discussed here:
http://www.st-and.demon.co.uk/AudioMisc/asymmetry/asym.html

Here's another quote:
"(a) audio signals are not always symmetric in electrical waveform (why should it always be? an oscillocope trace of a trumpet sound looks very asymmetrical & triangle in shape!)"

More:
http://www.klyne.com/pages/reviews4.html

As far as harmonic distortion is concerned, I think that might just be from the complexity of a brass instument maybe? See:
http://library.thinkquest.org/19537/Apps5.html

This quote was interesting from another site because it mentions "raspy," which is exactly what a trumpet sounds like when it is blasted, which is when I notice this the most:
"It can happen a lot with certain percussive instruments, it's very common on brass... Some vocalists - Especially the more "raspy" types. It can also be phase issues, could be compression settings... Not everything makes a perfectly balanced sine wave."

And although unrelated to my issue here, and although this seems obvious to me now, it's nonetheless a pretty interesting way to "hear" the seperation of your stereo image:

"To analyze music through A/B listening, connect the outputs of your source, such as a CD player, to two channels of a mixer. Pan both channels to the center, and then reverse the polarity on one of the channels. Any sounds that are equal in both channels of the mix are canceled out, because their polarities in one channel are inverted with respect to the same sounds in the other channel. The mono signal that remains contains the sounds that were panned to the left or right channel, but not both. (Sounds that are more pronounced in one channel than in the other in the stereo mix are reduced in volume in this inverted mono mix, but they are not eliminated completely.) Comparing this signal with the stereo mix, as well as with the complete (noninverted) mono signal, helps you learn more about the reverbs, backing parts, stereo spreads, and other details that are often masked by the more prominent parts of the mix."