Topic: What the? Strange Waveform?

[BayTaynt3d]

This could be the front of house sound engineer bumping up a fader on his main system EQ. I do this from time to time to see if I have a bump in the front of house or if its comming off the stage monitor mix and when I hear something on the verge of feedback I will do it to see if its me or the monitor engineer. But still its strange its only on the + side of the wave form. Hummm maybe it could also be a sampling error?. Here is another theory What if the +- voltages on your computers PCI buss were out if you sound card is internal or even if you have a external with a voltage rail problem the opamps would have a swing towards the + or - depending on the design of the circuit.
Hey maybe its just a screwed up waveform? how does it sound?


 

Chris Church

Alright, maybe I'm just a newb, but I've got about 20 taping sessions under my belt at this point, and I have yet to come across this. It was my first night out with my new CK91 caps, but I'm guessing this didn't have anything to do with the caps? Most of the night looks/sounds fine, but the attached pic shows a stretch where the waveform is shifted up and clipping only on the top. WTF? Is that what phase shifting looks like? I was running XY indoors and had the caps were very much on top of each other, so I didn't think phase would be a problem. Or is this something else? At one point, I thought the caps might have been touching each other (no windscreens), but I wasn't sure. There is no obvious vibration sounds or anything like that, so I'm not sure they were touching at all. Could someone please explain what the hell happened because I'm confused? Operator error? New caps? Weird acoustics in the venue? Corrupt file?

Yup. The lopsided thing is the weird part. Can what you describe make it asymetrical?

[CHURCH-AUDIO]

Well in theory Yes because when you start playing around with a Non digital EQ you introduce phase shift it could also be a sound engineer reversing the polarity of the midrange or high end drives on a system. Most sound engineers have total control over a sound system and can play with shit like that. But that is a long shot I would try using the same gear with a home stereo and see what the wave forms look like see if you can get it to do the same wave form in a controlled space.

This could be the front of house sound engineer bumping up a fader on his main system EQ. I do this from time to time to see if I have a bump in the front of house or if its comming off the stage monitor mix and when I hear something on the verge of feedback I will do it to see if its me or the monitor engineer. But still its strange its only on the + side of the wave form. Hummm maybe it could also be a sampling error?. Here is another theory What if the +- voltages on your computers PCI buss were out if you sound card is internal or even if you have a external with a voltage rail problem the opamps would have a swing towards the + or - depending on the design of the circuit.
Hey maybe its just a screwed up waveform? how does it sound?


 

Chris Church

Alright, maybe I'm just a newb, but I've got about 20 taping sessions under my belt at this point, and I have yet to come across this. It was my first night out with my new CK91 caps, but I'm guessing this didn't have anything to do with the caps? Most of the night looks/sounds fine, but the attached pic shows a stretch where the waveform is shifted up and clipping only on the top. WTF? Is that what phase shifting looks like? I was running XY indoors and had the caps were very much on top of each other, so I didn't think phase would be a problem. Or is this something else? At one point, I thought the caps might have been touching each other (no windscreens), but I wasn't sure. There is no obvious vibration sounds or anything like that, so I'm not sure they were touching at all. Could someone please explain what the hell happened because I'm confused? Operator error? New caps? Weird acoustics in the venue? Corrupt file?

Yup. The lopsided thing is the weird part. Can what you describe make it asymetrical?

[BayTaynt3d]

.

Alright, ALMOST ALL of the main act was fine. But on closer inspection, I found another short section that suffered the same fate. About 30 seconds worth. However, this sample might provide some insight possibly? The asymetry occurs exactly on time with the trombone (maybe it's the bass trombone) going for a really raspy lick. Maybe that mic was out of phase or something? Or maybe he was running through some sort of effects box? Could that cause this? You can download this 30 second MP3 and look at the waveform, it's not quite as pronounced as the waveform from the first act, but you can clearly see it, and you can clearly tell it occurs in sync with the trombone. This was the only 30 seconds out of more than two hours of the main act that I noticed anything. Maybe this had something to do with the venue? Or the PA? Or that trombone's phase or something? Is that possible?

-- DEAD LINK, SORRY --

.

[BayTaynt3d]

I should add that this was like a 15+ piece brass band, and this sample represents pretty much the only time that one trombonist soloed all by himself. So, it seems that out of two hours plus of music, it pops up only during his solo, which is kind of interesting. Maybe I'm reading too much into that, but maybe not?

[CHURCH-AUDIO]

I have heard the track and I think it has nothing to do with any sound engineer. It is from distortion I can hear it it's very hard to tell but I can hear it. I think it could be some strage power supply problem in a preamp or just plain old distortion it is electronic not accoustic. This type of distortion is called asymetrical hard clipping hard on the top of the wave form soft compression on the bottom. This from a preamp that ran out of gas what is your signal chain? exactly.

I should add that this was like a 15+ piece brass band, and this sample represents pretty much the only time that one trombonist soloed all by himself. So, it seems that out of two hours plus of music, it pops up only during his solo, which is kind of interesting. Maybe I'm reading too much into that, but maybe not?

[SparkE!]

what is your signal chain? exactly.

Here's the signal chain from a previous post:

Chain was: CK391 > UA5 (BM2+) > JB3

It was the first night out with the 391's (I usually ran C4s). And I also think I used a spare DVD battery that I got with the FP24 I bought a few weeks back. So, I had some new things in the mix that possibly could had contributed maybe?

The good news is that the first and second sets of the main act look fine (see attachments). Not sure what happened on the opener though. Strange. I wonder if the caps ended up touched each other in my XY on the opener if that could have done it? I was running XY on stage, and although the opener wasn't miced through the PA, there was a crapload of mics and cables on stage for the main act that had 20+ artists in it, every one of whom was mic'ed. Not sure if I could have had problems with that. The main act, I pulled back to the sweet spot for the PA and went DINA, so I had a pretty different setup for the second part.

I'm still questioning whether there's an electrolytic cap in the signal chain that is getting back biased by the signal so that its impedance is a function of the instantaneous signal voltage.  If that's the case, I'm suspecting that the problem is actually in the mics since he never had this problem before using these mics.  When you get a new problem the first time you use new equipment, you've got to suspect the new equipment first, or at least suspect that it's a compatibility problem between the new equipment and your old equipment.

[terrapinj]

have you tried to duplicate the problem in front of your stereo?

[SparkE!]

have you tried to duplicate the problem in front of your stereo?

This is the best question yet.

[CHURCH-AUDIO]

I looked at the wave form again but 100x magnification and it is harmonic distortion. Plain and simple Also I did suggest trying to use your home stereo and see if you could diplicate the problem but I highly doubt you would get the same SPL out of it.

Chris Church

what is your signal chain? exactly.

Here's the signal chain from a previous post:

Chain was: CK391 > UA5 (BM2+) > JB3

It was the first night out with the 391's (I usually ran C4s). And I also think I used a spare DVD battery that I got with the FP24 I bought a few weeks back. So, I had some new things in the mix that possibly could had contributed maybe?

The good news is that the first and second sets of the main act look fine (see attachments). Not sure what happened on the opener though. Strange. I wonder if the caps ended up touched each other in my XY on the opener if that could have done it? I was running XY on stage, and although the opener wasn't miced through the PA, there was a crapload of mics and cables on stage for the main act that had 20+ artists in it, every one of whom was mic'ed. Not sure if I could have had problems with that. The main act, I pulled back to the sweet spot for the PA and went DINA, so I had a pretty different setup for the second part.

I'm still questioning whether there's an electrolytic cap in the signal chain that is getting back biased by the signal so that its impedance is a function of the instantaneous signal voltage.  If that's the case, I'm suspecting that the problem is actually in the mics since he never had this problem before using these mics.  When you get a new problem the first time you use new equipment, you've got to suspect the new equipment first, or at least suspect that it's a compatibility problem between the new equipment and your old equipment.

[BayTaynt3d]

This weekend I'll record a test tone at home using both sets of mics and both caps on each mics (C4:card/omni & CK391/393) to see if I can recreate it. I'll also use my normal battery and the new (new to me, but old) battery I got with the FP24 I bought a while back. I'll do this all with the UA5 to see if I can force the issue. Any other things I should try?

[BayTaynt3d]

Also, so what is harmonic distortion? Is that gear related or FOH related or ?

[CHURCH-AUDIO]

If you look at the picture you will see your wave form and the wave form in the example are the same. I copyied this from this URL

http://en.wikipedia.org/wiki/Harmonic_distortion
there is some interesting info here hope this helps

Chris Church

Also, so what is harmonic distortion? Is that gear related or FOH related or ?

[SparkE!]

If your waveform is squashed on one side and expanded on the other (like yours was), that's a sign of even harmonic distortion.  That distortion could have been actually present in the acoustic signal you recorded or it could have been induced by non-linearities in your recording signal path.  If it was due to your own equipment, then when you play a pure tone through your speakers at HIGH levels and you record that sound, you will see not only the fundamental tone, but also mainly 2nd, 4th, 6th, 8th, ...., big even numberth harmonics.  You might also see odd harmonics, but the even harmonics will be larger in general when asymmetric clipping or asymmetric expansion occurs.  If you can't reproduce the problem when recording the sound from your home stereo (remember, it has to be LOUD!), then perhaps the problem was with the equipment at the venue where you recorded.

[CHURCH-AUDIO]

Thats true I have worked with alot of horn bands and heard this type of distortion. I think its caused by the horns them selves when air gets blown in it causes this very strange harmonic. Its almost like you can hear the spit inside the horn ( not joking ) Coltrain had that sound. Anyways Horn bands are a challange for any mic and preamp. When working with a all horn group or any horns  play it safe on the gain give it lots of room. Thats what i do with a console channel its no different with a set of sensitive room mics also on the AKG mics your using make sure if they have a 10 db pad you use it for this type of work. Even tough there are guys out there that will say it will increase your noise floor. I would always use a pad on the mic when working with any type of horns and condenser mics.

Chris Church

If your waveform is squashed on one side and expanded on the other (like yours was), that's a sign of even harmonic distortion.  That distortion could have been actually present in the acoustic signal you recorded or it could have been induced by non-linearities in your recording signal path.  If it was due to your own equipment, then when you play a pure tone through your speakers at HIGH levels and you record that sound, you will see not only the fundamental tone, but also mainly 2nd, 4th, 6th, 8th, ...., big even numberth harmonics.  You might also see odd harmonics, but the even harmonics will be larger in general when asymmetric clipping or asymmetric expansion occurs.  If you can't reproduce the problem when recording the sound from your home stereo (remember, it has to be LOUD!), then perhaps the problem was with the equipment at the venue where you recorded.

[BayTaynt3d]

Interesting stuff. I've just done a little reading, and I don't think I'm too worried about the harmonic distortion so much as what might be called, or at least looks like, DC OFFSET. Now maybe it is phase shifting or something else, and I plan on learning more this weekend, but I just found this interesting quote:

"Calling it "Centering" the wave is ambiguous -- perhaps this is something that Adobe should make note of. DC offset is easily diagnosed when there's a low-level signal, such as "open air," and the waveform sits off of the center line. Asymmetry is an entirely different matter, as you've all noted, and is usually caused by close-miking a voice, trumpet, trombone, or saxophone, all of which produce sounds with little puffs of air at the source. Andy & James, somewhere else it was noted that phase-shifting a voice could remove the asymmetry and allow a louder final result."

Now that's two "puffs" of air comments in a row, and all related to horns. Plus, it does look a little more "asymetrical" than strictly "off-center." Hmmm...