Uneven audio waveform

[katanna]

A random question here: look at this screenshot of the waveform of the recording of our service this morning: link. (The selected part is the first part of the service, the next 30 minutes is the sermon, and the last 10 minutes is the closing.)

Why is the waveform "uneven"? At parts, it is top heavy (like at the 6 minute mark), and at times it is bottom heavy (the entire sermon part is bottom heavy).

I think I understand the basics of what the waveform is showing you... but I don't know why it would be uneven like this. I have seen it several times over the last months, and it is making me curious. Thanks for answering my curiosity!

Matthew

[waynehoskins]

My bet would be that it's a result of compression. One part may be an auto-gain circuit while the other is compression at the console.

[katanna]

Here is the setup... (and don't kill me, I know it is crazy!)

Out of Aux 1 from the board to an XLR distribution amp. I then take the XLR from the amp and go into the XLR of our camera, and I take the headphone output of the camera to record it on the iMac... really I should go from the ampto the computer (IE: bypass the camera), but I am too lazy to solder a wire...

Matthew

[cmchamp]

Matthew
Does it sound OK? We use Audacity at my church, and I don't pay attention to the waveform peaks at all. As long as the signal going to your camera isn't saturated (clipping) you should be OK.
My only other observation, are you at all going from a stereo 1/8" to a mono 1/8"? If so, this might account for what looks like a phase shift (of set "heaviness").
C.

[audiomatic]

Why not bypass the camera and record a practice just to see if anything changes? If not, the camera isn't the problem.

[pdc]

Now I will address the real problem. It has nothing to do with his system or compression or anything else. The OP stated a desire to know why the waves were off center. The reason is phase.

Anytime you see a waveform heavy on one side or the other, you have something going on with the phasing of the source. I would almost bet you money that areas off center are where the pastor was standing over the lecturn or a reflective surface, in the line of fire of a speaker, etc.

This type of thing is common when drum kits are mic'd and the mic's are not in perfect phase. This is also common when recording instruments with intonation issues.

[katanna]

I am never going from a stereo to a mono. The Aux 1 goes to the distribution amp as a mono, and that mono signal goes into the camera twice (once as left, once as right). From there it stays stereo.

As far as phase, he is standing in the center of the stage, and there are no speakers anywhere near him. And during the sermon, the only two mics that are unmuted are his wireless and an SM58 under the piano. You can see him here in his robes, between the two speakers: link

As far as it sounding good, I think it sounds great. Go to our website: ArgyleUMC.org, click on the "Download Sermons" in the row of graphics at the bottom, click on today's sermon "Where has our religion failed?", and click on the first file in the list: "2009-07-12 Where has our religion f..." After the 30 second sound intro you can hear how his voice sounds. [/shameless plug]

As I said, I think it sounds fine, I am asking here not because it sounds bad, but because I am curious as to why it is doing this. Thanks for the replies!

Matthew

[pdc]

That waveform represents phasing, plain and simple. He does not have to be near speakers per se for it to occur. This is frequency dependent. This is why it is not happening all the time. It is not a constant, consistant phase shift. When he and the mic are in the right spot, it will happen. Your mic on the pastor is not only picking up the direct vocal but also the reflections from the room. The combination of the direct sound with the room sound (which can vary drastically by moving inches in any direction) and speakers (which radiate sound 360 degrees below 250Hz in most cases) is what is causing it. You will not likely encounter an issue with the sound of that one vocal alone. It would only be the combination of the same vocals that would allow you to notice the phasing effect.

[WORDpictures]

Glad you asked this Matt. I've noticed asymmetrical waveforms a few times while editing video recordings of live events. The waveform's appearance had me very concerned what was going wrong, though I don't recall noticing any audible problems either.

[katanna]

Thanks for the reply pdc, you answered my question... but now it begs the next question: If there anything "wrong" with it this way, and if so, what can be done about it?

Matthew

[pdc]

Originally Posted by katanna

Thanks for the reply pdc, you answered my question... but now it begs the next question: If there anything "wrong" with it this way, and if so, what can be done about it?

Matthew

No, there is nothing wrong really. It is what it is. Don't worry about it.

[katanna]

Originally Posted by pdc

No, there is nothing wrong really. It is what it is. Don't worry about it.

Thanks... that is what I figured, I just wanted to make sure!

Matthew