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Weird Looking Waveform

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Ev Tune
Weird Looking Waveform
on Apr 12, 2013 at 7:58:05 pm

I have a wireless Sennheiser lavalier mic that i'm having trouble with. Here is a short sample:






This distortion has happened a few times to me, but only while recording in a specific room. That would make me think that it's some kind of interference but it doesn't sound like interference to me. I'm no expert so I thought I would post it.


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Ty Ford
Re: Weird Looking Waveform
on Apr 12, 2013 at 11:55:42 pm

Hello Ev and welcome to the Cow Audio Forum.

Thanks for posting this. Interesting. Sounds like some strange peak at those moments. I haven't heard this sort of thing before. What in the room? Were the mics on the same person? Could it be that this was due to someone wearing a wireless lav and smart phone?

I'm sure we'll hear from others who may have encountered this.

Regards,

Ty Ford
Cow Audio Forum Leader

PS. check the room for wireless technology, see if you can turn it off and see if it goes away.

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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Brian Reynolds
Re: Weird Looking Waveform
on Apr 13, 2013 at 12:40:02 am

Ok can you give me more info on the record unit and the interface between the receiver and the recorder....

Would the input of the recorder be a 3.5mm socket?
Is the wave form you are showing just the Left channel?
Were you recording the mic receiver to both channels of the recorder?

Let me see if my hunch is correct ;)


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Peter Groom
Re: Weird Looking Waveform
on Apr 14, 2013 at 9:27:57 am

Im inclined more towards a recorder front end overload rather than a mic issue.
The waveform of the @good audio@ is nowhere near what would cause issue, but if the recorder gains were on the edge then it could well cause something like this.
Also check things like, there wasnt some incorrect powering going on on a device connected, like 40v being fed to a device that doesnt want it.

Peter

Post Production Dubbing Mixer


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Brian Reynolds
Re: Weird Looking Waveform
on Apr 14, 2013 at 12:40:35 pm

Let me mention my hunch, and it's only a hunch.... The audio was recorded on a device (DSLR or CamCorder) with a 3.5mm input socket.
The input cable was incorrectly wired where the Left ch effectively only recorded the positive side of the AC wave form and the Right ch recorded the negative side of that wave form.
If that occurred it would have given the recorder a slightly lower level than normal and when the level was pushed and a peak arose it clipped (as per example posted)
I'm sure that if we were to see the other channel it would be the opposite with downward clipping.
And if LEFT and RIGHT channels were mixed there would be a phase cancellation similar to many threads discussed on this forum.

As I said it's just a hunch. But I will admit I could be wrong.


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Richard Crowley
Re: Weird Looking Waveform
on Apr 15, 2013 at 4:52:26 am

It sounds RF-related to me. Furthermore, I am assuming from the original narrative that the same mic, transmitter, receiver, recorder, and associated cables are working properly in other rooms/venues, which would appear to rule out fundamental signal level, interface, or operational problems.

The OP did not exactly reveal any details about the location which is in question, so we are left to shoot in the dark here.


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Ev Tune
Re: Weird Looking Waveform
on Apr 15, 2013 at 12:31:34 pm

@brian
The equipment is a Sennheiser ew100 G2 lavalier (XLR) paired with a Panasonic AG-AC130, using only the left channel. The Panasonic is new, but I've recorded many talks with it without issue.

@richard
You are correct. The same mic, transmitter, receiver, recorder, and cables are working properly in other rooms. This has only happened twice, but it's been in the same room. The room setup is like every other venue we record in. It's usually just a classroom setting. From what I can tell, this classroom is in a clinical building with lab equipment. So who knows what is happening wirelessly in that area.

My question to you all is this: if it really is some kind of interference, wouldn't you expect the spikes be more random? The overloads appear to happen only while the presenter is speaking.

Thanks


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Ty Ford
Re: Weird Looking Waveform
on Apr 15, 2013 at 2:03:47 pm

Hi Guys,

I reached out to Sennheiser's Brian Walker. His reply follows.

Ty,
Starting to see a lot more of this kind of complaint. The problem is not limited to Sennheiser, it is one of those evils called the Laws of Physics. Most commonly, what is happening is that a person has been using the same frequency for years and now because of the re-pack a few years ago of DTV channels and new channels coming on the air, the wireless frequency is now in the middle of a DTV channel. DTV transmitters broadcast a digital signal of 1's and 0's adding up to a value of 8. Occasionally, that DTV transmitter will kick out a 6, 7 or 8 on or next to the frequency in use by the wireless mic. This is what causes the blip in the audio. Even though the transmitter may be miles away, those 6, 7 and 8 pulses are a lot of power and cause havoc. The solution is to re-scan and re-tune. This is a problem that plagues all wireless mic manufacturers. Please, please, please send your users having this problem with Sennheiser product my way. I am more than happy to help them out.

Thanks!


Brian Walker
Market Development - Professional Systems
Sennheiser Electronic Corporation
1 Enterprise Drive
Old Lyme, CT 06371

Office: 860-434-9190 x 585
Cell: 619-933-9255

http://www.sennheiserusa.com

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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Ev Tune
Re: Weird Looking Waveform
on Apr 15, 2013 at 2:38:40 pm

Thank you Ty. I will reach out to him.

I appreciate everyone's input.


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Richard Crowley
Re: Weird Looking Waveform
on Apr 15, 2013 at 7:59:47 pm

It is the ephemeral nature of RF waves that all sorts of things can cause similar symptoms. There could be an RF signal that is causing inter-modulation distortion only on the peaks, but not on the low-level signals. Remember that virtually all of our wireless mics us FM (frequency-modulation) and the higher-level the audio is, the wider bandwidth they take. So low-level audio could be "safe" while higher-level audio (and RF frequencies farther away from the center of the channel) could be "finding" this other RF who would rather have that frequency.

It seems likely that a simple scan for other users won't reveal this kind of interference. It will take actually testing the system with speech to identify which channels are unusable in that venue.

More people are getting spectrum analyzers to practice "defensive channel selection". Some are getting rather affordable, such as:

http://www.seeedstudio.com/depot/rf-explorer-3g-combo-p-1266.html?cPath=174


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