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Author Topic: Sennheiser Lav... very unique/strange RF issue  (Read 1538 times)

grantpichla

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Sennheiser Lav... very unique/strange RF issue
« on: November 20, 2014, 10:27:25 am »

Hey everyone,

I recently encountered some sort of RF error while recording a lecture for the University of Illinois using Sennheiser Lavalier microphones, model: “ew 100 G2”.  I was hoping to put this out there and see if anyone else had experienced it before.

I’ve dealt with typical RF issues; two devices trying to occupy the same frequency, distances between transmitter and receiver being too far, multiple transmitters sending the same frequency, etc.  I also understand the sound of volume peaking- my error at first felt like this, but proved not to be.

While on set, I had a single transmitter and receiver on the same channel, with levels averaging a little low at -12dB and -16dB.  What boggles me is that the first lecture we recorded sounded perfectly fine in the professor’s office in the 2nd floor of the building.  I have a sample of that 1st lecture here: https://www.dropbox.com/s/0cvrmdbpknukhrz/lav_in_first_lecture.aif?dl=0

For her 2nd lecture, we moved to a new room for a different look (also on the 2nd floor), and the recording sounded like this: https://www.dropbox.com/s/le6d48t223wxpcn/lav_cut_out_issues.aif?dl=0

The sound cuts in and out in a very strange way.  It was not due to peaking, and it doesn’t feel like typical RF interference.  I checked all the connections of wires on each device multiple times, no luck. I also changed frequencies from the high 600 range (something like 655) to the low (something like 628).  No luck.  I adjusted Squelch settings, which didn’t help.  I eventually ran over and grabbed a new lav kit and a Sennheiser shotgun microphone.  I decided to hook up the new lav kit (which occupied a 535.000 frequency) on channel 1 and the boom on channel 2.  This was my result: https://www.dropbox.com/s/yy5cfdmwf3nis1t/new_lav_on_left_and_shotgun_on_right.aif?dl=0

Same problem… although the shotgun channel will be usable.

Overall, I never understood what happened in that room… why her audio sounded so… compressed?  Almost like an auto gain control feel, but that was not the case.  Plus, the sound was perfect in her 1st lecture.  And if it would have been AGC, then we would hear more noise in the atmosphere bed rising up and down in between her statements.

Anyone ever experience this before?  I’m chalking it off as being in a room that was close to some sort of device center with a major amount of RF traffic… but who knows?!

Thanks,
Grant
grantpichla@gmail.com
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Jason Glass

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Re: Sennheiser Lav... very unique/strange RF issue
« Reply #1 on: November 20, 2014, 12:00:28 pm »

Hey everyone,

I recently encountered some sort of RF error while recording a lecture for the University of Illinois using Sennheiser Lavalier microphones, model: “ew 100 G2”.  I was hoping to put this out there and see if anyone else had experienced it before.

I’ve dealt with typical RF issues; two devices trying to occupy the same frequency, distances between transmitter and receiver being too far, multiple transmitters sending the same frequency, etc.  I also understand the sound of volume peaking- my error at first felt like this, but proved not to be.

While on set, I had a single transmitter and receiver on the same channel, with levels averaging a little low at -12dB and -16dB.  What boggles me is that the first lecture we recorded sounded perfectly fine in the professor’s office in the 2nd floor of the building.  I have a sample of that 1st lecture here: https://www.dropbox.com/s/0cvrmdbpknukhrz/lav_in_first_lecture.aif?dl=0

For her 2nd lecture, we moved to a new room for a different look (also on the 2nd floor), and the recording sounded like this: https://www.dropbox.com/s/le6d48t223wxpcn/lav_cut_out_issues.aif?dl=0

The sound cuts in and out in a very strange way.  It was not due to peaking, and it doesn’t feel like typical RF interference.  I checked all the connections of wires on each device multiple times, no luck. I also changed frequencies from the high 600 range (something like 655) to the low (something like 628).  No luck.  I adjusted Squelch settings, which didn’t help.  I eventually ran over and grabbed a new lav kit and a Sennheiser shotgun microphone.  I decided to hook up the new lav kit (which occupied a 535.000 frequency) on channel 1 and the boom on channel 2.  This was my result: https://www.dropbox.com/s/yy5cfdmwf3nis1t/new_lav_on_left_and_shotgun_on_right.aif?dl=0

Same problem… although the shotgun channel will be usable.

Overall, I never understood what happened in that room… why her audio sounded so… compressed?  Almost like an auto gain control feel, but that was not the case.  Plus, the sound was perfect in her 1st lecture.  And if it would have been AGC, then we would hear more noise in the atmosphere bed rising up and down in between her statements.

Anyone ever experience this before?  I’m chalking it off as being in a room that was close to some sort of device center with a major amount of RF traffic… but who knows?!

Thanks,
Grant
grantpichla@gmail.com

Hi Grant,

What the recordings sound like, to me, is the mic element not getting sufficient bias power.  However, this doesn't seem to be the case in the first lecture.  On some older gear, a dying battery could cause this, but on newer stuff the voltage regulation is usually quite good until the unit just completely stops working due to low voltage.  I would be surprised if a G2 didn't behave that way with a dying battery.

The situation you described, with performance changing in a different part of the building, consistently across different frequencies and different units in different bands, indeed points to broadband RF interference.  The most likely source of this type of RFIF in such a setting (provided that you checked an appropriate TV database for occupied channels) is a faulty lighting ballast.  If you ever see a flickering, partially lit, or dead vapor lamp in a fixture that is powered up near where you are having broadband RFIF, it is often the source of your problem.
« Last Edit: November 20, 2014, 12:14:39 pm by Jason Glass »
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grantpichla

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Re: Sennheiser Lav... very unique/strange RF issue
« Reply #2 on: November 20, 2014, 05:00:37 pm »

Hi Grant,

What the recordings sound like, to me, is the mic element not getting sufficient bias power.  However, this doesn't seem to be the case in the first lecture.  On some older gear, a dying battery could cause this, but on newer stuff the voltage regulation is usually quite good until the unit just completely stops working due to low voltage.  I would be surprised if a G2 didn't behave that way with a dying battery.

The situation you described, with performance changing in a different part of the building, consistently across different frequencies and different units in different bands, indeed points to broadband RF interference.  The most likely source of this type of RFIF in such a setting (provided that you checked an appropriate TV database for occupied channels) is a faulty lighting ballast.  If you ever see a flickering, partially lit, or dead vapor lamp in a fixture that is powered up near where you are having broadband RFIF, it is often the source of your problem.


Hi Jason,

The batteries seemed okay, but I think your idea of the lighting ballast sounds like the most likely culprit.  I recall a moment where I thought it was working again- that was when I was holding the transmitter and receiver by my camera... away from the light bank in the ceiling.  We used the overhead installed light as our primary key in the shoot, so there is a good chance that it was messing with the RF.  Thanks a ton for the input!

Grant
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Re: Sennheiser Lav... very unique/strange RF issue
« Reply #2 on: November 20, 2014, 05:00:37 pm »


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