Running an ENG G3 and a G2 at the same time (glitching/interference issues) at DVinfo.net

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Old February 2nd, 2010, 03:00 PM   #1
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Running an ENG G3 and a G2 at the same time (glitching/interference issues)

I need some help fast!

I've been running an ENG G3 and G2 wireless units on my shoots for Southern Souls

I make sure to scan for an open frequency on each receiver when I get to each new location. There is still violent/quick/loud interference noises, which aren't as bad outdoors as they can be indoors. But they happen at least 3-4 times in every song I shoot, which I cant have now that I'm filming bigger bands

Take a listen to Matt Sajn's second song "Soul Machines" you'll hear the noises at the beginning of that one, most takes where it was worse than that I had to ditch and couldnt put on the site. Indoors it is terrible, I almost cant use them indoors.

Why would this be? I've been reading some stuff on intermodulation but I don't exactly understand it, I've only found articles on people running two G2's at the same time so it doesn't really help me since both my units would have different memory banks

Please I need help to solve this fast!

Thanks for your time.
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Old February 2nd, 2010, 03:12 PM   #2
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Hi Mitch............

Try posting just the specific audio section file as compressed as you can, even my link is so slow with the video, I'm going to collect my pension before it downloads.



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Old February 2nd, 2010, 03:23 PM   #3
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Sounds like pretty typical RF hits to me. I doubt it was caused by using the two wireless together. Could you tell if it happened with just one of the mics or did it happen to both of them? If both, at the same time or at random on each? (You did record each mic iso, deferring mixing to post, so you could localize such a problem didn't you?) Did you insure that all cell phones in the vicinity of the set were completely powered off, not just set to silent?
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Old February 2nd, 2010, 07:53 PM   #4
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What frequency band mics where you using that would help? What buildings/places where nearby if you can remember? What recording device?



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Old February 2nd, 2010, 07:54 PM   #5
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Those little fwips, eh? Sounds like dropouts to me, and probably not cell interference.

Can you better describe your setup? I see the guitar mic, but where's the talent mic? Which transmitters for which lav? Where are your receivers mounted (be as specific as possible), and how far apart? What frequencies for each, on that specific shoot? Did the orientation of the antenna on the Tx's match those on the Rx's?

Intermodulation, as I understand it (and I am no expert!), is basically from two transmitting frequencies being too close to one another. The primary result of intermodulation can be a decimation of your range, which can cause dropouts. It's a decent theory as to why we seem to hear more dropouts in the beginning than the end. If the Tx's are being blocked by your body, that could also cause dropouts due to decreased effective range, even up close.

The rules of thumb I've heard around intermodulation include:
- Don't have transmitters and receivers in the same bag/cart on the same block (referring to blocks as established by Lectrosonics, also used by Zaxcom)
- If there is <1MHz of difference between two signals, that's what'll kill your range. Even a 10 MHz difference could halve your effective range, leading to increased chances of dropouts. If you don't have a diversity receiver, this number may be higher.
- 25MHz difference between two good diversity receivers and/or transmitters is a good rule of thumb.

That's how I've heard it and I've not been steered wrong by that advice, but others may be able to further clarify or offer more advice!
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Old February 3rd, 2010, 06:37 AM   #6
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At least in that one video, the metal car bodies in close proximity to the transmitters and especially between the transmitters and receiver early in the piece would have a huge influence on the signal strength and multipath.
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Old February 3rd, 2010, 08:23 AM   #7
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Sounds like range/ diversity problems to me, I find the Sennheisers prone to it and that car door would do you no favours...
I noticed in the 1 other video (les Mammographs) you had the pack in the arse pocket, and then he turned towards you so basically the signal had to pass through his body and his guitar to reach you, not to mention the amount of jiggling about that happens there, maybe clipped to his hip would be better
I have a Senn G2 but i only use it in static situations or as a backup because I've found it prone to dropout, for that level of work I think you need better radios tbh
If thats not an option then try to place the pack in the place with clearest line of sight to the receiver possible

Love the website by the way , bookmarked
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Old February 3rd, 2010, 10:43 AM   #8
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To avoid intermodulation problems you need to use freqs in the same bank of 4...see the manual regarding finding open freqs within a given bank..../B. Vaughan
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Old February 4th, 2010, 06:13 AM   #9
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Using only two frequencies, IM-free is not such a big issue - you just have to make sure that the two frequencies are as far apart as possible preferably 500kHz or more away from each other would be best.

Make sure that the aerials are hanging free - touching the body can attenuate the signal by about 70dB.

Remember that the G2 camera receiver is not diversity - the G3 is diversity.

If you use more than two then you *have* to use frequencies in the same memory bank - but G2 and G3 banks are different or differently numbered. In the case of a G2 and G3 mix you need to use a G2 frequency bank - if the same frequency set is not in the G3, then program the G2 bank of your choice into the G3 User Bank.

The G3 is better than the G2 and you must always use the frequency set of the lower specified equipment.

I hope this helps.
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Old February 12th, 2010, 09:54 AM   #10
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Quote:
Originally Posted by Nathan Moody View Post
Those little fwips, eh? Sounds like dropouts to me, and probably not cell interference.

Can you better describe your setup? I see the guitar mic, but where's the talent mic? Which transmitters for which lav? Where are your receivers mounted (be as specific as possible), and how far apart? What frequencies for each, on that specific shoot? Did the orientation of the antenna on the Tx's match those on the Rx's?

Intermodulation, as I understand it (and I am no expert!), is basically from two transmitting frequencies being too close to one another. The primary result of intermodulation can be a decimation of your range, which can cause dropouts. It's a decent theory as to why we seem to hear more dropouts in the beginning than the end. If the Tx's are being blocked by your body, that could also cause dropouts due to decreased effective range, even up close.

The rules of thumb I've heard around intermodulation include:
- Don't have transmitters and receivers in the same bag/cart on the same block (referring to blocks as established by Lectrosonics, also used by Zaxcom)
- If there is <1MHz of difference between two signals, that's what'll kill your range. Even a 10 MHz difference could halve your effective range, leading to increased chances of dropouts. If you don't have a diversity receiver, this number may be higher.
- 25MHz difference between two good diversity receivers and/or transmitters is a good rule of thumb.

That's how I've heard it and I've not been steered wrong by that advice, but others may be able to further clarify or offer more advice!
I realize this is a slightly old thread, but wanted to clear up a few things here.

1. I agree with Nathan that the "hits" heard in the video are just basic reception problems. IMO the reason is *multipath nulls* caused by A) the receiver is non-diversity (probably the G2 system) and B) there are lots of metal reflective surfaces around (outdoors -the cars, indoors -the walls, floor, metal in the building's structure) and C) that during the nulls, there is some other competing signal either intermod products, TV transmissions, or other wireless systems like what's on stage or used for communication in the building.

2. Intermod problems are caused by mathematical interactions between frequencies when radio signals mix within devices such as transmitters, receivers or antenna systems. 3rd order intermod products are the biggest culprit because they can be generally within the band of the system you are using and they are strong enough to appear at your receiver and overpower your transmitter's signal during a multi-path null.

IM products are NOT caused by two or more frequencies being too close together. They can be caused by two frequencies being quite far apart, as long as other criteria are met (transmitters are physically close to each other, transmission signal is too strong for receivers or antenna system, etc.) An example of an Intermod-prone setup is to have two belt packs right next to each other on the talent's belt - one for the guitar and one for the voice, for instance. If this is how you have them set up, I would suggest physically separating them as much as possible - on either side of the torso is a good start and should help a great deal.

3. The rule of thumb for separation between your wireless mic frequencies is generally 400kHz or more. You can get down to 300kHz if you HAVE to, and you have a good, well-filtered system. Any of the high-end systems from Shure, Sennheiser & Lectrosonics will handle this. For stuff in the <1000/ch. price range, 400 or maybe 500kHz between channels is a good idea. This is not because of intermod, but because the actual transmission signals will step on each other - they are about 200kHz wide, maybe more. I've done experiments where I've gotten the channel separation down to 250kHz and it is on the verge of not working, while 300kHz seems to work fine.

4. If you are using a "bag" system and have talent coming in, then a re-transmission going out of the bag, yes it is a good idea to separate these by as much as possible using different frequency blocks for your bag receivers (talent) and bag transmitters (camera hop). Otherwise, the bag/hop transmitter signal will "de-sense" the receivers, reducing range.

To get your channels to be intermod-free, a real frequency coordination is in order. This requires that you use a software package like Shure's Wireless Workbench, Sennheiser's similar product, or a good 3rd-party product like IAS from Professional Wireless or RF Guru from Stage Research. This also means that you need to do a scan or get a list of what the local area TV channels are and any other wireless in your proximity, like Clear-com, systems on stage used by musicians, and stuff used on other cameras, etc. These software packages will help step you through all of this.

A properly coordinated wireless mic system should perform flawlessly. With one caveat - back to the multi-path null issue. Receivers without diversity reception are particularly vulnerable to this problem. Your G2 system is not diversity, while your G3 system is. Even still, small receivers like the G3 or Lectrosonics UCR401, etc. - the antennas are quite close together and only offer so much improvement over non-diversity systems. There is improvement, but not as much as when using a well-designed antenna system with proper spacing.

So, there you have it - wireless 101.
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