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Author Topic: 'Crosstalk' on UHF-R systems  (Read 4629 times)

Samuel Rees

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'Crosstalk' on UHF-R systems
« on: August 04, 2016, 01:53:13 pm »

I've been doing a lot of RF work recently, almost always using UHF-R and WWB 6 for frequency coordination and monitoring. I generally am having a pretty pleasant time, all things considered in this Washington DC crowded environment.

I have observed a problem occasionally where there will be RF and audio crosstalk between channels. I mean by this that two receivers will seem to pickup & meter audio from one transmitter. Transmitter A will be set to frequency X, and receiver A set to frequency X, and this will operate no problem. Receiver B will be set to frequency Y, yet still seem to meter RF and audio from transmitter B, with bad signal quality. I'm the past I written this off as frequencies too close or something, but I've now observed it many times when frequencies are multiple MHz away. This has happened when coordinating with WWB using a less than default exclusion threshold. It happened to me yesterday. I re-did all of my frequencies this morning using an even lower exclusion and 'robust' analysis, which I have not used before, and I've had good results. But, I've had good results loads of times with standard.

I would love to understand this phenomenon. Is this an intermodulation issue? I recently have been borrowing an RF explorer and was very interested to see how proximity affected the intermodulation, which I didn't realize happened to the extent that it does. Wouldn't WWB 6 be able to place frequencies far enough away to prevent this every time? Maybe it will better, in robust? Or is something else at work here?

As long as we are talking about WWB 6.... I've been setting the exclusion essentially as low as I can while acquiring the number of frequencies I require. Is this how you all are doing it?

Thank you!

Sam
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John Sulek

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Re: 'Crosstalk' on UHF-R systems
« Reply #1 on: August 04, 2016, 03:26:36 pm »

I've been doing a lot of RF work recently, almost always using UHF-R and WWB 6 for frequency coordination and monitoring. I generally am having a pretty pleasant time, all things considered in this Washington DC crowded environment.

I have observed a problem occasionally where there will be RF and audio crosstalk between channels. I mean by this that two receivers will seem to pickup & meter audio from one transmitter. Transmitter A will be set to frequency X, and receiver A set to frequency X, and this will operate no problem. Receiver B will be set to frequency Y, yet still seem to meter RF and audio from transmitter B, with bad signal quality. I'm the past I written this off as frequencies too close or something, but I've now observed it many times when frequencies are multiple MHz away. This has happened when coordinating with WWB using a less than default exclusion threshold. It happened to me yesterday. I re-did all of my frequencies this morning using an even lower exclusion and 'robust' analysis, which I have not used before, and I've had good results. But, I've had good results loads of times with standard.

I would love to understand this phenomenon. Is this an intermodulation issue? I recently have been borrowing an RF explorer and was very interested to see how proximity affected the intermodulation, which I didn't realize happened to the extent that it does. Wouldn't WWB 6 be able to place frequencies far enough away to prevent this every time? Maybe it will better, in robust? Or is something else at work here?

As long as we are talking about WWB 6.... I've been setting the exclusion essentially as low as I can while acquiring the number of frequencies I require. Is this how you all are doing it?

Thank you!

Sam

How physically close are the two transmitters when this is happening? Are they laying on a table beside each other ready to be deployed?
All bets are off if you have a bunch of transmitters stacked up on a table without any shielding between them. Folks on high rf count shows use baking pans to help isolate the transmitters from each other.
Try war gaming your set up by switching off one transmitter at a time and making sure that the receiver rf meters go dark for the transmitter powered off.
This will help find issues with the frequencies you are using.
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Samuel Rees

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'Crosstalk' on UHF-R systems
« Reply #2 on: August 04, 2016, 03:34:29 pm »

How physically close are the two transmitters when this is happening? Are they laying on a table beside each other ready to be deployed?
All bets are off if you have a bunch of transmitters stacked up on a table without any shielding between them. Folks on high rf count shows use baking pans to help isolate the transmitters from each other.
Try war gaming your set up by switching off one transmitter at a time and making sure that the receiver rf meters go dark for the transmitter powered off.
This will help find issues with the frequencies you are using.

In this exact case I'm working with a rack of 5x UR4Ds with a shure antenna distro and shure paddles. I've repeatedly had this happen in similar situations. With both transmitters off, both channels will show no noise. This generally comes up when I first turn them all on. That's why I suspect the mysterious intermod issue, because the transmitters are usually pretty close at this time. Maybe it's as simple as that.

This has happened to me enough that I'm pretty confident I've unintentionally controlled for different systems and etc. It's true, I didn't 'war game' everything, I scanned them all in WWB, did exclusions, set a low exclusion threshold, got a bunch of frequencies and deployed them via WWB. Then, if I'm unlucky, this happens, and I switch the offending channel to a backup frequency.

Shouldn't a robust frequency distribution setting in WWB be able to anticipate this problem when calculating freqs?

« Last Edit: August 04, 2016, 03:39:05 pm by Samuel Rees »
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Mac Kerr

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Re: 'Crosstalk' on UHF-R systems
« Reply #3 on: August 04, 2016, 03:44:29 pm »

In this exact case I'm working with a rack of 5x UR4Ds with a shure antenna distro and shure paddles. I've repeatedly had this happen in similar situations. With both transmitters off, both channels will show no noise. This has happened to me enough that I'm pretty confident I've unintentionally controlled for different systems and etc. It's true, I didn't 'war game' everything, I scanned them all in WWB, did exclusions, set a low exclusion threshold, got a bunch of frequencies and deployed them via WWB. Then, if  I'm unlucky, this happens, and I switch the offending channel to a backup frequency.

I feel like other people must have experienced this, as it happens to me somewhat often.

How are you determining there is a problem? Is there audible interference when all transmitters are turned on? Are you seeing lights on a receiver that has no transmitter on? Transmitter intermods happen when multiple transmitters are close together, if you have them next to each other on a table, or pick up all the transmitters in one hand, that is a worst case scenario. You will get the strongest intermods under those conditions. Seeing a couple of lights on a receiver that has no transmitter turned on is not ideal, but generally not a disaster either. Wider spacing between frequencies may reduce that, and with 6 channels you should be able to get enough space to avoid that, but once all the transmitters are all on you will probably be fine. War gaming by turning off 1 transmitter at a time is an important part of checking that everything you have calculated is fine, but be sure the transmitters are not all together when you do it since that will really make intermods.

Mac


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Samuel Rees

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Re: 'Crosstalk' on UHF-R systems
« Reply #4 on: August 04, 2016, 04:27:23 pm »

How are you determining there is a problem? Is there audible interference when all transmitters are turned on? Are you seeing lights on a receiver that has no transmitter on? Transmitter intermods happen when multiple transmitters are close together, if you have them next to each other on a table, or pick up all the transmitters in one hand, that is a worst case scenario. You will get the strongest intermods under those conditions. Seeing a couple of lights on a receiver that has no transmitter turned on is not ideal, but generally not a disaster either. Wider spacing between frequencies may reduce that, and with 6 channels you should be able to get enough space to avoid that, but once all the transmitters are all on you will probably be fine. War gaming by turning off 1 transmitter at a time is an important part of checking that everything you have calculated is fine, but be sure the transmitters are not all together when you do it since that will really make intermods.

Mac

Today this is a 10 channel system as I'm generally working with 8-24 channels.

I know there is trouble because when I turn on transmitter A, full RF signal appears on receiver A, AND partial signal appears on receiver B. Also if I talk into transmitter A, some inconsistent audio signal will appear on receiver B.  I have not experimented with it much, as I'm usually busy and just change the channel.
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Ike Zimbel

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Re: 'Crosstalk' on UHF-R systems
« Reply #5 on: August 04, 2016, 04:47:06 pm »

I've been doing a lot of RF work recently, almost always using UHF-R and WWB 6 for frequency coordination and monitoring. I generally am having a pretty pleasant time, all things considered in this Washington DC crowded environment.

I have observed a problem occasionally where there will be RF and audio crosstalk between channels. I mean by this that two receivers will seem to pickup & meter audio from one transmitter. Transmitter A will be set to frequency X, and receiver A set to frequency X, and this will operate no problem. Receiver B will be set to frequency Y, yet still seem to meter RF and audio from transmitter B, with bad signal quality. I'm the past I written this off as frequencies too close or something, but I've now observed it many times when frequencies are multiple MHz away. This has happened when coordinating with WWB using a less than default exclusion threshold. It happened to me yesterday. I re-did all of my frequencies this morning using an even lower exclusion and 'robust' analysis, which I have not used before, and I've had good results. But, I've had good results loads of times with standard.

I would love to understand this phenomenon. Is this an intermodulation issue? I recently have been borrowing an RF explorer and was very interested to see how proximity affected the intermodulation, which I didn't realize happened to the extent that it does. Wouldn't WWB 6 be able to place frequencies far enough away to prevent this every time? Maybe it will better, in robust? Or is something else at work here?

As long as we are talking about WWB 6.... I've been setting the exclusion essentially as low as I can while acquiring the number of frequencies I require. Is this how you all are doing it?

Thank you!

Sam
It sounds to me like you are seeing intermods, which as others have mentioned, can be VERY active when the transmitters are all piled in a heap, in a bucket (I've seen this), or a milk crate, or laid back-to-back on a console. You may know all of this already, but, these two articles might have some nugget of info that helps.
http://www.prosoundweb.com/article/avoiding_intermod_the_importance_of_wireless_frequency_coordination/
http://www.prosoundweb.com/article//whoa_you_cant_do_that/
Best,
Ike
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~Ike Zimbel~
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Alteros Inc
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Samuel Rees

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Re: 'Crosstalk' on UHF-R systems
« Reply #6 on: August 04, 2016, 04:57:10 pm »

I know a little bit, and I saw intermods on a live frequency analyzer for the first time last week. I'm trying to confirm that is what is happening here. Next time I'll try separating the transmitters before I jump to changing frequencies. If that does it, intermod sounds like the correct diagnosis.

Follow up question:

1) If I determine this to be intermodulation, how should I proceed? Is intermod like this relatively normal in crowded situations with close transmitters, or is it something to be avoided at all costs? Maybe if all this time I had just set the transmitters a little farther apart on the table, everything would have proceeded along perfectly.

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Ike Zimbel

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Re: 'Crosstalk' on UHF-R systems
« Reply #7 on: August 04, 2016, 05:13:18 pm »

I know a little bit, and I saw intermods on a live frequency analyzer for the first time last week. I'm trying to confirm that is what is happening here. Next time I'll try separating the transmitters before I jump to changing frequencies. If that does it, intermod sounds like the correct diagnosis.

Follow up question:

1) If I determine this to be intermodulation, how should I proceed? Is intermod like this relatively normal in crowded situations with close transmitters, or is it something to be avoided at all costs? Maybe if all this time I had just set the transmitters a little farther apart on the table, everything would have proceeded along perfectly.
A real good start is to just lay them one up - one down (ie: with each mic facing the opposite direction on the table from its neighbour). Yes, intermod is always there, or potentially there when the transmitters get close enough to generate it. Coordination programs like IAS and WWB exist to prevent you from allocating frequencies on ones that will potentially be occupied by intermods. In fact what you are seeing is an intermod that is close enough to light up the adjacent channel, but the reason it sounds like crap is because it's not right on the frequency, just the same as if you tune the Rx to say, 500.525 and the Tx to 500.575 or similar.
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~Ike Zimbel~
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Alteros Inc
Radio Active Designs
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ca.linkedin.com/pub/ike-zimbel/48/aa1/266

Samuel Rees

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Re: 'Crosstalk' on UHF-R systems
« Reply #8 on: August 04, 2016, 05:16:21 pm »

A real good start is to just lay them one up - one down (ie: with each mic facing the opposite direction on the table from its neighbour). Yes, intermod is always there, or potentially there when the transmitters get close enough to generate it. Coordination programs like IAS and WWB exist to prevent you from allocating frequencies on ones that will potentially be occupied by intermods. In fact what you are seeing is an intermod that is close enough to light up the adjacent channel, but the reason it sounds like crap is because it's not right on the frequency, just the same as if you tune the Rx to say, 500.525 and the Tx to 500.575 or similar.

That all makes sense. I am using WWB whenever possible. Sometimes the freqs are a lot farther apart than those two.

Sam
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Mac Kerr

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Re: 'Crosstalk' on UHF-R systems
« Reply #9 on: August 04, 2016, 05:37:17 pm »

A real good start is to just lay them one up - one down (ie: with each mic facing the opposite direction on the table from its neighbour). Yes, intermod is always there, or potentially there when the transmitters get close enough to generate it. Coordination programs like IAS and WWB exist to prevent you from allocating frequencies on ones that will potentially be occupied by intermods. In fact what you are seeing is an intermod that is close enough to light up the adjacent channel, but the reason it sounds like crap is because it's not right on the frequency, just the same as if you tune the Rx to say, 500.525 and the Tx to 500.575 or similar.
That all makes sense. I am using WWB whenever possible. Sometimes the freqs are a lot farther apart than those two.

Sam

It isn't the carrier that is close to the other channel, it's the intermods. I am not very familiar with WWB, but if you were using IAS you would see an indication of the intermods being calculated in the graph when you are looking for frequencies. On Panorama there were millions of intermod frequencies. I just did a look for G10 PMS1000s on one of the Panorama coords and it came up with 16 choices of freqs, with a possible 31 million 3rds. As you reduce what the software eliminates those intermods end up closer and closer to your actual carrier frequencies.

Mac
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