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Author Topic: Combining Digital & FM IEMs  (Read 473 times)

Mark Hannah

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Combining Digital & FM IEMs
« on: October 28, 2020, 08:08:07 am »

Hello All,

Has anyone combined digital and FM modulation together? I recall combiners were being tested and approved to be used with digital transmitters when the Duets first came out. Thus the reason for wondering what happens with the two in the active stages of a combiner. Whether yes or no, I'd appreciate some details. Pros/cons.

Additional details that might affect your response...

An upcoming event on Sunday is using Duets for some of the house musicians. Guest vocalist is bringing his PSM 1000 rig. I cannot get an answer on if
  • He has own combiner & antenna
  • Single or duo transmitter
  • If duo and no combiner, will both transmits be used?
I can open up two ports on the Duets' TX-8 and I can be prepared with another antenna if a single unit. If two channels, I might have to fight (budget) to get a separate combiner.

Thank you.
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Henry Cohen

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Re: Combining Digital & FM IEMs
« Reply #1 on: October 28, 2020, 01:31:18 pm »

Has anyone combined digital and FM modulation together? I recall combiners were being tested and approved to be used with digital transmitters when the Duets first came out. Thus the reason for wondering what happens with the two in the active stages of a combiner. Whether yes or no, I'd appreciate some details. Pros/cons.

Additional details that might affect your response...

An upcoming event on Sunday is using Duets for some of the house musicians. Guest vocalist is bringing his PSM 1000 rig. I cannot get an answer on if
  • He has own combiner & antenna
  • Single or duo transmitter
  • If duo and no combiner, will both transmits be used?
I can open up two ports on the Duets' TX-8 and I can be prepared with another antenna if a single unit. If two channels, I might have to fight (budget) to get a separate combiner.

In general, combining analog and digital signals within a single combiner is not an issue, as long as you treat all carriers as if they were digital and adhere to some basic guidelines:

- Ensure the combiner is capable of handling wideband signals. As it regards the RAD TX-8, it's best to use the TX-8U. The older TX-8 generally does not perform well with digital signals regardless of input or output power levels.

- Keep the input levels low, generally 20mW max for most combiners. The TX-8U can handle 25mW in and set to 100mW output with sufficient frequency separation, but 50mW is best. The Lectro M2C can handle 50mW input with sufficient frequency separation.

- Minimum frequency separation should be 600-700kHz for lower input levels, 800kHz-1MHz for higher input levels. Keep adjacent frequencies on non-adjacent input ports. (In either ascending or descending order of frequencies, combiner input port utilization should be 1, 5, 3, 7, 2, 6, 4, 8.)

If using a single M2T with both channels operating, set both channels to 50mW and use a passive 2-way combiner to feed a single antenna, presumably a directional with passive gain. Isolators on the output of each channel transmitter, before the 2-way combiner, is strongly recommend, as well as on the output of the active combiner.


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Henry Cohen

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Mark Hannah

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Re: Combining Digital & FM IEMs
« Reply #2 on: October 28, 2020, 10:44:02 pm »

In general, combining analog and digital signals within a single combiner is not an issue, as long as you treat all carriers as if they were digital and adhere to some basic guidelines:

Quote
Minimum frequency separation should be 600-700kHz for lower input levels, 800kHz-1MHz for higher input levels. Keep adjacent frequencies on non-adjacent input ports. (In either ascending or descending order of frequencies, combiner input port utilization should be 1, 5, 3, 7, 2, 6, 4, 8.)

Treat as digital because of their constant bandwidth usage and thus one of the reasons for the 600-700k separation?

Quote
If using a single M2T with both channels operating, set both channels to 50mW and use a passive 2-way combiner to feed a single antenna, presumably a directional with passive gain. Isolators on the output of each channel transmitter, before the 2-way combiner, is strongly recommend, as well as on the output of the active combiner.

Would you also take the same approach to passively combining a PSM-1000? Unfortunately, I'm not in a position to get isolators at this late date.

The combiner is a TX-8U. I'd like it to have 3x Duets (6 frequencies) and 2x PSM (2 frequencies).

Need to do some work in IAS based on your response and with an outdoor scan from next to the loading dock.
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Andrew Broughton

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Re: Combining Digital & FM IEMs
« Reply #3 on: October 29, 2020, 11:54:12 am »


Is that a typo or can you clarify this statement?

Keep the input levels low, generally 20mW max for most combiners. The TX-8U can handle 25mW in and set to 100mW output with sufficient frequency separation, but 50mW is best. The Lectro M2C can handle 50mW input with sufficient frequency separation.


NM, First was input, 2nd was output. Carry on.
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Henry Cohen

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Re: Combining Digital & FM IEMs
« Reply #4 on: October 29, 2020, 12:25:43 pm »

Treat as digital because of their constant bandwidth usage and thus one of the reasons for the 600-700k separation?

Yes. Keeps the amps in the combiner from saturating.


Quote
Would you also take the same approach to passively combining a PSM-1000?

Yes. Isolators should always be used when combining transmitter outputs, especially on the outputs of combiners, regardless of transmission scheme.


Quote
The combiner is a TX-8U. I'd like it to have 3x Duets (6 frequencies) and 2x PSM (2 frequencies).

Should not be a problem. Keep the Duets at 25mW and the PSM's at 50mW; keep the TX-8U output settings at 50mW.
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Henry Cohen

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Mark Hannah

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Re: Combining Digital & FM IEMs
« Reply #5 on: October 29, 2020, 07:13:57 pm »

Yes. Isolators should always be used when combining transmitter outputs, especially on the outputs of combiners, regardless of transmission scheme.

Can you share a few sources for isolators?

The 50 Ohm devices I found have limited bandwidth, i.e. nothing 470 - 608 MHz, and rated to 100W which isn't necessary. Prices at $400+. I'm not too surprised about the limited bandwidth based on a conversation I had with Sennheiser regarding their digital transmitters.
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Henry Cohen

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Re: Combining Digital & FM IEMs
« Reply #6 on: October 30, 2020, 01:52:50 pm »

Can you share a few sources for isolators?

The 50 Ohm devices I found have limited bandwidth, i.e. nothing 470 - 608 MHz, and rated to 100W which isn't necessary. Prices at $400+. I'm not too surprised about the limited bandwidth based on a conversation I had with Sennheiser regarding their digital transmitters.

Limited bandwidth is a natural characteristic of ferrite based device, but at the low power levels we need, there are some 200MHz wide offerings in the UHF core TV band. Expect to pay >$300.00.

https://rflambda.com/search_isolator.jsp  (Usually has stock of the 400-600 MHz and 500-700 MHz)
http://www.mwdevices.com/isolators-std-bandwidth.html
https://www.dorado-intl.com/product-category/broadband-high-power/broadband-high-power-isolator/
https://mcli.com/products/isolators-circulators/connectorized-broadband-2-port-isolator-and-3-port-circulator
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Henry Cohen

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Mark Hannah

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Re: Combining Digital & FM IEMs
« Reply #7 on: November 03, 2020, 07:29:24 pm »

Limited bandwidth is a natural characteristic of ferrite based device, but at the low power levels we need, there are some 200MHz wide offerings in the UHF core TV band. Expect to pay >$300.00.

With an unknown/uncertain future, isolators will need to go on my growing wish list.

Thank you very much for all your responses on my recent posts.

Regards
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ProSoundWeb Community

Re: Combining Digital & FM IEMs
« Reply #7 on: November 03, 2020, 07:29:24 pm »


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