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Author Topic: How To Figure Analog RF Mic Packing into TV Channels?  (Read 619 times)

Dan Mortensen

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How To Figure Analog RF Mic Packing into TV Channels?
« on: September 08, 2017, 07:12:25 pm »

I've decided to try for the RF Mic license and am amassing microphones to add to the ULX-D and ULX that are left in my inventory after the 600MHz auction. I'll get help for the legal part of it after acquiring enough mics.

The bulk mics that I've chosen are the  JTS R-4's , which are similar packaging (4 receivers in one rack space) to the ULX-D's that I love. They seem to be reasonably physically robust and sound fine to my ear, although not as fine as the ULX-D. They are very much more affordable, though, and I think will work for my purposes when the ULX-D's or ULX's are not appropriate, particularly for rentals.

Needless to say, they are made in China, by the same company that makes Audio Technica and Audix and some others according to the mfr's rep. So there is some pedigree there.

However, the data on them is sketchy at best, and the American importer clearly does not have wireless microphone technical expertise in-house. As one example, they have only been importing and stocking the 506-542 band. Period.

I've ordered more in the 470-506 bandwidth, where there are more open channels here, but since they are being custom built in China and then slow-boated here, that has taken months so far, but I'm not in a hurry, so that's fine. I have informed the importer through the rep that they will not be able to have a major presence in the market by only stocking one frequency band.

The mics come with a printout of suggested groups of usable frequencies withing that bandwidth.

The most conservative group has 14 choices, and the two most packed groups have 22 frequencies. Rather than type all the possibilities for you to see, I'm attaching a picture.

It appears to me that there are at most 6 frequencies usable within a 6MHz TV channel, and that at only one or maybe two segments of a group. Usually it seems like it's only 3 or 4 mics per 6MHz, although I haven't searched through all possibilities.

The spacings between frequencies in all groups seem more random than I expected, but maybe there's a reason for that?

I'm using RF Explorer and Vantage to capture signals and Wireless Workbench to plan frequencies, so the next post has some pictures of the JTS transmitter at low power (the choices are "Low" and "High", with no values) vs. Shure ULX-D at 20mw (which seems to match the JTS for amplitude) and 10mw. The pics should be titled appropriately so you can tell the difference. All three are 6MHz wide, although the mics are not set to the same frequency. I hope you can still see what you need to.

Specific questions:

1) How to tell Wireless Workbench about these microphones so it can know how many can pack in a TV channel?

2) More generally, how does anyone know how many mics can pack into a given space? What characteristics determine that and how does one recognize them?

Any thoughts? Thanks in advance.

Oh, and I should say that the breadth of JTS's mic offerings is ridiculous. They must have hundreds if not thousands of products, covering lots of areas of need. I haven't tried anything else yet.

« Last Edit: September 08, 2017, 07:21:35 pm by Dan Mortensen »
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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels
« Reply #1 on: September 08, 2017, 07:13:36 pm »

Pics:
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Henry Cohen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #2 on: September 08, 2017, 07:58:17 pm »

The spacings between frequencies in all groups seem more random than I expected, but maybe there's a reason for that?

There is: IM math.


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Specific questions:

1) How to tell Wireless Workbench about these microphones so it can know how many can pack in a TV channel?

Create your own equipment profile in WWB. In the Filtering & Intermods tab, it's the fields for More Robust, Standard and More Frequencies: This is where to enter occupied channel bandwidth (OBW) including guardband.


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2) More generally, how does anyone know how many mics can pack into a given space? What characteristics determine that and how does one recognize them?

This is determined by:
a) Primarily the parameter of selectivity. How well the receiver can pick out the desired carrier while ignoring co-channel noise and inband adjacent RF energy.
b) Secondarily the spurious emissions of the transmitter; how much unwanted/unnecessary RF energy (spurs, harmonics, inband noise) is there in addition to the the required carrier and spectral mask.

Much of this information can be found in the equipment's FCC Authorization filing and test report. This can be found by searching the OET Equipment Authorization database using the FCC ID on the equipment label.

No FCC ID number? Then it's not legal to sell or operate in the US.
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Henry Cohen

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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #3 on: September 08, 2017, 11:50:23 pm »

Thanks, Henry, this is informative.

Regarding


Much of this information can be found in the equipment's FCC Authorization filing and test report. This can be found by searching the OET Equipment Authorization database using the FCC ID on the equipment label.

the receiver has a sticker with "FCC ID INGR-4"

Entering "ING" in the first search field and "R-4" in the second but filling in none of the other blank fields yields a result of "No records found", as does adding the frequency range (506-542) and or changing the records searched from "All Granted Statuses" to "All Grants, Dismissals, and Denials".

Does this confirm my worst suspicions? Or do I need to fill all those other fields that I couldn't be expected to know?

For reference, here's a picture of the only thing I could identify on a Shure ULXD4Q as anything to do with the FCC, and I don't see an ID on it at all, unless it's that blocky thing.

Edit: Further searching for either of the strings starting with 616A or 328A also yields the "no applications on file that match" screen.
« Last Edit: September 08, 2017, 11:58:21 pm by Dan Mortensen »
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Henry Cohen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #4 on: September 09, 2017, 08:29:51 am »

The FCC ID number of interest is on the transmitter (not the receiver).

You can also try the search using the OEM's name ("Applicant Name"). If there are a lot of results, you can then sort by upper or lower frequency to help locate the unit of interest.
« Last Edit: September 09, 2017, 08:49:04 am by Henry Cohen »
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Henry Cohen

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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #5 on: September 09, 2017, 05:11:14 pm »

The FCC ID number of interest is on the transmitter (not the receiver).

Ah, so it's the characteristics of the transmission and not the ability of the receiver to sort things out that is relevant.

Looking at the tags on the transmitters of the JTS and the ULXD do get some letters and numbers.

Entering the JTS info still results in no matches; the Shure gets links to a gigantic trove of application information and engineering data. Downloading the test results gets a 128 page paper mostly showing spurious frequency generations.

So if Shure doesn't gather it from non-Shure manufacturers and put it in WWB, the user is obliged to enter it in? If the user wants to predict the mic performance, I guess....

Looking at your earlier advice

"In the Filtering & Intermods tab, it's the fields for More Robust, Standard and More Frequencies: This is where to enter occupied channel bandwidth (OBW) including guardband."

I go to the ULXD data (since I don't have any for JTS) and look at the picture of the transmission spectrum (below) which has a lot of data, but do not see anything labeled OBW. What is it?

And regarding the other adjustable parameters in that Filtering & Intermods tab, how does one know what values to add?

Clearly I need to ask JTS about this FCC ID number business.

Thanks, Henry.
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Henry Cohen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #6 on: September 09, 2017, 05:53:09 pm »

Ah, so it's the characteristics of the transmission and not the ability of the receiver to sort things out that is relevant.

Of course. The transmitter is the intentional radiator, and thus the potential polluter of spectrum. In a near-perfect engineering world, the receiver would have no emissions whatsoever when work properly.


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Entering the JTS info still results in no matches

That could be an indication that the JTS gear you're looking to purchase is not in fact authorized for sale or use in the US.


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. . . the Shure gets links to a gigantic trove of application information and engineering data. Downloading the test results gets a 128 page paper mostly showing spurious frequency generations.

Not only has Shure been filing for equipment authorizations for decades, but their submissions are quite comprehensive.
 

Quote
So if Shure doesn't gather it from non-Shure manufacturers and put it in WWB, the user is obliged to enter it in?

Yes. Just as with IAS and Sennheiser's program.


Quote
Looking at your earlier advice

"In the Filtering & Intermods tab, it's the fields for More Robust, Standard and More Frequencies: This is where to enter occupied channel bandwidth (OBW) including guardband."

I go to the ULXD data (since I don't have any for JTS) and look at the picture of the transmission spectrum (below) which has a lot of data, but do not see anything labeled OBW. What is it?

It's the spectral mask in the image you posted labeled "FCC/Industry Canada Occupied Bandwidth".


Quote
And regarding the other adjustable parameters in that Filtering & Intermods tab, how does one know what values to add?

Again, it's dependent primarily on the front end selectivity of the receiver; basically how wide is the filter and/or detector window. In the case of ULXD, Shure states that for regular mode, channel spacing should be 350kHz minimum (and 125kHz in high density mode). So in the case of ULXD, you'll see the intermod and filter values in WWB are all 350kHz, as digital systems in and of themselves have no consequential intermodulation and thus no IM products with which to contend.

In the case of analog systems, it can simply be a matter of testing to see how far off frequency the transmitter is with the receiver still showing signal on the meters and a correctly tuned transmitter unable to capture the receiver. Generally, mostly owing to fairly well established design practices and the off the shelf chipsets available, it's a fairly safe bet for mid-tier wireless products to use 500kHz, 400kHz and 300kHz for "More Robust", "Standard", and "More Frequencies" respectively. But you should confirm yourself when you wargame.


Quote
Clearly I need to ask JTS about this FCC ID number business.

A search for just "JTS" in the OET database returns 43 results, including a few operating in the 470-600MHz range. You might see the model number you're looking to purchase.
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Henry Cohen

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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #7 on: September 09, 2017, 07:45:18 pm »

A search for just "JTS" in the OET database returns 43 results, including a few operating in the 470-600MHz range. You might see the model number you're looking to purchase.

Dang, I swear I entered that on another computer and got zilch (yes, I blame my tools...), but on this computer got the same 43 results you say.

That's promising.

Unfortunately, the most recent of the results is from 2014 and the product I'm looking at is new this year AFAIK, so it's not there. How much of a lag is there in the FCC posting its actions?

Still, one of the 2014 results was interesting as it describes their test procedures just as thoroughly as the Shure document did.

Where Shure does their tests in a lead lined room IIRC, JTS does them on a roof in Taiwan.

Exhibit 6 here

They have a similar page (pics posted below) to the one I posted earlier which seems to specify the OBW as 89.7MHz, calling it Delta 1, and then shows the plot points between which it appears, calling that D1.

I don't see anything named similarly on the Shure plot, just a red symmetrical stair step with no labeling. How to read that one? (Edit: Not sure what you mean by "Spectral Mask".)

(Edit 2: Ah, is the -26db point defined as the OBW? So the Shure is like 175kHz OBW?)

And why are the D1 points at the top of that step in the emissions mountain and not further down, either at the bottom of that step or at the top of the next step down, or the one below that?

Thanks again for all this, I feel like I'm learning a lot of useful info.
« Last Edit: September 09, 2017, 07:55:47 pm by Dan Mortensen »
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Henry Cohen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #8 on: September 10, 2017, 03:16:25 pm »

Unfortunately, the most recent of the results is from 2014 and the product I'm looking at is new this year AFAIK, so it's not there. How much of a lag is there in the FCC posting its actions?

It varies. If you're referring to a testing lab uploading the test results and pass/fail status to the OET, it can be a matter of days to months. Depends on OET's backlog.


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Still, one of the 2014 results was interesting as it describes their test procedures just as thoroughly as the Shure document did.

Where Shure does their tests in a lead lined room IIRC, JTS does them on a roof in Taiwan.

OEM's very rarely do their own testing as it must be done by an FCC certified testing laboratory (CTL). The reports to OET must contain very specific information, so submissions for like products will be very similar in form and information.


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(Edit: Not sure what you mean by "Spectral Mask".)

The trace showing the emission signature/outline.


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(Edit 2: Ah, is the -26db point defined as the OBW? So the Shure is like 175kHz OBW?)

It's a scaled dBc versus distance from center frequency, and it varies from service to to service. By way of examples see Part 15 (15.209) versus Part 74 (74.861(e)(6)(i-iii))


Quote
And why are the D1 points at the top of that step in the emissions mountain and not further down, either at the bottom of that step or at the top of the next step down, or the one below that?

That's the way the CTL chose to annotate the emissions trace (and somewhat forced upon them by the analyzer's marker functionality).
« Last Edit: September 10, 2017, 03:19:49 pm by Henry Cohen »
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Henry Cohen

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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #9 on: September 11, 2017, 06:59:06 pm »

It varies. If you're referring to a testing lab uploading the test results and pass/fail status to the OET, it can be a matter of days to months. Depends on OET's backlog.


OEM's very rarely do their own testing as it must be done by an FCC certified testing laboratory (CTL). The reports to OET must contain very specific information, so submissions for like products will be very similar in form and information.


Update: The rep talked to the importer who said that my request for new frequencies resulted in needing to do the testing which has caused the delay, and that they ABSOLUTELY get everything FCC certified and wouldn't sell anything that wasn't.

I have passed along the data that I need and will report back when something changes.

Thanks again for the help. I would not have known what to ask for without this thread.
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