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Author Topic: How To Figure Analog RF Mic Packing into TV Channels?  (Read 5206 times)

Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #10 on: September 15, 2017, 03:42:22 am »



The trace showing the emission signature/outline.


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))


JTS replied:

(Occupied Bandwidth)" is +/- 75 KHz. This is the standard of frequency modulation."

Spacing:

        "More frequencies: 250 KHz

        "Standard : 300 KHz

        "More robust: 400 KHz.

        "These data are measured with 10 mW transmission power." So "Low" vs. "High".

So now I have all I need to know to set up WWB to predict?

Thanks again!
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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #11 on: November 30, 2017, 03:32:03 am »

Since last writing about this subject, another thread in this forum asked about Intermods, and I posted some results of my scans of 2-transmitter intermods from the JTS mics. Those results, while interesting and enjoyable to generate and capture, did nothing to give numbers to put into the Filtering and Intermods page of WWB. Asking other people who know more about the subject than me similarly did not provide anything specific about coming up with values for the empty boxes on that page.

Two days ago I called Shure Support, or a number that I'd seen posted on Shure forums by Shure for Shure Support, and the number turned out to be the Director of Manufacturing or something like that, and he generously took my information and passed it on to a couple of Support guys, including Tim Vear, who had so generously helped me at the last AES convention with understanding WWB, as reported earlier ITT.

Tim called yesterday and we talked for over an hour, I think, and this is a summary of the relevant part of that conversation to the best of my recollection, originally written a few hours after the conversation. We also talked at length about ULX-D High Density mode and why/why not to use it, Inclusion Groups, and some other things.

It turns out that the Filtering and Intermods tab is decipherable by sections. (Unfortunately the format of this forum always puts attached pictures at the bottom of the post, so you may have to scroll back and forth between the words and the picture to see what I'm talking about.)

The part labeled “Filter” is for the front end of the receiver, and specifically the filter that tunes to the transmitter frequency and hopefully blocks out the rest, beyond a certain distance specified in the “Start” “End” section. That information (how much distance spacing) is not released by manufacturers to the public, but similarly priced systems are likely to have similar performance. He suggested that the JTS is somewhere between the SLX and the ULX Shure systems, which are pretty gentle in their filtering and need wide spacing between channels.

The pull down menu above that (“Tracking” or “Fixed”) for the front end filter will almost always be “Fixed”, because the “Tracking” electronics are very expensive. ULX-D has Fixed, for example, so it’s not expensive enough!

Lectrosonics has a choice when ordering between Tracking and Fixed, and in WWB you can switch back and forth in the Lectrosonics profile to see the differences between Tracking and Fixed in the Filter Selectivity.

The “Intermod Source” should always be checked.

Channel Spacing is the only thing the Manufacturer provides usually, and it can also be inferred by going through the Group charts that they generally provide with each mic, to observe the minimum spacing on the Group(s) with the most available channels within the group. The JTS has two groups with 22 possible channels, and the minimum spacing is 700kHz in one and 750kHz in the other. Presuming that they selected the frequencies in the groups from measurement using all relevant parameters, those numbers are reasonable for More Frequencies, and they are not very high numbers at that. If they didn’t get their channel spacing conclusions from measurements….

The Intermod spacings refer to the input filter selectivity, how much spacing from the center frequency will filter out the 3rd order harmonics without affecting the primary frequency, and various 5th, 7th, and for sure 9th and beyond are ignored. 5th’s and 7th’s are sometimes close enough to the center frequency to maybe cause problems, but the higher harmonics are for sure by definition far enough outside the input filters to be ignored. Those filters will wipe them out.

That Spacing spec has nothing to do with the width of the harmonic itself.

He thought that I will be safe if I use the values of the SLX (worst Shure bandwidth hog), and probably OK if some of the values approach those of the ULX, which is a little better than the JTS in most ways.

Cool, huh?

He described how I could reverse engineer the mics to get more accurate values, but that it would take a LOT of time, trial and error. I’ve been spending a lot of time on it already as I return to Student mode, and we’ll see if I’m motivated to pursue this further or just accept the SLX/ULX values.

All this, of course, is if I remember correctly what he told me. It all made sense and put a lot of pieces together, so I think what I wrote is what he said.

I’m still in the process of getting all these new mics working together and organized, and after we hung up I spent a couple hours entering values for their highest channel count Groups and getting the mics to work together, which was fun to see. Having all 20 on at once, and turning one at a time off to see if there was visible interference, was real interesting.

Happily, they don’t seem to generate clear interference, although I wasn’t impressed with the signal stability as I could get visible wavering that was clearly related to how my hand was moving through space. Wave hand, lots of meters drop.

Thinking about it since, I only had one of the antenna inputs connected through to all five rack units (4 receivers each). The JTS R4, like the ULX-D, uses antenna diversity rather than true diversity, so the antenna switcher is making the jump to the other antenna when the signal gets worse without first checking to see that the second antenna has better signal than the first. Since I didn't have the second antenna hooked up (only one input on my scanner, so I only thought I needed one antenna connected throughout, with the last rack unit's Cascade output going to the scanner input), there was nothing for it to go to. You would think that would result in a visible drop out, but that if there had been a second antenna hooked up there would have been a good signal there (transmission distance was about 10 feet).

Speaking of those Cascading antennas from rack unit to rack unit so you don't need a distro, unlike Shure who recommends 3 units cascaded max, JTS is comfortable with something like 12 or more rack units cascaded. I measured signal strength from 2 to 5 units together and didn’t see significant signal strength reduction, but I don’t yet have a generator to check that or cable integrity. I plan to get one first of the year.

Lastly, since I don't think I'll have anything more to report in this thread, I earlier learned the hard way why it's better to connect the scanner to the actual antennae that the microphones use rather than using an antenna that attaches directly to the scanner.

I was doing a gig a few weeks ago with lots of wireless mics (31, mixed brands) and for the first time decided to connect scanner to mic antenna. All seemed as it should be. I needed to change something, which required the scanner to source from to the other side of the last ULXD4Q in the ULXD branch of antenna distro.

Instead of the nice squarish waves of TV channels and spikes of mic channels, the display was a nearly flat line with a few short spikes sticking up.

The mic screens were all displaying normally and sound was coming from all of the mics, so further research waited until after loadout in the shop.

Long story slightly shorter, it turned out that one whole side of the antenna distro was not working, and I have no idea how long that's been the case. That distro is about 4 or so years old, and I've never noticed an issue with it.

The funny thing is that it is pretty clearly OEM'd by JTS and sold under another name. Connectors and silk screening are exactly the same as an older Audix distro that I have, although the power switch is a different format, while the latter looks EXACTLY in every way like the new JTS distro that came with the new mics. The Audix must be at least 15 years old!

I mentioned both the similarities and the problem to the JTS US importer (who has coincidentally been assured that our new mics both have FCC listing and that the paperwork will be with us this week), and he immediately sent a return label so his repair guy can look at it and advise on repair cost. He didn't have to do that but generously did anyway.

So from now on I'm going to only connect scanner to mic antennas, and am looking for a way to be able to connect to both antennae at once. Maybe I'll start another thread about that.

Thanks for reading.
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Scott Helmke

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #12 on: November 30, 2017, 10:30:56 am »

Cheap antenna distros, don't get me started.  I've done a couple repair calls to a school that has 10 channels of Audio-Technica 3000 mics. All the mics and receivers have been fine, but I think at least 3 of the 4 distros have failed. One of them was producing RF interference all on its own somehow. 
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Jason Glass

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #13 on: November 30, 2017, 12:12:37 pm »

Speaking of those Cascading antennas from rack unit to rack unit so you don't need a distro, unlike Shure who recommends 3 units cascaded max, JTS is comfortable with something like 12 or more rack units cascaded. I measured signal strength from 2 to 5 units together and didn’t see significant signal strength reduction, but I don’t yet have a generator to check that or cable integrity. I

The main reason for limiting the number of cascades is not reduced signal strength as much as it is increased noise and distortion, which reduce CNR.  Good cascades are designed to amplify just enough to make up for inherent spltter losses and achieve unity gain. When too many are stacked, each cascade amp not only adds more noise at idle, but also compounds slight input overload conditions that might otherwise be tolerable into harmful levels of distortion.

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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #14 on: November 30, 2017, 02:57:46 pm »

The main reason for limiting the number of cascades is not reduced signal strength as much as it is increased noise and distortion, which reduce CNR... When too many are stacked, each cascade amp not only adds more noise at idle, but also compounds slight input overload conditions that might otherwise be tolerable into harmful levels of distortion.

Interesting.

I'm guessing my scanner does not register distortion, but it does show a noise floor. Am I watching for the noise floor to rise when measuring from one rack unit down to the next/last in line?

If so, the signals present would stay close to their same levels but not stick up as high from the noise floor?

Thanks.
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Dan Mortensen

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #15 on: November 30, 2017, 02:59:11 pm »

Cheap antenna distros, don't get me started.

Actually, please do get started. What is a good non-cheap one? And what approximate price level is non-cheap?
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Jason Glass

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #16 on: November 30, 2017, 03:15:31 pm »



Interesting.

I'm guessing my scanner does not register distortion, but it does show a noise floor. Am I watching for the noise floor to rise when measuring from one rack unit down to the next/last in line?

If so, the signals present would stay close to their same levels but not stick up as high from the noise floor?

Thanks.

Yes, in RF, one of the ways distortion appears is as a rise in the noise floor.

It also doesn't necessarily require a single signal above a threshold to cause distortion; it can also be caused by the vector sum of all energy entering the device at every frequency.  This is why places with heavily crowded spectrum, such as Dallas, are so challenging for inexpensive wireless equipment. The expensive stuff has better filtering to keep out of band energy from entering active circuits, and those circuits are more linear than the cheap stuff.

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Scott Helmke

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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #17 on: November 30, 2017, 06:16:54 pm »

Actually, please do get started. What is a good non-cheap one? And what approximate price level is non-cheap?

We've been very satisfied with Shure units - UA844, UA845, etc.  I can't recall one of either, out of the dozen or two we own, going bad.  MSRP $499 for a UA844, which nowadays is a 5-output splitter for two antennas. UA845, also a 5-output but a little more professional (no line lump) costs roughly twice that.
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Re: How To Figure Analog RF Mic Packing into TV Channels?
« Reply #17 on: November 30, 2017, 06:16:54 pm »


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