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Author Topic: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)  (Read 922 times)

Diogo Nunes Pereira

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Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« on: May 07, 2018, 06:19:16 am »

Hi all.

This last week I had the opportunity to visit RF World at the Eurovision Song Contest in Lisboa. Sennheiser is providing all mics and IEMs for the show and invited a bunch of EU techs to attend a seminar and tour the ESC backstage. Lucky to be among them... pretty impressed with the ESC setup.

All wireless mics are Digital 6000 systems and during the seminar they demonstrated the ability to have lots of transmitters in close proximity.

Scans from this setup were IMD free, and all transmitters were equally-spaced 600kHz. (To bad I didn't take a photo of the analyser on-screen. You'll have to trust me on that one...)

The ability to equal-space digital transmitters in frequency gives some great advantages over analog and I thought all digital wireless produced no IMD. I did make this statement in conversation and a Sennheiser gentlemen told I was wrong - that not all digital wireless produce no IMD, that their systems are free but not all brands digital RF systems perform the same...  ???

Luckily enough, a friend in Lisbon I went to visit after the seminar had a Axient Digital demo-rack in the warehouse, and I had some time to kill before taking to the airport. So I gave it a go and made some measurements:

I had four Axient digital transmitters (2 packs + 2 handhelds) in close proximity. Equally spaced them 400 kHz, because WWB profile allows for 350k spacing in the "More Frequencies" compatibility setup. With 2mW power some very low IMD could be perceived, but at high power (20mW) in the bunker I was in IMD was well noticeable.

So it does seem not all digital wireless is the same. Now, will you help understand why?

All scan files and additional screenshots in my dropbox. Link:

https://www.dropbox.com/sh/sfx28bn9eol7lh2/AADt2rMzNn5e0h3x-YVOn1-Ia?dl=0
« Last Edit: May 07, 2018, 09:22:36 am by Diogo Nunes Pereira »
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Diogo Nunes Pereira
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Dan Currie

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #1 on: May 07, 2018, 09:29:54 am »

I haven't had a chance to measure a Digital 6000.  However, my intuition is the IMD are there, just under the noise floor of his SA.

Here are picts of 2x AD1 on top of each other.  The only difference is I used my hand to cover the antenna in the first picture.  This effectively lowered the noise floor of the SA revealing the IMD.



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Jean-Pierre Coetzee

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #2 on: May 07, 2018, 10:44:09 am »

I agree with Dan, I don't see how changing from analogue to digital changes the laws of physics.
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Jason Glass

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #3 on: May 07, 2018, 11:54:44 am »

Hi all.

This last week I had the opportunity to visit RF World at the Eurovision Song Contest in Lisboa. Sennheiser is providing all mics and IEMs for the show and invited a bunch of EU techs to attend a seminar and tour the ESC backstage. Lucky to be among them... pretty impressed with the ESC setup.

All wireless mics are Digital 6000 systems and during the seminar they demonstrated the ability to have lots of transmitters in close proximity.

Scans from this setup were IMD free, and all transmitters were equally-spaced 600kHz. (To bad I didn't take a photo of the analyser on-screen. You'll have to trust me on that one...)

The ability to equal-space digital transmitters in frequency gives some great advantages over analog and I thought all digital wireless produced no IMD. I did make this statement in conversation and a Sennheiser gentlemen told I was wrong - that not all digital wireless produce no IMD, that their systems are free but not all brands digital RF systems perform the same...  ???

Luckily enough, a friend in Lisbon I went to visit after the seminar had a Axient Digital demo-rack in the warehouse, and I had some time to kill before taking to the airport. So I gave it a go and made some measurements:

I had four Axient digital transmitters (2 packs + 2 handhelds) in close proximity. Equally spaced them 400 kHz, because WWB profile allows for 350k spacing in the "More Frequencies" compatibility setup. With 2mW power some very low IMD could be perceived, but at high power (20mW) in the bunker I was in IMD was well noticeable.

So it does seem not all digital wireless is the same. Now, will you help understand why?

All scan files and additional screenshots in my dropbox. Link:

https://www.dropbox.com/sh/sfx28bn9eol7lh2/AADt2rMzNn5e0h3x-YVOn1-Ia?dl=0
The typically low IMD strengths of digital modulation schemes are not strictly by virtue of them being digital, or even AM. They are far more the result of tech advancements making their hardware more linear, better shielded, and more narrowly bandpass filtered at the desired frequencies of operation.

Sent from my Nexus 5 using Tapatalk

Lyle Williams

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #4 on: May 07, 2018, 04:38:50 pm »

Intermodulation products are created, but are scattered far and wide by all the source signals being constantly moving.
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Henry Cohen

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #5 on: May 07, 2018, 10:09:11 pm »

The typically low IMD strengths of digital modulation schemes are not strictly by virtue of them being digital, or even AM. They are far more the result of tech advancements making their hardware more linear, better shielded, and more narrowly bandpass filtered at the desired frequencies of operation.

This is key.
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Henry Cohen

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Henry Cohen

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #6 on: May 07, 2018, 10:15:31 pm »

Intermodulation products are created, but are scattered far and wide by all the source signals being constantly moving.

Not moving, Rather spread out, akin to a fully deviated FM signal. Presuming highly linear gain stages, if there is any mixing of of signals, it will result in an overall rise in the noise floor, not defined carriers experienced with typical FM systems.
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Henry Cohen

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Henry Cohen

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #7 on: May 07, 2018, 10:17:56 pm »

I agree with Dan, I don't see how changing from analogue to digital changes the laws of physics.

Same physics, but different constituent carrier spectral mask, thus the results will be different form that of analog.
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Chris Eddison

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #8 on: May 08, 2018, 02:04:41 am »

I was speaking to a Sennheiser rep last week at a trade show here in the UK and he too mentioned about 6000 being intermod-free. He said that they were using isolators on the RF output to ensure nothing got ďback up emí ď (Dadís army quote for anybody this side of the Atlantic).
I was actually looking at the G4 EW100ís in the 1.8GHz frequencies and he said these had the same isolators as the 6000 because there was space on board due to the higher frequencies and therefore smaller component sizes.
Iím seriously considering buying some of these G4 systems to move me away from 2.4GHz (which Iím currently using with Line6, so the prospect of simply spacing my channels by 600KHz is quite appealing.
« Last Edit: May 08, 2018, 03:24:02 am by Chris Eddison »
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hugovanmeijeren

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #9 on: May 08, 2018, 07:57:54 am »

He said that they were using isolators on the RF output to ensure nothing got ďback up emí ď

That is correct, Sennheiser 6000 series is 'intermodulation free' because of extremely linear rf amps and the use of rf isolators in all transmitters.
I'm not absolutely sure whether the Shure Axient Digital is also using isolators, but I don't think they have them...
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Dan Currie

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #10 on: May 08, 2018, 09:11:46 am »

That is correct, Sennheiser 6000 series is 'intermodulation free' because of extremely linear rf amps and the use of rf isolators in all transmitters.
I'm not absolutely sure whether the Shure Axient Digital is also using isolators, but I don't think they have them...

The quick test I did was a worst case scenario with the packs as close together as they could get.  The next question would be does it affect rf performance.  In the case of the AD the answer is no.  If someone around the Detroit area has a 6000 series available I would be more than happy to do a direct A/B comparison.     
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Diogo Nunes Pereira

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #11 on: May 09, 2018, 04:32:05 pm »

Not moving, Rather spread out, akin to a fully deviated FM signal. Presuming highly linear gain stages, if there is any mixing of of signals, it will result in an overall rise in the noise floor, not defined carriers experienced with typical FM systems.

That was my understanding also, and what I would expect: a raise in the noise floor... but upon measurement, and as the scans/screenshots show, you can see defined carriers caused from the digital transmitters in close proximity.

At 20mW (true, in a low ceiling, very bunker-like stretch of a warehouse) it does seem to me the power of the IMD products can't be disregarded, and the equipment profile in WWB6 for Axient Digital (ULXD and QLXD also) does disregard IMD, at least in the "more frequencies" scheme. Having 4 Tx as close as I did aren't common practice fortunately.

That is correct, Sennheiser 6000 series is 'intermodulation free' because of extremely linear rf amps and the use of rf isolators in all transmitters.
I'm not absolutely sure whether the Shure Axient Digital is also using isolators, but I don't think they have them...

This makes sense... circulators probabilly are key to their design.

The quick test I did was a worst case scenario with the packs as close together as they could get.  The next question would be does it affect rf performance.  In the case of the AD the answer is no.  If someone around the Detroit area has a 6000 series available I would be more than happy to do a direct A/B comparison.     

During my tests, while producing intermod at a given frequency and then firing the transmitter allocatted to that same frequency didn't seem to impact the audio/link-quality. Receiver still reported 5 stars quality and sounded good. Range would still be an issue of course... I get the part where physics are still physics ;)
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Diogo Nunes Pereira
elraval@gmail.com
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"I envy not those who own charriots, horses or land. I envy only those who drink water from every spring." - Popular Song from Portugal

Jason Glass

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #12 on: May 10, 2018, 05:44:50 pm »

I was speaking to a Sennheiser rep last week at a trade show here in the UK and he too mentioned about 6000 being intermod-free. He said that they were using isolators on the RF output to ensure nothing got ďback up emí ď (Dadís army quote for anybody this side of the Atlantic).
I was actually looking at the G4 EW100ís in the 1.8GHz frequencies and he said these had the same isolators as the 6000 because there was space on board due to the higher frequencies and therefore smaller component sizes.
Iím seriously considering buying some of these G4 systems to move me away from 2.4GHz (which Iím currently using with Line6, so the prospect of simply spacing my channels by 600KHz is quite appealing.
Isolators such as these are precisely why Lectrosonics products from the last decade or so can operate at 100 mW in close proximity and not kludge the noise floor. It's also why their freq blocks have such narrow tuning bandwidth. It's extremely difficult to engineer isolators with wide bandwidth and small size at UHF freqs.

Sent from my Nexus 5 using Tapatalk
« Last Edit: May 11, 2018, 10:46:02 am by Jason Glass »
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Lyle Williams

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Re: Digital Wireless IMD (Sennheiser 6000 vs. Axient Digital)
« Reply #13 on: May 11, 2018, 07:44:12 am »

Not moving, Rather spread out, akin to a fully deviated FM signal. Presuming highly linear gain stages, if there is any mixing of of signals, it will result in an overall rise in the noise floor, not defined carriers experienced with typical FM systems.

I remember struggling to find the right word to cover both DSSS and other schemes.  Moving wasn't quite the word I was after, but I worried about wording my answer in a way that was only meaningful to someone who already "got it".
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