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Author Topic: Digital Wireless is Intermodulation Free???  (Read 12363 times)

brian maddox

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Digital Wireless is Intermodulation Free???
« on: November 22, 2015, 06:25:21 pm »

Okay, so someone on Facebook was dissing the Axient system since it was a analog and therefore subject to all the weaknesses of Analog Wireless Microphone systems.  Companding, poor sound quality, etc.  I disagree, but it's subjective so....

But then he stated that "analog systems create interfrequency modulation, and digital systems don't, because they use linear filters. Which is why digital is much more spectrum-efficient..."

Wait, what? 

So i asked him to explain and he basically just restated what he'd said before without really explaining it.  He did reference the Sony DWX wireless system repeatedly and so i looked that system up and found THIS quote on their brochure...

"WiDIF-HP enables large-scale multi-channel operation. Thanks to a digital modulator, WiDIF-HP realizes inter-modulation-free, equally spaced channel allocation, which enables a signi cant increase in the number of simultaneous digital wireless systems in comparison with current analog wireless systems."

This was accompanied by a little graphic that seemed to indicate that only Analog transmission systems created Intermod products whereas Digital transmission systems did not.

[here's the brochure if you want to see it for yourself...  https://pro.sony.com/bbsccms/assets/files/cat/audio/brochures/digitalwirelessmicsystem_a1179a.pdf  ]

So, none of this sounds right to me, but i don't feel like i know enough to be definitive about it.  So can any of our resident RF experts explain to me what is being claimed here?  Is this just a case of Sony saying something as a marketing tool that is being taken in a different way by someone that doesn't really understand RF theory?  Or am i the one that just doesn't get it?

Thanks in advance.
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brian maddox
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Henry Cohen

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Re: Digital Wireless is Intermodulation Free???
« Reply #1 on: November 22, 2015, 07:58:26 pm »

Yes, Virginia, for all intent and purposes digital modulation schemes are intermodulation free. That is why there are now adjacent TV channels within the same market and adjacent cellular/PCS channels within the same cell. Whether or not they are more spectrally efficient is a matter of application and execution.

A constant carrier, non-hopping digital transmission scheme essentially utilizes the entire channel bandwidth (1/2 power of about 150kHz for a wireless mic; 5.4MHz for ATSC DTV; 6.25, 12.5 or 25kHz for 2-way LMR), basically a Bart Simpson haircut. Perhaps the best way to think of this is that within this transmission mask is an infinite number of carriers, and when an infinite number mixes together, and with another infinite number of carriers from an adjacent or near adjacent channel, the result is an infinite number of intermodulation products - and that simply results in a higher RF noise floor, not defined carriers to which a wireless mic (com, IEM or IFB) might accidentally be tuned.

To mitigate the RF noise floor rise and out of band emissions in general, the transmitter's final power amplifier does need to be a far more linear, and have a higher saturation point, than that typically used for analog modulations. So, returning to my favorite analog (pun intended) of the audio system component, a proper RF PA or preamp will be akin to a good audio amplifier: it won't change, add to or subtract from the sound; it'll just make it louder.

About that spectral efficiency claim for digital . . .  For the most part, when designed and implemented properly for the right application, digital transmission schemes can be more spectrally efficient. Higher cellphone channel counts per cell is a terrific example, as is adjacent channel DTV transmitters. But both suffer a negative: With cellular phones voice quality is down and latencies are up compared to analog; with DTV, digital does not propagate as far and because group delay is always an issue with a broadband signal, VHF is really problematic in urban areas.

A more relevant example is that of digital wireless mics. Many of the wireless mic systems require rather large guard bands which limits the number of frequencies within a given 6MHz TV channel. Given a single TV channel, one can get more channels of [higher quality] analog mics, coms, IEMs or IFBs coordinated in. It's only when attempting to occupy multiple adjacent TV channels and the intermod math is rising exponentially that digital's efficiencies bear fruit. But the corresponding costs are latency, shortened battery life or lower RF power output, possible audio quality, and monetary cost. (I should say there is one manufacturer at this time which does produce a well priced digital product for which no guard band is required between channels, and that results in some awesome spectral efficiencies, but it too can suffer some of the aforementioned trade offs.)

So digital is not a panacea. While there are definite advantages, there are also costs involved, just as in any engineered product. The question is as always, do the benefits outweigh the negatives. As spectrum reductions continue, and that which remains becomes more and more congested, people are beginning to re-examine those trade-offs.
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Lyle Williams

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Re: Digital Wireless is Intermodulation Free???
« Reply #2 on: November 23, 2015, 03:11:57 pm »

Digital intermodulation for free?  Sounds like a bargain.  I like anything that is free.  :-)

As mentioned above, digital intermod gets scattered so widely that it doesn't feel/behave/impact the same way as traditional narrowband intermod.
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Henry Cohen

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Re: Digital Wireless is Intermodulation Free???
« Reply #3 on: November 23, 2015, 06:07:26 pm »

Digital intermodulation for free?  Sounds like a bargain.  I like anything that is free.  :-)

It's not free, as I indicated. The question is whether the costs are worth it to you (or so inconsequential in your application it seems "free").
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Henry Cohen

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brian maddox

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Re: Digital Wireless is Intermodulation Free???
« Reply #4 on: November 23, 2015, 07:48:18 pm »

Well, this has been quite enlightening.  Just when i think i understand something i find out i don't.

Which is kinda nice.

So, just to make sure i understand this.  When i do a Frequency Coordination for a bunch of ULX-D systems in WWB, the software isn't really worrying about Intermods as it would be if these were analog systems.  Instead it's mostly trying to figure out the most efficient use of the bandwidth available given the presence of DTV and number of channels required, etc.  ?

And how are things affected when digital and analog are used in the same environment?  Like, you know, on every show i ever do?  Once you've mixed in ULX-D with BTR800's and UHFR and who knows what else, how does this complicate things?  I'm assuming the WWB continues to do the math necessary to make it all work reasonably well.  I'm just curious as to what is going on under the hood.

Oh so many questions...  But i do like to know things...
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brian maddox
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       ....for you are crunchy, and taste good with ketchup...'

Scott Holtzman

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Re: Digital Wireless is Intermodulation Free???
« Reply #5 on: November 23, 2015, 08:23:33 pm »

Well, this has been quite enlightening.  Just when i think i understand something i find out i don't.

Which is kinda nice.

So, just to make sure i understand this.  When i do a Frequency Coordination for a bunch of ULX-D systems in WWB, the software isn't really worrying about Intermods as it would be if these were analog systems.  Instead it's mostly trying to figure out the most efficient use of the bandwidth available given the presence of DTV and number of channels required, etc.  ?

And how are things affected when digital and analog are used in the same environment?  Like, you know, on every show i ever do?  Once you've mixed in ULX-D with BTR800's and UHFR and who knows what else, how does this complicate things?  I'm assuming the WWB continues to do the math necessary to make it all work reasonably well.  I'm just curious as to what is going on under the hood.

Oh so many questions...  But i do like to know things...


Here are a few things that helped me in the journey to understand RF.

Even a RX is a TX from an intermodulation perspective.  Any time an amplifier is present be it an IF amp in an RX or an output amplifier in a TX, carriers can combine (mix or heterodyne) and produce spurious (incident) emissions.  Intermod products follow certain properties so once you know all the active frequencies you can model the potential products.

In an analog system a carrier is transmitted on a single frequency.  This causes a very critical and noticeable impairment in the recovered signal.

Digital systems that use a single carrier and FSK modulation follow almost the same rules, the recovered bitstream is so degraded you know immediately that the channel is compromised.

Spread spectrum radios on the other hand mitigate information by following a preset (pseudorandom) sequence through a given number of channels.  The bursts at any given frequency/time are very brief and contain a small amount of the original sample.  In fact on a spectrum analyzer, a high performance spread spectrum digital radio looks like a high noise floor, or simply broadband noise.

As the number of systems sharing the occupied bandwidth increases collisions occur.  This is the nature of the design.  Once the collisions increase to the point that the error rate (BER) of the recovered data stream is such that the decoded signal is degraded is when you have an RF problem on a spread spectrum system.

In theory mitigating interference in a spread spectrum radio system is a matter of making sure different offsets are assigned to the coding sequence (the pseudo random mentioned above, how the RX and the TX know where to look for each other) and finding the quietest chunk of spectrum to work with.

I tried to find a table of the frequency hopping schemes of the different vendors digital systems.  I am sure most of the low end are FSK, as the production systems are pushed into higher frequency spectrum spread spectrum techniques, and their interference mitigation will become common place.

To sum up, if the digital radio is on a single radio channel, intermod has the same characteristics for FM and FSK modulation.

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Henry Cohen

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Re: Digital Wireless is Intermodulation Free???
« Reply #6 on: November 23, 2015, 08:33:57 pm »

So, just to make sure i understand this.  When i do a Frequency Coordination for a bunch of ULX-D systems in WWB, the software isn't really worrying about Intermods as it would be if these were analog systems.  Instead it's mostly trying to figure out the most efficient use of the bandwidth available given the presence of DTV and number of channels required, etc.  ?

I believe so, but I don't use WWB [with ULXD's] enough to remember how it deals with the digital carrier and analog intermods. I vaguely recall the equipment profile for the ULXD essentially had only the required channel bandwidth (based on regular versus high density modes) and tuning steps, with the intermod product separations set to 0 (or maybe that's what I did?).

It's easy enough to run a couple of scenarios with the ULXD mixed with analog gear, then replace the ULXD's with analog channels attempting to use the same frequencies and seeing what WWB returns.


Quote
And how are things affected when digital and analog are used in the same environment?  Like, you know, on every show i ever do?  Once you've mixed in ULX-D with BTR800's and UHFR and who knows what else, how does this complicate things?  I'm assuming the WWB continues to do the math necessary to make it all work reasonably well.  I'm just curious as to what is going on under the hood.

In practice, because the digital frequencies do not contribute to intermod products, the digital channel can be placed anywhere that has 175kHz bandwidth (or 125kHz in high density mode) clear of analog channels and their intermod products. Using IAS, I generally coordinate them in the same zone as the analogs but, as stated above, have 0kHz for their intermod spacing. I also tend to do them last as they are arguably one of the most frequency flexible/agile devices.

The one aspect to be aware of is that a digital channel contains more aggregate RF energy than an analog channel of the same power rating. This means there's a greater chance of saturating analog TX final PA's and analog receiver front ends that don't have good selectivity and high saturation specs.
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Henry Cohen

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Henry Cohen

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Re: Digital Wireless is Intermodulation Free???
« Reply #7 on: November 23, 2015, 08:51:30 pm »

To sum up, if the digital radio is on a single radio channel, intermod has the same characteristics for FM and FSK modulation.

Unless I'm misunderstanding your intention, not really. It's sort of the difference between trying to make your way through a dense forest of large trees (analog intermod products) versus walking through some moderately tall grass (increased noise floor from multiple digital carriers.

On a pedantic note, digital carriers are not modulated: They are a constant emissions mask regardless of whether or not there is data, or how much. On the other hand, if your talking about an analog FSK signal being transmitted over digital or analog carrier, that's another matter.
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Henry Cohen

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Scott Holtzman

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Re: Digital Wireless is Intermodulation Free???
« Reply #8 on: November 24, 2015, 01:40:02 am »

Unless I'm misunderstanding your intention, not really. It's sort of the difference between trying to make your way through a dense forest of large trees (analog intermod products) versus walking through some moderately tall grass (increased noise floor from multiple digital carriers.

On a pedantic note, digital carriers are not modulated: They are a constant emissions mask regardless of whether or not there is data, or how much. On the other hand, if your talking about an analog FSK signal being transmitted over digital or analog carrier, that's another matter.
You are right, the carrier is deviated from the center frequency in both cases.  In retrospect I think my coments were even more confusing.  Modulation is an AM/SSB term.

Sent from my SM-T800 using Tapatalk

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Scott AKA "Skyking" Holtzman
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Henry Cohen

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Re: Digital Wireless is Intermodulation Free???
« Reply #9 on: November 24, 2015, 07:29:14 pm »

Modulation is an AM/SSB term.

And for FM, and PWM
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Re: Digital Wireless is Intermodulation Free???
« Reply #9 on: November 24, 2015, 07:29:14 pm »


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