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Digital Wireless is Intermodulation Free???

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brian maddox:
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...  ]

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.

Henry Cohen:
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.

Lyle Williams:
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.

Henry Cohen:

--- Quote from: Lyle Williams on November 23, 2015, 03:11:57 PM ---Digital intermodulation for free?  Sounds like a bargain.  I like anything that is free.  :-)
--- End quote ---

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").

brian maddox:
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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