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[Scott Holtzman]

FM deviates from the center freq. 

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[Diogo Nunes Pereira]

Great detailed explanation going on here... I'd vote this feed up for sticky.



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[DavidTurner]

FM deviates from the center freq. 

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Hence the term FREQUENCY MODULATION.

[Scott Helmke]

Digital wireless is more like what they used to do in modems to fit more bits onto a phone line, the more you can fit the more it looks and sounds like noise. But it's still a form of modulation.

[Scott Holtzman]

Well yeah, modulation  is the process of placing information on a carrier.  It's a decidedly analog term. 

It all depends how far the signal stays in the digital domain as to the correct terminology.  Data can be modulated on an analog carrier (what I think you are referring to, multi phase approaches like QPSK). 

Since audio requbyires both a high bit rate and is relatively intolerant of compression it would take an awful lot of RF spectrum to transmit a 20-20k audio stream with 100db of dynamic range as a digital Bitstream over a FSK or a QPSK carrier.

Wideband spread spectrum solves these issues by sharing radio spectrum, compensating for collisions and using complex math (called pseudo random coding)  CDMA uses in cellular is a brilliantly executed example of this technology at work.

Now that I talk this out I am surprised that the vendors have not come out with a spread spectrum Pico cell technology for large productions.

For those who love math and or Bell Labs history the behavior of data encoding was derived by some smart dudes at Bell Labs.  Claude Shannon and Nyquist both published extensively on the subject.

These equations work across many domains, AF, RF and even Optical cuz it's all just waves dude.

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[brian maddox]

Well yeah, modulation  is the process of placing information on a carrier.  It's a decidedly analog term. 

It all depends how far the signal stays in the digital domain as to the correct terminology.  Data can be modulated on an analog carrier (what I think you are referring to, multi phase approaches like QPSK). 

Since audio requbyires both a high bit rate and is relatively intolerant of compression it would take an awful lot of RF spectrum to transmit a 20-20k audio stream with 100db of dynamic range as a digital Bitstream over a FSK or a QPSK carrier.

Wideband spread spectrum solves these issues by sharing radio spectrum, compensating for collisions and using complex math (called pseudo random coding)  CDMA uses in cellular is a brilliantly executed example of this technology at work.

Now that I talk this out I am surprised that the vendors have not come out with a spread spectrum Pico cell technology for large productions.

For those who love math and or Bell Labs history the behavior of data encoding was derived by some smart dudes at Bell Labs.  Claude Shannon and Nyquist both published extensively on the subject.

These equations work across many domains, AF, RF and even Optical cuz it's all just waves dude.

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so.  at the risk of being "that guy".

links?

[Scott Helmke]

Well yeah, modulation  is the process of placing information on a carrier.  It's a decidedly analog term.

Well, it's a decidedly analog medium. 

Now that I talk this out I am surprised that the vendors have not come out with a spread spectrum Pico cell technology for large productions.

I believe it exists in the intercom world - HME for instance, unless you're talking about something different.

[Henry Cohen]

Well yeah, modulation  is the process of placing information on a carrier.  It's a decidedly analog term.

And really, there is no such thing as actual digital RF. The movement of electromagnetic waves - photons - is a purely analog occurrence. It's whether the information on the carrier is digital or analog (and each requires it's own emissions mask).

Wideband spread spectrum solves these issues by sharing radio spectrum, compensating for collisions and using complex math (called pseudo random coding)  CDMA uses in cellular is a brilliantly executed example of this technology at work.

Now that I talk this out I am surprised that the vendors have not come out with a spread spectrum Pico cell technology for large productions.

Several reasons: Latency for starters. A lack of appropriate off the shelf (or market size to warrant custom development of) vocoder chipsets to provide the audiio quality wireless mics and IEMs require. Cost.

But the picocell concept actually is already in the market place: Clear-Com's Cell-Com and FreeSpeak, and Riedel's Acrobat. Also, arguably, the Tempest 2400 now that it has base station roaming.

[Henry Cohen]

I believe it exists in the intercom world - HME for instance, unless you're talking about something different.

Unless you're referring to HME's Clear-Com products, the HME brand has never marketed any cellular architecture devices.

[Scott Holtzman]

I knew of the intercom products but Henry made some great points WRT the challenges.

It still would be slick CDMA uses a concept called a rake receiver,  multiple "carriers" PN sequences are received from different cell sites.  The most usable signal is voted up.  Since these "fingers"  (CDMA term not mine) are maintained concurrently the controller always has link to theoretically eliminate jerk offs (my term for a failed handoff between cells ) This technique would work really well in a closed picocell,  mesoscale network. 

As far as links.  I learned RF before this newfangled Internet.  Hard to believe IS95 (CDMA One)  is frigging 20 years old. 



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