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2.4??

Old 09-19-2011, 02:59 PM
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FlyingBrick50
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Question 2.4??

Hey Folks

I been wondering, exactly how does the 2.4 freq work? How many freqs do they use when they are hopping around? I was told they only use 10 sub freqs? Does not sound right to me, how can you fly that many planes and only use 10? Hmmmmmmm





I have a perfect flying record,....I have not left any planes up there,...........
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Old 09-19-2011, 06:33 PM
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kyleservicetech
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Originally Posted by FlyingBrick50 View Post
Hey Folks

I been wondering, exactly how does the 2.4 freq work? How many freqs do they use when they are hopping around? I was told they only use 10 sub freqs? Does not sound right to me, how can you fly that many planes and only use 10? Hmmmmmmm
OMG, that is not a simple question, and it is not easy to answer. I'm by no means an expert in 2.4 Ghz radios, having fumbled around with reading about it for the past few years. Did pick up an inexpensive "Spectrum Analyzer" awhile back that shows interesting stuff.
http://www.wattflyer.com/forums/showthread.php?t=63497

There are two main types of 2.4 Ghz radios, the Futaba type that is "Frequency Hopping" all across the 2.4 Ghz band. This design goes back to World War II, it is not a new design.

The Spektrum/JR DSSS system is a very complex mathematical processing type of transmission that finds two "Unused" channels in the 2.4 Ghz radio band, and transmits on those two. But, the DSSS system spreads the bandwidth of the transmitted signal by that mathematical process, making it very resistant to getting hit, even by someone on the same frequency.

If you want some real brain numbing reads, Google for Direct Sequence Spread Spectrum.
http://www.cs.clemson.edu/~westall/8...d-spectrum.pdf (No I don't understand all of this either. )

And now, we've got the Spektrum/JR DSMX, which looks to be a combination of Frequency Hopping and DSSS. From what I've read, DSMX generates their Frequency Hopping signals based on an internal "Serial Number" of the transmitter. So no two DSMX transmitters generate the same order of transmitted signals.

You can find tests on line through Google that JR did with 100 2.4 Ghz transmitters all turned on at the same time with no interference problems.
http://www.horizonhobby.com/DSMX/

What made all of this possible in the first place is the electronic integrated circuits that provide a complete 2.4 Ghz radio system on a single chip, with only a few external parts. That, plus the microcontrollers (Miniature computers) that control the whole mess. This stuff all has some of its origins in something many people carry in their pocket. The common cellphone.

So, which is better, the Spektrum/JR? Or the Futaba? They both work well. Before buying your system, check with your fellow modelers and find out what they are using.

As for the club I belong to, all but two club members that have gone to 2.4 Ghz have gone to either Spektrum or JR. One has a Futaba system, another has another brand. The club member that had the Futaba system lost a $$$$ turbine jet when the Futaba transmitter just shut it self off while flying. That little issue repeated itself several times while ground checking, twice when I personally was checking it out. The battery was NOT the cause. Futaba has checked it out, found nothing wrong with it. But, that is only one sample of a Futaba 2.4 transmitter problem, and it can't be used to judge overall quality of a product.
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Old 09-19-2011, 07:36 PM
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Originally Posted by FlyingBrick50 View Post
How many freqs do they use when they are hopping around? I was told they only use 10 sub freqs?
Spektrum DSM and DSM2 use 80 channels (slices might be a better word). They, as pointed out above do not hop around.

Other systems generally use 40-80 slices of the 2.4GHz spectrum band as they hop around.

Most importantly for we mere RC mortals we can be pretty happy with the fact that even though we may not know exactly how they work, they indeed do work. All of them have a little bit different spin and a lot of fancy marketing words to make one brand sound better than that other guy.

But at the end of the day I forget all that and go fly.

Mike
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Old 09-19-2011, 09:42 PM
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kyleservicetech
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Originally Posted by rcers View Post
But at the end of the day I forget all that and go fly.
LOL
Same as our cell phones. 99.9% of the cell phone users don't know how they work, and don't care.

Agreed, most all RC radio systems work well. And those that don't do not survive long in todays market.

That said, it would not make sense to put a $1000 RC system in a $100 Foamie, or a $100 RC system in a $1000 model.
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Old 09-21-2011, 05:20 PM
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Regardless of number of 'slices' system uses ... the main thing to remember is that the Rx recognises the ID code sent by the Tx at start of each data sequence. If Rx misses part of the sequence it holds at last known .. detects Id code on another slice and updates ... This happens so fast that it means we don't notice any loss in control while hops are made. But if signal is lost completely or garbled for a number of sequences - then you get the "brown-out" that some claim.

One of the reasons for different systems of frequency hopping is the power allowed to be transmitted ... I cannot remember the breakdown on this - but I seem to remember that basing on limited number of 'slices' allowed higher power, but use the whole spectrum of frequency with multiple 'slices' then means lower power allowed. Found one link : http://www.rcmodelreviews.com/dsmj.shtml

My oft scorned FlySky system uses 16 'slices' ... the FrSky similar .. both being AFHS ...
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Old 09-21-2011, 06:49 PM
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Originally Posted by kyleservicetech View Post
The Spektrum/JR DSSS system is a very complex mathematical processing type of transmission that finds two "Unused" channels in the 2.4 Ghz radio band, and transmits on those two. But, the DSSS system spreads the bandwidth of the transmitted signal by that mathematical process, making it very resistant to getting hit, even by someone on the same frequency.
Exactly. Although the math behind the spreading is pretty complicated, the principle is simple. Here's what you need to know:

1) The TX and the RX share an "encryption key" and the signal is "encrypted" (it's usually referred to as "scrambled", but the principle is exactly the same).
2) The signal is scrambled to look like noise. If you transmit a signal, your RX will be able to decrypt and make sense of it, but to your neighbors RX it will just look like background noise.
3) Here comes the kicker: Noise plus noise equals more noise. Mathematically, we'd say that the signals are independent, i.e. they share no "pattern". This means there's no interference between the two signals, even though they're on the same frequency. But if you have the right key to decrypt a signal, you'll still be able to extract it from the noise.
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