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Old Aug 31, 2007, 02:39 PM
Old fart. VAAATSP
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Futaba SS, is it FHSS or DSSS

An FCC document has been found that brings into question the true type of Spread Spectrum used by the Futabs FASST systems. All of Futabas advertisements say they use frequency hopping, but this document clearly states that the modules use direct sequence. Since this subject has taken another thread off subject, I have taken the liberty to move it here.

Later;

D.W.
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Old Aug 31, 2007, 02:44 PM
Old fart. VAAATSP
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Following is a reply brought over from the other thread:

Quote:

During the FCC test, frequency hopping was disabled. Read section 3.4, 4.4, 5.4, 6.4, 7.4 and 8.4 in the provided document.
This information is not that hard to miss, is it

Do yourself a favour and chill down.
Stress is not good for the heart.

In addition, if Futaba wasn't FHSS, they would not have gotten this certification.

Again, FHSS and DSSS do still not exclude each other.

Cheers,

Julez
All of your examples are subject to interpretation. Nowhere does it state that it was changed from FH to DS for testing. Only that it is locked to certain frequencies. This was probably done so they would know what frequency to center their analyzer on and not have to hunt around during the test. The only paragraph that plainly states modulation type is 1.1, and it states DSSS.

I don't have any idea how the other document (certification) would prove the type of modulation used. Care to elaborate?

Could this be why Futaba says their 6 channel receiver will not work with the modules? Perhaps the 6EX is FHSS, and the modules are DSSS. Anyone know where to find the FCC documents for the 6EX?



Later;

D.W.
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Old Aug 31, 2007, 02:54 PM
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I think it uses DSSS on each channel, plus a 2ms pseudo-random frequency hopping from channel to channel as well.
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Old Aug 31, 2007, 02:56 PM
You can call me FANBOY!
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All you need is the FCC ID for the 6EX and you can look it up on that same site. And what Julez said in the other thread is correct, it can be both. Often this is done to provide the benefits of both types of systems. So it very likely is DSSS that FHSS's.
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Old Aug 31, 2007, 03:20 PM
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6EX says the same thing.

https://fjallfoss.fcc.gov/prod/oet/f...ive_or_pdf=pdf
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Old Aug 31, 2007, 03:30 PM
Old fart. VAAATSP
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Quote:
Originally Posted by chrisf testpilot
Thanks! Is it not necessary to reapply for type acceptance if a the modulation type of a system is modified?

Later;

D.W.
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Old Aug 31, 2007, 03:32 PM
Old fart. VAAATSP
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Quote:
Originally Posted by mmormota
I think it uses DSSS on each channel, plus a 2ms pseudo-random frequency hopping from channel to channel as well.
Perhaps, but where can we find definitive information on this? Not saying you are wrong, but would like to see Futaba's explanation of it.

Later;

D.W.
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Old Aug 31, 2007, 03:43 PM
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Hi!

Quote:
Could this be why Futaba says their 6 channel receiver will not work with the modules? Perhaps the 6EX is FHSS, and the modules are DSSS.
This is certainly not the case, sorry. There is at least one known case where a TM7 module worked with a 6ch RX, despite Futabas claims of incompatibility.

Quote:
I don't have any idea how the other document (certification) would prove the type of modulation used. Care to elaborate?
Yes. In Europe, as opposed to the USA, the regulation about the maximum output power of 2.4GHz devices depends on wether DSSS or FHSS is used.
Using DSSS, the maximum spectral power density is 10mW/MHz, while it is 100mW/MHz for FHSS applications. As the FCC document states, the power is more than these 10mW.
Thus, the only way of getting declared as being conform to the EU regulation for the Fasst system, is transmitting with FHSS.
Here is the document:
EN 300 328
See section 4.3.2.2, that is what I'm talking about.

Cheers,

Julez
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Old Aug 31, 2007, 03:46 PM
You can call me FANBOY!
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You wont, Futaba has been as they always have, very tight lipped about it. And the incompatibility issue could very well be something Futaba programmed in to the RX and Tx.
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Old Aug 31, 2007, 03:49 PM
VE7FM
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I would love to see Futaba's explanation as well.
The first FASST system Futaba designed actually had switches to select the 2.4Ghz channel to use, pretty much the same as many 72Mhz syth systems. Obviously that wasn't going to sit well or play nice with the other 2.4Ghz systems so they changed it to FHSS. Every 2.4Ghz device they have approved for RC use is listed by Futaba as being DSSS. I haven't looked into the rules that define what a FHSS system really is but it appears by changing the channels of a DSSS system often enough it qualifies as FHSS.

I have looked at how often the Futaba modules transmit, the ground system transmits once per PPM frame, so anywhere from 50 to 125 times per second depending on the radio its connected to. So if it really switches channels every 2ms it would seem most of the channel changes are pointless.

edit:

Here is a great link, its the theory of operation from the newest Nomadio ground radio system. Its DSSS but they change the channels to make it FHSS. There is still one big difference between the Nomadio and the Futaba. The Nomadio has full two-way communication(XPS wasn't first...), the Futaba is one way only.
https://fjallfoss.fcc.gov/prod/oet/f...ive_or_pdf=pdf
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Old Aug 31, 2007, 04:09 PM
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Compatibilityof FUTABA RX-TX-Modules

I have just seen here:
http://www.rcuniverse.com/forum/m_6289503/tm.htm

that it seems, all FUTABA Transmitters are able to work with all RX (at least those, which are on the sale right now)
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Old Aug 31, 2007, 04:22 PM
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Hi!
Quote:
I have looked at how often the Futaba modules transmit, the ground system transmits once per PPM frame
Do you know how long such a transmission lasts?

Quote:
So if it really switches channels every 2ms it would seem most of the channel changes are pointless.
Unless - most of the seemingly superflous transmissions are precautionary retries, so that in case one or more transmission do not make it through, there are in fact so many of them that the data of a whole frame hardly can get lost.
I explained it here.

Cheers,

Julez
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Old Aug 31, 2007, 04:57 PM
VE7FM
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It would be great if they did send the same packet on multiple frequencies but if you measure the current drain of a Futaba TX module you'll clearly see when its transmitting.
This also works with Spektrum and XPS modules.

I can post some scope shots later but am at the shop right now.
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Old Aug 31, 2007, 05:49 PM
Old fart. VAAATSP
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Quote:
Originally Posted by Julez
Hi!


This is certainly not the case, sorry. There is at least one known case where a TM7 module worked with a 6ch RX, despite Futabas claims of incompatibility.

Cheers,

Julez
Yes, I had seen the post about getting the 6 channel receiver to BIND with a 7 channel module, but the last time I looked (it has been a few days) there was still no information as to whether it was actually usable or not.

Thanks for the information about the European documentation. Makes me feel a little better about the situation, even though I still haven't seen or heard Futaba's slant on things. Just for grins, I may post a question on one of the Futaba direct support forums just to see what they say.

Later;

D.W.
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Old Aug 31, 2007, 08:26 PM
Sink Stinks
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I'm no RF engineer (and I did NOT sleep at a Holiday Inn Express last night) but it seems pretty clear to me that they locked in individual channels to test and certify the unit. This is fairly standard practice. When ham radios get certified they do no test it on every possible frequency, but just on selected frequencies in each band.

But I forget, this is RCG and it is all about the "Gotcha'" factor. Better still if those yelling gotcha have not a clue about what they are looking at.

Oh well.
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