Discussion Futaba SS, is it FHSS or DSSS

[Rudy Vienna]

Does this help? http://www.rcuniverse.com/forum/m_6289503/tm.htm

Answer of Bax was revised (without leaving a trace!!) after a short discussion.

[Julez]

Hi!

Steve told me, that he used the current drain of his modules to find out, when they transmit and when not.

I figured, that each current burst has to lead to a voltage depression.
So I hooked my soundcard oszilloscope to the voltage supply of my transplantend T6EX Fasst module, and voilá:
Clean, rectangular pulses of voltage depressions.
These have to come from the module needing power to transmit a burst, as these pulses were not visible, when I started my TX on PCM mode, leading to the module blinking "error" at its LEDs, and likely not transmitting anything.

However, I am not too sure how to interprete these pulses.
My radio has a 43Hz framerate, meaning that one frame lasts 23ms.

Each transmitting burst of the module lasts 2ms, with a framerate of 125Hz. Every 8ms, a new burst starts.
For every frame, one has at least 2 bursts; 2.875 to be precise.

The bad news is: Something is wrong with Futaba claiming that "the frequency shifts every 2ms". When the module does not transmit, changing the frequency is pointless, right

This makes, however, perfect sense when one takes into consideration the fixed frametime of the reciever, which is 16ms. So the reciever is updated twice for every frame. These numbers fit together so nicely, it simply has to be true

If there are any other interpretations possible, please post them.

I also asked the Futaba support about this.
I wonder what they have to say.

Cheers,

Julez

[d_wheel]

Originally Posted by Silent-AV8R

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.

Wow! The above sentence is a REALLY good example of a "gotcha"! Thanks!

Later;

D.W.

[TheSteve]

Julez,
Interesting find, I wonder if its sending the full data in each packet or if it sends half the channels in the first packet and half in the second.

I know on the ground FASST system it did one transmit burst for each set of PPM pulses(3 channel radio running at 70Hz)

EDIT - here is a picture of the TX pulses(ch1) along with the current spikes(ch2) that mean the module is transmitting.

[Julez]

Steve,

You are spot on with your assumption.
Fasst does not transmit all channels with each burst.
I mixed a switch on channel 2 and 5 at my transmitter. When I flip the switch, the pulswidth change of both channels occurs synchronous in the PPM pulsetrain of my TX, as verified with the scope.

If Fasst transmitted all channels' information with each burst, the pulsewidth change would also occur synchronous in each reciever frame.
It does not.
In some cases, channel 5 is updated earlier than channel 2.

My explanation is the following: After each sampling of the pulsetrain, the gained information is stored in the Fasst module. This information is used when the module transmits the 1st burst for the first channels, and when it transmits the 2nd burst for the last channels.
However, these bursts with a length of 2ms occur every 8ms.
Thus it is possible, that the pulsetrain information is updated between the 1st and the 2nd burst. In this case, the last channels get "fresher" information than the first ones.
Exactly this event one can see in the scope screenshot.
In the 2nd frame, channel 5 is updated, while channel 2 still has its old value. Finally, in the 3rd frame, channel 2 is also updated.

One can discuss the advantages and disadvantages of this technique.
The advantage is, that the average latency of all channels is reduced.
The disadvantage is, that there can be a latency difference of up to 16ms between individual channels in one and the same frame.

Perhaps some heli guy can elaborate on "swashplate dance"?

However, only transmitting half of the channels in each burst is surprisingly few information. Perhaps this contributes to the robustness of the transmission?

Cheers,

Julez

Edit: The above information was deleted within minutes in the RCU Futaba support forum

[ricoalonso]

Julez,
I understand you have a Futaba 6-Ch FASST , correct? Any updates as to any problem you may have encountered? For example: power, loss of link, etc.??

[Julez]

Originally Posted by ricoalonso

Julez,
I understand you have a Futaba 6-Ch FASST , correct?

In a way, yes.

No problems to this day. The best RC investion I ever made. I did not regret it one second.

[Rudy Vienna]

Me too, no problems.

Rudy

[ricoalonso]

Thanks for the updates. Now, if only we can hear soon about the FASST Modules (Tx/Rx) it would be nice.

[krysta]

Hello DW,

We here at Futaba are also aware of the posting regarding the ability to bind the TM-7 module to the R606FS. Please our statement regarding this on the FAQ's on our website at:

http://www.futaba-rc.com/faq/faq-receivers-q923.html


Sincerely,
Krysta
Product Development & Support Specialist
Futaba Service Center USA

Originally Posted by d_wheel

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.

[bjpaul]

I have heard that the R606FS receiver can be forced to work with the TM-7 module. The 2.4GHz website states that they are not compatible, how do you explain this?

While the R606FS and TM-7 module can be made to work together be a series of steps, Futaba does not recommend this practice. Doing so could be very dangerous if hit with interference or loss of battery power as the receiver would not re-link on its own to the module without the repeated series of steps.

Well I guess it's not just Chinese manufactures that can massacre the English language.

[Julez]

Very interesting, thanks!

[krysta]

Thanks, for pointing out my typo, I will have this corrected.
Krysta

Originally Posted by bjpaul

Well I guess it's not just Chinese manufactures that can massacre the English language.

[d_wheel]

Originally Posted by krysta

Hello DW,

We here at Futaba are also aware of the posting regarding the ability to bind the TM-7 module to the R606FS. Please our statement regarding this on the FAQ's on our website at:

http://www.futaba-rc.com/faq/faq-receivers-q923.html


Sincerely,
Krysta
Product Development & Support Specialist
Futaba Service Center USA

Thanks Krista. Makes perfect sense.

Are there any problems using the R607FS receiver with a T6EX transmitter?

Later;

D.W.

[TheSteve]

Nice find Julez. I think I will have my own 8 channel system transmit after the first 4 channels and again after the second 4. The wireless module overhead is such that it will work fine.
Gotta wonder why Futaba didn't make the 6 channel receiver 100% compatible with the modules.