The LEVEL of interference required to kill the DSM2 is likely to also kill other types.
Does that statement appear true to you?

My reasoning goes like this:

Lets say, you have a DSSS system with one channel placed randomly somewhere in the band.
Now, you have interference. At first, only on one channel, then on 2, then on 3, and so on, placed randomly. The risk of having a lockout rises linearly with the amount of channels being swamped.
It is better with the multichannel DSSS system, but you also have a lockout, when the channels of the system happen to be swamped.

When using the FHSS system, you will still have control when all but one channels are saturated. However, the average latency will rise with the amount of channels that are unusable because of the noise.

Assuming that a random number of random channels will suddenly be saturated with noise during the flight, the yellow area will represent the total risk of a lockout in a prolonged time.

As you can see, this area is smallest when using an FHSS system.

Bruce has pointed this out in better english than mine:
http://www.flyinggiants.com/forums/f...tml#post409160

To come back to your original question I quoted:
Given a definite amount of channels which are suddenly swamped, the DSM2 system will suffer less lockouts than a conventional one-channel DSSS system.
But, a FHSS system will suffer even less.
Thus, it is safe to say, that when a FHSS system will be killed, it is likely that all other DSSS system will be killed, too.

Where I live -in a vast wasteland of wild Indians and coyotes, we flyers using XPS/FASST and DSM2 -simply see no interferrence problems

Normally, as you correctly state, we are usually flying on the far left side of the graphs. But it is never wrong to prepare for the worst.
If it was not beneficial, Spektrum would not have chosen 2 channels instead of one. One could conclude, that 3 channels are even safer, 4 even more safer, and so on. A DSSS system transmitting on all channels at the same time would be the safest.
Unfortunatelly, more than one of these supermultichannel DSSS systems would step on each others toes, making the operation actually less safe. This is why Spektrum scans for free channels on startup.
So, a system designer has to weight between the advantages of using many channels for safety, and the disadvantage of many channels disturbing each other, when multiple systems are used.

The best possible solution would be, to be somehow able to use all channels, without multiple systems disturbing each other.

And this is why I like FHSS so much: It actually makes use of all channels, but it only uses one channel at a time, thus not disturbing other systems.


Generally about the graphs: When talking about FHSS, in this case I mean a DSSS/FHSS hybrid like Fasst or Airtronics. There are no pure FHSS systems on the RC market that I know of.

Concerning the Latency of FHSS in presence of strong interference: Fasst will have a maximum latency of 0.288s, when only one channel can be used to get the information through. Although it is probably not nice to fly with such a latency, it is possible.
I once had the BEC of a cheap chinese ESC overloaded, and I had about 0.1s of control every second. It was not nice, but I was able to bring the plane down with a soft landing. <0.3s latency look like a piece of cake compared.

Bruce, Ian, Frank, Holger, your comments would be appreciated. All others too, of course

Cheers,

Julez

Originally Posted by richard hanson

The LEVEL of interference required to kill the DSM2 is likely to also kill other types.

Interesting question. This should be tested onetime.

In fact both systems (Spektrum and FASST) seems to work interference-free. Even in Germany, where there is a power limitation for the Spektrum-systems, users of Spektrum-system are reporting of at least 1 kilometer range at ground and, of course even more in the air.

This is finally all, what counts: Does a system work? Is it reliable? Do I have problems installing the system?

The technical way, the technical aspects are nice to know, but..........see above.

Greetings
Udo

The technical way, the technical aspects are nice to know

And this is exactly why this thread was started.

Thanks for the answers - Julez and Udo.
Being basically a lay person where this subject is concerned --I have to look at results with both systems -- as FIRST attempt to rationaize which may have a demonstrable advantage.
So far -I have not seen interference as a worrisome factor for either system - again looking at results.
The prime problem for either that I see, is how the user (typically a layperson as applies to this field, ), follows information provided. Some will --some won't
Good info is supplied by the mfgrs but --not always followed - This I know from many years of dealing with modelers and generally consumers as a group.
The RESULTS--when the 2.4 systems are properly used -is simply put - excellent.
My preferrece for the DSM2 is really solidified by the features offered which are NOT related to the situation (comparison of type of signal)we have been discussing.
As a layperson-I like features like the Model Match-which is one more layer of insuring safe operation -especially in the case of electric powered models --
I love these discussions especially when they are not accompanied by wild eyed rants --

Even the 72 mhz systems are far more likely to have a failure due to poor installation or the RX switch just going bad than from interference...

So the higher interference resistance of 2.4 ghz means that you probably will NEVER see an interference based crash.

I have had one definitive 72 mhz interference crash... back when flying AM. Every other "glitch" induced impact got traced to something in my own airplane that failed.
(most offten a switch... buy good switches and be careful how you install them.)

Many people CLAIM a "glitch" caused thier crash... when the only glitch was between the brain and the stick.

We have a known issue of an area at our field that (for Futaba) gives full down elevator (we seem to all have the same servo direction on elevator) for any 72 mhz frequency associated with "channels" 50 and up. That area of airspace is a "tube" appx 75 ft dia well above the trees and a little to the north of our normal flight area... planes that experience the glitch are recoverable if not dead-stick. (if dead stick it could cause inability to clear the trees heading to the runway)
So even our KNOWN interference issue isn't a plane killer...
(we think it has to do with a couple of microwave towers... since it is in the line between towers)

We also have a "ridge" at the end of the runway... that due to the way the wind blows can produce a WICKED "rotor" and has driven unwary pilot's planes into the dirt... looking like full down elevator was applied.

As far as can be determined from looking at photos of receivers and tracing patents, the Futaba receiver uses a single receiver and an antenna switch which relies on some form of packet preamble to decide whether it needs to switch to the other antenna. This sounds pretty slick for a static system but may have issues for a moving system.

Do any of the experts out there have an opinion on this?

Seems to work for them...

"Seems to work for them..."

Nice to know you have a sence of humor but but I was hoping for something a little more scientific. XPS seems to work for a lot of people too............

Well, I just had a look again at the patent.
My interpretation is, that during the transmission of the preamble, which consists of a number of bits with the same value, the antennas are switched at a time. Afterwards, the antenna is selected, which recieved the higher number of correct bits.

This sounds pretty slick for a static system but may have issues for a moving system.

Your concern is, that during the transmission of the actual data, the reception environment might have changed to an extent, that the antenna selection due to the preamble reception method might not be the best anymore in this new environment?

I just did some quick calculations. As we now, a burst does not last longer than 2ms. It is well possible that such a burst might be further divided into multiple packets with multiple preambles.
But for the sake of argument, lets just assume, that one burst of 2ms only consists of one preamble and one packet.
Now, how much does the reception environment change in 2ms?

Lets say, we have a plane moving at 100mph.
In 2ms, it would move stunning 3.5 inches.

Now we have a plane that makes 2 rolls / sec.
In 2ms, it would bank stunning 1.44°.

I think we can agree, that this change of position and orientation is not sufficient to fundamentally change the reception environment.

Cheers,

Julez

Originally Posted by richard hanson

There is yet another thread whre the guys are trying to evaluate radios by turning off tx
Why not simulate tx efficiency by removing cells from the tx then timing how long it takes to replace them
Sorry -but some of these tests are just -plain- meaningless.

Suppose you're flying your 135 mph jet and your mom calls, you have to put your girlfriend on hold to pickup your mom's call because your dad has not taken a dump in 6 days and you want to know how he is doing. After speaking with your mom, you go to put your cell phone back on your belt clip but it gets tangled in your ipod headphone wires, as you free it up you realize you had your girlfriend on hold but when you go to resume her call you find she's hung up on you. Oh oh, now she's mad at you and you know how bad that can be, so you you go to call her back but accidentally call your wife instead and call her by your girlfriends name. In a panic you intend to press the 'end call' button on your cell but shut off your transmitter by mistake and have lost control of your 135 mph jet. A fast reboot time would be important here!

Now aren't you glad people are doing "shut off" tests for Real Life situations like these?

Originally Posted by CyberPilot

A fast reboot time would be important here!

And especially with that AR6100 receiver in the 135 mph jet!

Ivan

Julez,

"Lets say, we have a plane moving at 100mph.
In 2ms, it would move stunning 3.5 inches."

3.5 inches = 8.9 cms ~70% of a wavelength at 2.4 Ghz so yes I do believe the distance is significant and remain unconvinced of the robustness of the system under some conditions.

Of course, in use you can lose a lot of packets and still be under "full control". If Futaba provide a flight logger like the Spektrum we won't actually know how effective the system really is.

Originally Posted by CyberPilot

Suppose you're flying your 135 mph jet and your mom calls, you have to put your girlfriend on hold to pickup your mom's call because your dad has not taken a dump in 6 days and you want to know how he is doing. After speaking with your mom, you go to put your cell phone back on your belt clip but it gets tangled in your ipod headphone wires, as you free it up you realize you had your girlfriend on hold but when you go to resume her call you find she's hung up on you. Oh oh, now she's mad at you and you know how bad that can be, so you you go to call her back but accidentally call your wife instead and call her by your girlfriends name. In a panic you intend to press the 'end call' button on your cell but shut off your transmitter by mistake and have lost control of your 135 mph jet. A fast reboot time would be important here!

Now aren't you glad people are doing "shut off" tests for Real Life situations like these?

If you are playing with that much other electronic stuff....

Your hands aren't on the TX sticks anyway.

So you won't notice the radio system getting sluggish (if it does)

3.5 inches = 8.9 cms ~70% of a wavelength at 2.4 Ghz so yes I do believe the distance is significant and remain unconvinced of the robustness of the system under some conditions.

Ok, but this would affect all other systems, too.
Unless, of course, you tell me that individual bits, and not only whole packets are compared in other systems.

But I think we can agree, that 2.4GHz equipped planes, once out of the Fresnel Zone, do not have problems with reflected radiation from the ground.
Thus, the only things reflecting stuff are conductors in the plane itself, and these do not change within 3.5 inches of movement.

Futaba's original antenna patent was intended for their wireless modems. That is, two vertical antennas spaced apart. The antenna patent allows for selecting the antenna which has the best signal - not in a multipath fading situation. This is the kind of problem teamdavey is concerned about - moving a fraction of a wavelength.

In the RC application, the antennas are at 90 degrees. The antenna switching allows mainly for polarization diversity. The receiver actually does not know why one antenna has a better signal than the other - fading, polarization, blocking, etc. Some frames will be lost. That's why WLAN protocols have acknowledge and retry.

The idea is that two antennas have a lower probability of missing a data frame than one - analogous to the entire Spektrum argument about two channels.