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That bane called interference

Old 06-07-2012, 09:59 PM
  #26  
JetPlaneFlyer
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fmw,

I'm no expert but i don't think it's that simple, not by a long shot. If other transmitters transmitting on the same frequency was a problem then you would not be able to fly two Spektrum systems at the same time.. When the reality is that you can fly many at the same time without any issue.

Unlike previous technologies 2.4Ghz does not need dedicated frequencies for each system in use. Many systems can co-exist on the same frequency without interfering. This is because each transmitter and receiver pair are 'bound' together. The signal carries a special code that the receiver looks for. The receiver ignores any other signal that doesn't carry the correct code so it only responds to signals sent by the Tx with which it's 'bound'.

Steve
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Old 06-07-2012, 10:03 PM
  #27  
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More info in this thread:

http://www.wattflyer.com/forums/showthread.php?t=63497
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Old 06-08-2012, 04:38 PM
  #28  
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Originally Posted by JetPlaneFlyer View Post
fmw,

I'm no expert but i don't think it's that simple, not by a long shot. If other transmitters transmitting on the same frequency was a problem then you would not be able to fly two Spektrum systems at the same time.. When the reality is that you can fly many at the same time without any issue.

Unlike previous technologies 2.4Ghz does not need dedicated frequencies for each system in use. Many systems can co-exist on the same frequency without interfering. This is because each transmitter and receiver pair are 'bound' together. The signal carries a special code that the receiver looks for. The receiver ignores any other signal that doesn't carry the correct code so it only responds to signals sent by the Tx with which it's 'bound'.

Steve
The earlier Spektrum technology picked two frequencies randomly when you booted the radio. They weren't the same two every time. The chances of two transmitters choosing the same two was remote, hence, no interference. If two did choose the same two it would cause interference. Also, this random frequency selection would mean that it is also rare for the transmitter to choose one of the two frequencies to match the local cell tower or whatever. But when it does, it creates interference. This was tested with a spectrum anaylyzer by engineers at our flying field. It really is that simple.

The problem is fixed with the new generation Spektrum radios but it was a problem with the older ones. After they installed a new cell tower 3 miles from our field, several Spektrum using members had to upgrade their radios. That solved the "brownout" problems 100%.
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Old 06-08-2012, 06:11 PM
  #29  
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Originally Posted by fmw View Post
The earlier Spektrum technology picked two frequencies randomly when you booted the radio. They weren't the same two every time. The chances of two transmitters choosing the same two was remote, hence, no interference. If two did choose the same two it would cause interference. Also, this random frequency selection would mean that it is also rare for the transmitter to choose one of the two frequencies to match the local cell tower or whatever. But when it does, it creates interference. This was tested with a spectrum anaylyzer by engineers at our flying field. It really is that simple.

The problem is fixed with the new generation Spektrum radios but it was a problem with the older ones. After they installed a new cell tower 3 miles from our field, several Spektrum using members had to upgrade their radios. That solved the "brownout" problems 100%.

This might make a heluvalotta sense... I had days at that site when I could bind in 3 seconds, other times I would cycle power on the rx 3-4 times, no bind, then power cycle the TX and it would bind right away when I powered up the RX, and always have a great range check afterwords.

The day I crashed was, i think, the one and only time I ever forgot to range check.

Dave
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Old 06-08-2012, 06:23 PM
  #30  
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Originally Posted by fmw View Post
The chances of two transmitters choosing the same two was remote, hence, no interference.
Actually the chances were not remote - especially as you had more systems on. They used 2 of 40 channels so with just a few transmitters on they indeed might be on exactly the same channel. But that was not really an issue.

Proof is in the testing. On DSM and DSM2 (fixed channel selection) you could have many (100+) systems on before the noise floor really started to cause any issues and even then they were minor issues. Pretty slick eh?

Originally Posted by fmw View Post
If two did choose the same two it would cause interference.
This is absolutely wrong. The system also had a GUID it sent. So your TX and RX "know each other". The receiver only listens to the TX it is "bound" too. That is what binding does - links the two together. The joy of spread spectrum systems is you can have two or three or more on the same channel but they don't listen to all the signal - just the one meant for them.

This is why Spektrum could happily exist with "other" frequency hopping systems that would transmit right on YOUR channel. Again no biggie - until you start getting signal saturation. Then things "slow down". But you still don't likely crash planes. But you might start seeing the plane get sluggish to controls (bad).

The signal saturation (noise) of all the systems in that 2.4GHz range is were we started to see issues at the HUGE events. Huge means 500+ pilots. There with the chance of 100-200-300 systems "ON" at the same time just saturate the entire band with signals. The systems all coped with that remarkably well - even DSM/DSM2 with fixed channels. Spektrum DMS2 was at 'more' risk so they introduced DSMX that hops. Most agree that is vastly better.

Hitec goes one step further than anyone. They not only hop - but can adapt. So as they see saturation on specific channels, it avoids hopping to those channels - excellent!

Originally Posted by fmw View Post
Also, this random frequency selection would mean that it is also rare for the transmitter to choose one of the two frequencies to match the local cell tower or whatever.
Cell systems don't use 2.4GHz - so no issue with cell phones.

Originally Posted by fmw View Post
But when it does, it creates interference.
Again - NOPE.

Those spread spectrum engineers are pretty sharp folks!

Mike
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Old 06-08-2012, 06:27 PM
  #31  
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Interference from a non-RC-tx might conform closely to the mechanism described, though.

Plenty of 2.4 gig stuff out there.

Dave
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Old 06-08-2012, 06:43 PM
  #32  
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Originally Posted by CNY_Dave View Post
Interference from a non-RC-tx might conform closely to the mechanism described, though.

Plenty of 2.4 gig stuff out there.

Dave
Yes but none of it is bound to your TX. Your receiver "knows" your transmitter. So it only listens to that. Make sense?

Now as the noise floor increases - it gets hard to listen because so many "people" are talking - that slows it down - but it does not stop listening. So interference is not really the right word.

Mike
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Old 06-08-2012, 06:52 PM
  #33  
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Originally Posted by rcers View Post
Yes but none of it is bound to your TX. Your receiver "knows" your transmitter. So it only listens to that. Make sense?

Now as the noise floor increases - it gets hard to listen because so many "people" are talking - that slows it down - but it does not stop listening. So interference is not really the right word.

Mike
What I mean is, another TX that's non-RC has a good chance of swamping the signal (raising the noise floor).

Yeah, I'm slopping with the terminology, used to be somewhat into this stuff when the company I was working for was doing some front-end stuff for L3 and their spread-spectrum stuff.

Dave
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Old 06-08-2012, 06:58 PM
  #34  
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Originally Posted by CNY_Dave View Post
What I mean is, another TX that's non-RC has a good chance of swamping the signal (raising the noise floor).

Yeah, I'm slopping with the terminology, used to be somewhat into this stuff when the company I was working for was doing some front-end stuff for L3 and their spread-spectrum stuff.

Dave
Yea but one device - likely operating at TINY power outputs should not cause a good RC system any grief at all.
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Old 06-08-2012, 06:58 PM
  #35  
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Originally Posted by rcers View Post
This is absolutely wrong. The system also had a GUID it sent. So your TX and RX "know each other". The receiver only listens to the TX it is "bound" too. That is what binding does - links the two together.
+1.. That's precisely the way I understand it. Otherwise what does binding do

IMHO 'interference' and 'brownout' are very much over-used, in most cases it's simple pilot error.
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Old 06-08-2012, 07:03 PM
  #36  
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Originally Posted by rcers View Post
Yea but one device - likely operating at TINY power outputs should not cause a good RC system any grief at all.
Assuming it is one device, and operating at a low power output.
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Old 06-09-2012, 02:45 AM
  #37  
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Originally Posted by rcers View Post
Actually the chances were not remote - especially as you had more systems on. They used 2 of 40 channels so with just a few transmitters on they indeed might be on exactly the same channel. But that was not really an issue.

Proof is in the testing. On DSM and DSM2 (fixed channel selection) you could have many (100+) systems on before the noise floor really started to cause any issues and even then they were minor issues. Pretty slick eh?



This is absolutely wrong. The system also had a GUID it sent. So your TX and RX "know each other". The receiver only listens to the TX it is "bound" too. That is what binding does - links the two together. The joy of spread spectrum systems is you can have two or three or more on the same channel but they don't listen to all the signal - just the one meant for them.

This is why Spektrum could happily exist with "other" frequency hopping systems that would transmit right on YOUR channel. Again no biggie - until you start getting signal saturation. Then things "slow down". But you still don't likely crash planes. But you might start seeing the plane get sluggish to controls (bad).

The signal saturation (noise) of all the systems in that 2.4GHz range is were we started to see issues at the HUGE events. Huge means 500+ pilots. There with the chance of 100-200-300 systems "ON" at the same time just saturate the entire band with signals. The systems all coped with that remarkably well - even DSM/DSM2 with fixed channels. Spektrum DMS2 was at 'more' risk so they introduced DSMX that hops. Most agree that is vastly better.

Hitec goes one step further than anyone. They not only hop - but can adapt. So as they see saturation on specific channels, it avoids hopping to those channels - excellent!



Cell systems don't use 2.4GHz - so no issue with cell phones.



Again - NOPE.

Those spread spectrum engineers are pretty sharp folks!

Mike
As I understand it, cell phone frequencies include the 1.9 Ghz band, quite a ways from our 2.4 Ghz RC radio frequencies.

As for cost, methinks it's a lot more expensive to design the Spektrum/JR type of DSSS (Direct Sequence Spread Spectrum) modulation than the frequency hopping systems. Frequency hopping has been around since World War II.

These Spektrum/JR systems actually use wide band transmission on their radio signals. With wide band transmission, a single frequency narrow band interferring signal has less an effect on reception.

There is a lot of high level mathematics behind the DSSS modulation systems. What made this system possible were the inexpensive microcontrollers that control the 2.4 Ghz transceiver microchips. Now, Spektrum has come out with their DSMX system that combines wide band with frequency hopping. These systems, both DSSS and FHSS (Spektrum and Futaba) are far and away more bullet proof than the original 72 Mhz systems. Just ask anyone that lost an airplane when someone else turned on their 72 Mhz frequency.

One other thing on this issue. That is flying a 2.4 Ghz receiver over another pile of 2.4 Ghz transmitters located some distance away. The relative signal strength goes down very quickly when your model is perhaps 1/4 or 1/2 mile from the transmitter. And if that receiver is directly over a transmitter in another area, the receiver has to deal with signal levels several hundred times stronger than the signal level of your transmitter. Doing this with the old 72 Mhz receivers usually resulted in total loss of control.

Take a look:
http://en.wikipedia.org/wiki/Spread_spectrum

And
http://en.wikipedia.org/wiki/Direct-...pread_spectrum
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