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Old 06-09-2012, 02:45 AM
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kyleservicetech
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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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