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Dec 10, 2015, 11:09 PM
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Does the Receiver Really Transmit and the Transmitter Receive?


I am interested in learning more about how during binding, the transmitter knows what mode the receiver is capable of.

Apparently the receiver transmits this information back to the transmitter. Does this occur only during binding or is the receiver transmitting all the time?

I have searched these forums without success for more detail on how it works. It's probably because I haven't been able to come up with good search terms.

One suggestion was asking the FCC, but that didn't seem appropriate. Is there someone here who could please explain it to me?

Thanks!
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Dec 10, 2015, 11:12 PM
S.A.D. member
ivanc's Avatar
Most if not all 2.4GHz receivers transmit during binding/linking at very low power levels.

All telemetry receivers transmit at full power the telemetry data back to the transmitter and the telemetry transmitters receive that information.

Ivan
Dec 11, 2015, 02:05 AM
TigreJohn
Basically, with a non-telemetry system, the bind signal from the TX transmits a code to the receiver telling it to "remember" it. If any subsequent 2.4 signal that the receiver gets that doesn't have that code on the front end, the RX says the heck with it.
Dec 11, 2015, 08:12 AM
AndyKunz's Avatar
I didn't say "ask the FCC" - I said "read the FCC filings." The documentation there gives an overview of the entire system. There are also many explanations here on how the bind process works, as well as telemetry (not available in all receivers).

Andy
Dec 11, 2015, 02:33 PM
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Quote:
Originally Posted by AndyKunz View Post
... "read the FCC filings." The documentation there gives an overview of the entire system. There are also many explanations here on how the bind process works, as well as telemetry (not available in all receivers).

Andy
That sounds good to me Andy. I did some looking, but am not having much luck. Might you have a link to the specific FCC filing?

Thanks!
Dec 11, 2015, 02:49 PM
Registered User
Fauropitotto's Avatar
You can search for documents through here: http://apps.fcc.gov/ecfs/comment_search_solr/search

For example, using the FCC ID on the TM1000 : "BRWDAMTX11", I was able to find the documentation here - https://fccid.io/BRWDAMTX11

Or the AR7000 receiver with sat: https://fccid.io/BRWDASRX10
Dec 11, 2015, 03:21 PM
AndyKunz's Avatar
You can search this forum for posts made by me with the term "negotiate" as the search key.

You'll find lots of posts, like this: https://www.rcgroups.com/forums/show...ostcount=28426

So what are you really looking to learn?

Andy
Dec 11, 2015, 10:04 PM
Registered User
Thread OP
Quote:
Originally Posted by AndyKunz View Post
You can search this forum for posts made by me with the term "negotiate" as the search key.

You'll find lots of posts, like this: https://www.rcgroups.com/forums/show...ostcount=28426

So what are you really looking to learn?

Andy
What I'd most like to read is a couple of paragraph explanation in not too technical terms on how the binding and negotiating works for DSM2 / DSMX radios. Does the receiver transmit all the time? At the same power as the transmitter? Does the transmitter listen all the time? I'm guessing that the binding process would be more simple to explain than the frequency hopping part, but I'd sure be interested in hearing about the frequency hopping too if that's not too much trouble. Thanks!
Dec 11, 2015, 11:24 PM
S.A.D. member
ivanc's Avatar
Quote:
Originally Posted by BLT Aviation View Post
What I'd most like to read is a couple of paragraph explanation in not too technical terms on how the binding and negotiating works for DSM2 / DSMX radios. Does the receiver transmit all the time?
No, only when it is told to do so by the transmitter.
Quote:
At the same power as the transmitter?
No, much less during binding/linking
Quote:
Does the transmitter listen all the time?
No. They communicate with each other in a specific way which might be OEM proprietary information.
Quote:
I'm guessing that the binding process would be more simple to explain than the frequency hopping part, but I'd sure be interested in hearing about the frequency hopping too if that's not too much trouble. Thanks!
Ivan
Dec 11, 2015, 11:50 PM
If it flies, I can crash it.
rocketsled666's Avatar
In general, multi-model radios all work the same way when it comes to binding. You put the RX in to bind mode (or it goes in to BIND by itself if it doesn't see a matching TX signal after some amount of time), and you push a BIND button on the TX to bind them. The Receiver has a "GUID" code, a large binary number that uniquely identifies it (the manufacturer ensures that each RX it makes has a different GUID). The RX transmits that code during binding. The TX receives the code and assigns it to the currently selected model. Then, after binding, when you go to use the TX and RX, the TX transmits that GUID code along with the information on stick and switch positions. The RX receives the signal from the TX no matter what model is selected by the TX, but it will only respond to transmissions with the correct GUID. So if you select a different TX model by mistake, or someone else turns on their similar TX, the RX in your model will not respond.

For higher end radio systems, the two way communication that occurs at binding occurs just once when the BIND function is enabled on the TX and RX. But I've seen some cheaper Chinese models that have to go through the bind process every time they power up.

Spread Spectrum can work in a couple of different ways, but all use multiple radio channels within the band of available channels, which is what makes them "spread spectrum". Simple spread spectrum transmits on multiple channels at once, typically two, so if one channel experiences interference the signal still gets through on the other. The TX and RX decide on the frequencies to use when they first "see" each other at start up (by looking at signal quality across the band before choosing the best). Frequency hopping uses one channel at a time. But the TX and RX agree on a pseudo-random sequence of channels and they switch between them on a rapid basis, so if there is interference on one frequency the radio moves off to another channel before it can cause more than an imperceptible glitch in the control of the model. Each bound TX/RX pair uses a different sequence when in operations, so multiple radios on the same frequency, even if they're the exact same brand, can peacefully coexist.
Dec 15, 2015, 01:03 PM
Registered User
Thread OP
Great summaries of how it all works. Very helpful. Thanks!
Mar 10, 2016, 08:57 PM
Florida
++ Rocketsled666... nicely done.
Mar 10, 2016, 09:38 PM
S.A.D. member
ivanc's Avatar
One misconception - "spread spectrum" does not necessarily mean that multiple channels are used, spread spectrum can use a single channel. What that means is that a signal with particular bandwidth is spread in the frequency domain resulting in a signal with much wider bandwidth. For example direct sequence spread spectrum (DSSS) does not need more than one channel. The fact that one DSSS system uses 2 channels and another changes channels constantly does not mean that more than 1 channel is needed for the system to function - 2 or more channels in those cases add redundancy to reduce interference and jamming.

Ivan
Mar 10, 2016, 11:14 PM
If it flies, I can crash it.
rocketsled666's Avatar
DSSS modulates the signal around a fixed center frequency. It's like FM transmission except the modulation is a function of a pseudo random number sequence and not the content of the data being transmitted. The modulated signal bandwidth ends up being much wider than normal and the signal itself looks like white noise. The bandwidth is wide enough it's hard to argue it's "one channel" any more (in fact, you could argue if it's not spread out to encompass multiple "traditional" channels or it's not really "spread spectrum").

Technically you're right in that it uses a "single" fixed center carrier frequency with a very wide bandwidth. But I don't think it matters. I'd be happy to learn otherwise, but I don't think any RC system uses DSSS.

Older systems (like DSM or FAAST) use two frequencies, chosen by the TX when it initiates a connection to the RX. All transmitters have to be "good citizens" and only pick channels that are not being used, irrespective of whether it's a Spektrum or Futaba (or other) radio on the used channels. Having selected its channels, the TX then transmits on both and the RX chooses the channel with the best signal. It is "spread spectrum" in that the same signal occupies two channels. That's what makes it "spread".

Newer systems (like Spektrum's DSMX or Futaba's FHSS) are true frequency hopping systems. They sequence through a large group of channels very quickly using a pseudo random sequence. Over a larger period of time, the signal from the TX appears to use the entire band of frequencies rather than a wide bandwidth centered around one frequency, or two separate fixed frequencies. The good thing about frequency hopping is that every TX uses a different random sequence. So if two TXs happen to use the same channel at the same time, their signal doesn't get through to the RX. But the TXs move to new/different frequencies within milliseconds and the "collision" results in at worst a minor glitch at the RX end.

Hollywood actress Hedy Lamaar patented a Spread Spectrum Frequency Hopping anti-jamming radio system in WWII. The digital technology required to select channels didn't exist back then, her system used a player piano-like paper roll system to sequentially select the channels chosen by the TX and RX.

RC spread spectrum systems all tend to be very similar because the RC manufacturers are using the same (or similar) chipsets to implement their radios. The "protocol" (how the data being transmitted is encoded/decoded) changes from one manufacturer to another, which is why their systems aren't interchangeable, but the basic operating principals are the same with few exceptions.
Mar 11, 2016, 08:32 AM
AndyKunz's Avatar
Quote:
Originally Posted by rocketsled666 View Post
I'd be happy to learn otherwise, but I don't think any RC system uses DSSS.
Actually, yes, Spektrum does. So does JR for DMSS. We both use spreading (making it wider than FSK). We also use multiple center frequencies (hopping).

Dig a little deeper.

Andy


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