GONZO Posted December 15, 2014 Author Share Posted December 15, 2014 It doesn't know what channels are free. I'll see if I can explain, hopefully correctly. With Tx and Rx switched on the Tx starts transmiting on the hopping sequence as predetermined by the PRN(pseudo random number) code stored in the Tx software. The Rx listens on a channel, any channel, until it recieves a signal that it recognises as from its Tx. This only take a very small fraction of a second to happen. It then starts to hop in the same sequence as the Tx because it also has the same PRN stored in its software. Any channel clashes that occur may or may not cause a loss of a very small part of the overall data. Various digital processes help in the recovery of the correct data, de spreading etc. Any data loss from a single event is so small as to have no material effect. I'm not that good at FHSS and DSSS but there are numerous tutorials on the web that explain it much better than I could by those that understand it much better than I. Somebody on here, more conversant with this will put things right, I'm sure. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted December 15, 2014 Share Posted December 15, 2014 Posted by Simon Chambers on 14/12/2014 19:39:28: No they don't all LBT. However some have always done so. I know for certain that Hitec 2.4GHz system has already LBT. Si. But I didn't think that Hitec was LBT as per the latest spec which is basically listen before each transmission, I understood that on start up the Hitec system checked the airways and then selected the least busy channels to hop between, but didn't listen before each hop. Quote Link to comment Share on other sites More sharing options...
Simon Chambers Posted December 15, 2014 Share Posted December 15, 2014 Posted by Frank Skilbeck on 15/12/2014 14:45:21: Posted by Simon Chambers on 14/12/2014 19:39:28: No they don't all LBT. However some have always done so. I know for certain that Hitec 2.4GHz system has already LBT. Si. But I didn't think that Hitec was LBT as per the latest spec which is basically listen before each transmission, I understood that on start up the Hitec system checked the airways and then selected the least busy channels to hop between, but didn't listen before each hop. The selection of channels on startup on Hitec was only on the first firmware versions - even then, it was only in a special "scan" mode. Later versions got rid of this and the Scan mode became effectively a "retune" mode. This does a scan of the current environment and picks a new batch of channels to hop on. As the channel hop list is defined when binding (on Hitec), you then have to rebind all your receivers. LBT on the other hand is when the transmitter hops channel (around every 20ms on the Hitec system), before it actually sends the data packet, it acts as a receiver to listen for any other signal transmitting on the channel it is on. It doesn't actually process any of the signal, just looks at the signal strength. If its above a certain level, it won't transmit on that channel. It then hops to the next channel in the next 20ms slot, and does this cycle again. Si. Quote Link to comment Share on other sites More sharing options...
GONZO Posted December 15, 2014 Author Share Posted December 15, 2014 Allan B Link to a post by Simon Chambers that explains things better **LINK** Quote Link to comment Share on other sites More sharing options...
gangster Posted December 15, 2014 Share Posted December 15, 2014 As Simon says the LBT should not concern us both tx and Rx will be happily reunited within 50 ms if the slot is in use hardly a serious latency much quicker than you me or the slop in most servos will ever notice Quote Link to comment Share on other sites More sharing options...
Simon Chambers Posted December 15, 2014 Share Posted December 15, 2014 Given that most of us are flying in the middle of a field with little, so no other 2.4GHz traffic, there won't be a lot for LBT to have to avoid! The only potential problem would be a large number of WiFi access points in close proximity or an analog 2.4GHz FPV Video Transmitter. These are high powered (especially FPV transmitters which don't conform to CE spec), broadband (take a large amount of the 2.4G spectrum) and older stuff (and all video transmitters) certainly don't LBT! Si. Quote Link to comment Share on other sites More sharing options...
GONZO Posted December 15, 2014 Author Share Posted December 15, 2014 But, what about public displays? Cosford spings to mind with the large number of houses nearby. I and most people I know don't change thier system Tx that often, years even decades can pass. These units will continue, quite legally, to be used along side the later LBT systems with possibly unforseen consequecies. Being in my late 60's I think it highly unlikely I would wish to buy anything new. This activity is unlike consumer electronics with its high 'churn' except maybe in crash damaged items. On the point of clubs being in the middle of large fields, some are quite close to large urban developments. The Rhyl club, for one, is well under a mile from the costal urban development of the North Wales coast. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted December 15, 2014 Share Posted December 15, 2014 Posted by GONZO on 15/12/2014 16:17:20: Allan B Link to a post by Simon Chambers that explains things better **LINK** Thanks GONZO, your 12:35:04 post was good enough So, on a busy field, the receiver might be ignoring a large percentage of the signals it receives, because they're not from its bound transmitter. But it doesn't matter because it takes no action in response to the "foreign" signals, and it'll get another good one from its own transmitter within a fraction of a second. It does sound like a bit of a scattergun approach when you think about it, so preventing the transmitters from transmitting on a frequency that's already in use must go a little way to cleaning up the airwaves. Quote Link to comment Share on other sites More sharing options...
Simon Chambers Posted January 9, 2015 Share Posted January 9, 2015 Fix is out for some of the FrSky FASST receivers: **LINK** How to guide: http://www.frsky-rc.com/download/down.php?id=47 Will require updating the firmware. They've been pretty quick going really getting this out imo. Si. Edited By Simon Chambers on 09/01/2015 13:49:34 Quote Link to comment Share on other sites More sharing options...
GONZO Posted January 9, 2015 Author Share Posted January 9, 2015 I've decided on my position. As my FASST is via modules (2 of TM-8, 2 of TM-14 and a RASST-12) and I have quite a few FASST compatible Rx already the decission was vertually made for me. The modules can not have the firmware updated and only some of my compatibles can. So, I've increased my holding of Rx using the original FASST, whilst stocks are still available, to the point where I think I'm well provided for long into the future. The only problem with this is if 'they' decide to make the spec requirements retrospective at some time in the future. Quote Link to comment Share on other sites More sharing options...
GONZO Posted January 9, 2015 Author Share Posted January 9, 2015 This post may be of interest to others as it sums up the current position for other makes **LINK** HTH Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted January 9, 2015 Share Posted January 9, 2015 Posted by GONZO on 15/12/2014 20:21:45: But, what about public displays? Cosford spings to mind with the large number of houses nearby. I and most people I know don't change thier system Tx that often, years even decades can pass. These units will continue, quite legally, to be used along side the later LBT systems with possibly unforseen consequecies. Just as a matter of interest, my Multiplex system has been 1.8.1 compliant since early last year and I've flown with it at several large aerotow events, 2 to 3 tugs running, 6 gliders in the air and many more on the ground being set up or waiting in line for a tow. No problems to report. It's only after looking at the logger after the event that you note that the LQI lost (skipped) 2 to 3 frames per flight, which when you consider there are over 50 frames a second it's not noticeable. With a LBT system it checks the freqency channel before transmitting and if it's occupied then doesn't transmit, the existing systems transmit anyway, but it's most likely that if there was another transmission on the frequency then the signal would be corrupted. Quote Link to comment Share on other sites More sharing options...
GONZO Posted January 10, 2015 Author Share Posted January 10, 2015 Good to know. I had my doubts about mixing LBT and non LBT. Lets hope that all systems are as robust. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted January 10, 2015 Share Posted January 10, 2015 you could argue that without LBT then both systems would interfere with each other with LBT then only the set wanting to transmit when it sees something else on the same frequency skips a transmission. So in theory a non LBT system is at an advantage over a non-LBT system in that if it's already transmitting a LBT system won't interfere with it, but the LBT systems aren't similarly protected from the non-LBT systems, but if everything was non-LBT then both systems would be affected. I can't see why all LBT systems wouldn't be as robust, all they are doing is checking to see if there is interference before transmitting Quote Link to comment Share on other sites More sharing options...
Simon Chambers Posted January 10, 2015 Share Posted January 10, 2015 When I first reverse engineered the Hitec 2.4GHz system all the way back in 2011, I noticed that it LBT too. DSMX reportedly has always done so too. Its not like Spectrum ever has had any problems with signal loss... Oh wait The other thing is that it also depends on where the LBTsignal strength threshold is set. I suspect most will be pretty low and will still transmit when other transmitters are on in the patch. Si. Quote Link to comment Share on other sites More sharing options...
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