Being dragged screaming and kicking towards 2.4!

[Simon Chaddock]

As a lone flyer I have no problem whatsoever with 35mHz and its cheap!

However now 2.4 has become the norm (you can't fly at the Nats on 35) I have reluctantly decided to 'give it a go'.

So I promptly did what you are not supposed to do and bought second hand off ebay and a basic one at that - a Sky-706 made by Skyartec - who?

7 channel with two 7 channel receivers and in 'show room' condition with little or no sign of much use - I wonder why?

Obviously not 'high end' and a search of this site shows no posts on this or for Skyartec in general - hmmm.

Seems to be fully programmable by a PC but of course no cable (so on order from Skyartec)

Down loaded the programming software and the detailed instruction manual but the chinglish is as bad as I have seen.

Under the heading for double rate it says (and its verbatim!)

can switch kinds rudder of aileron. elevator and direction to rudder use beyond initial rudder angle sensibility exponent

"-" one side is fuselage centre zone becomes slow."+" one side is fuselage centre zone becomes quick

No joke, that's all there is - good spelling though!

This could prove an interesting journey but has anybody any experience of this equipment?

[Biggles' Elder Brother - Moderator]

Simon - I wish you the very best of luck mate!

BEB

[John Olsen 1]

So you plan to put this in charge of an aeroplane?

At least I am well out of harms way!

[avtur]

Just my 2p ...

The radio is about as fundamental as it gets in terms of flight safety ...

Given the affordability of new 2.4 gear off the shelf, known and reputable brands, I would not buy anything else.

[Oldbaldfella]

You could have had my nearly new Futaba tx for 60 quid............... and we KNOW that's reliable!

[ken anderson.]

best of luck from me also...hope you are going to do the maiden well away from any 3rd party property or people etc.....

ken anderson ne..1... tin hat dept....

[Mark Powell 2]

Simon,

People are taking the mickey, but take no notice. The techniques of 2.4 Ghz transmission/receiving are well understood in the electronics world, if new to modelling. Also, with the exception of Spectrum, which has its own (somwhat inferior) way of dong things, and Futaba's Fasst systems (very likely to be obsolete soon) - Ripmax have stated that all future transmitters will support FHSS and S-FHSS, so it looks like they  may not support FASST) they all work in near enough the same way. So use it, with one reservation. I notice that the receivers have only one aerial. This can be blocked by metal or carbon things in the plane Most sysems use two aerials, and the best ones, such as Multiplex, use two complete receivers in one case.

But you do have a problem. As unlike 35Mhz, all these systems are completely incompatible with each other. I suspect deliberately. Are you REALLY going to eventually buy more Skyartec receivers for your other planes? If not, you are going to have to buy a 'proper' system sooner or later anyway, so you have gained nothing by buying this.

Your Chinglish is not perfect. You imply that they are talking about dual rates. I suspect the second phrase is about expo 

Finally, as you are a lone flyer,why not stick with 35Mhz. As a matter of fact, I can sell you about fifteen absolute top receivers, mainly Multiplex, but some JR and Futaba, as a very reasonable price (not too reasonable, I have to try and recover some of their cost). I find they have not worked too well as doorstops

Edited By Mark Powell 2 on 16/08/2012 08:07:19

Edited By Mark Powell 2 on 16/08/2012 08:15:55

[NigelH]

A lone flyer should stick to 35MHz ? Very bad advice. When I started flying (self-taught) I used my Futaba 6EXA 35Mhz TX not knowing that there was a club not that far away. That was quite a while ago before I got involved with the BMFA, the club, etc. If you cannot be 100% sure that there will be nobody else nearby with a 35MHz TX, 2.4GHz is more than likely the best option.

[Simon Chaddock]

Thanks for all the advice.

As you might have guessed it was cheap so the fact that it might not be compatible with any other (but then so are some of the major brands!) and with two RXs I can atleast use it it in two planes.

Risk flying with it? Experiance has taught me never to fly anything I cannot affort to loose (or break) and so far every RC plane I have built I still have, although not all are still flyable!

Mark

Reading the manual again it appears you might be right as the section heading is "D/R EXP". Fortunately the PC software programming package is rather more obvious to use!

.

Edited By Simon Chaddock on 16/08/2012 11:56:24

[Rich too]

Posted by NigelH on 16/08/2012 09:32:18:

A lone flyer should stick to 35MHz ? Very bad advice. When I started flying (self-taught) I used my Futaba 6EXA 35Mhz TX not knowing that there was a club not that far away. That was quite a while ago before I got involved with the BMFA, the club, etc. If you cannot be 100% sure that there will be nobody else nearby with a 35MHz TX, 2.4GHz is more than likely the best option.

Took the words out of my mouth. I am a lone flyer and had that experience - 2.4 is a godsend.

I agree with other posters, given how cheap mainstream equipment is I would ditch it and buy a "brand". jmho.

Rich

[David Ashby - Moderator]

Post deleted, forum language infringement.

[Rich too]

Posted by Mark Powell 2 on 16/08/2012 07:58:20:

Futaba's Fasst systems (very likely to be obsolete soon) - Ripmax have stated that all future transmitters will support FHSS and S-FHSS, so it looks like they may not support FASST)

Really? That's inconvenient

Rich

[Mark Powell 2]

Yes. You can spot it in an advert, though I suspect they did not do it on purpose.

"Both the Acro Wot Foam-E FTR and RTF Version can be operated with a Futaba S-FHSS transmitter and 'ALL FUTURE FUTABA AERO-HELI RADIOS". (My big letters). So obviously all future Futaba radios will therefore have S-FHSS, won't they? Only the expensive 18 channel one has both that and Fasst. The other Fasst radios are all pretty old. S-FHSS for sure, new Fasst ones unlikely.

Lone Flyer on 35Mhz.

You are all probably right, I did not think of that. But I get the impression that he has being doing it for a long time without problems. So have I, but I have moved to 2.4 totally now. Myself and a friend have been doing it for over 30 years, on a field he owns next to his house. first on 27, later on 35. But my oldish Multiplex transmitter has a built in scanner that continually scans all 35Mhz frequencies.  But I do take the point. 

Edited By Mark Powell 2 on 16/08/2012 13:12:09

[Trevor Crook]

Mark, can you elaborate on your assertion that the Spektrum system is "somewhat inferior"? I know that there has been much debate on the merits of DSM2 (broad spectrum using two frequencies) versus frequency hopping narrow spectrum, but technical assessments of the current DSMX system (broad spectrum frequency hopping) suggest it offers the best of both worlds. I've been using the older system for a few years with no problems. Don't want to turn this into a "this brand is better than that" thread, I'm just interested in what your reasoning is.

[Trex700e]

Buy a Spektrum DX8 and you will not be sorry!

[Mark Powell 2]

Trevor,

I don't want to turn it that way either. I use Multiplex, but I am tempted to buy a small Spektum because of the wide availability of so many of these complete models already fitted with a Spektrum receiver.

DMS2 is a bit dodgy because it starts off by finding two clear frequencies and uses (i believe) just one of them. If it experinces interference it uses the other. If that becomes interfered with it tries to swap back, but that could still be subject to interference. So it is stuck.

Multiplex, Futaba, new JR etc. use the full (about 50) channels available, and continually swap about 500 times a second whether it is interfered with or not. If it hits a bad one it is going to swap anyway within, at worst, the next two milliseconds so it does not matter. Could if there were lots of bad ones out of the fifty, then you might see a slowing of response but not a totall loss of control.

Spektrum DSMX does the same, but only across about five channels. These five are each a tiny 'bunch' of the overall fifty, but it still does not use all of them. It in effect picks five wider band channels rather than fifty narrow ones, but their 'wide band' is still not very wide.

None of it is true spread spectrum at all really. But is works. Mainly because 2.4, for all purposes (mainly medical and instumentation) is very low power and thus short range. We don't usually fly in the middle of an industrial park or in hospital grounds, so all we have to cope with is other modellers.

[Trevor Crook]

Good summary Mark. Spektrum DSM2 uses 2 frequencies as you describe, but the DSSS system it uses is spread over a wide band, with sophisticated noise rejection techniques, so is a much more robust data link than a narrow band signal as used in a FHSS system. As you say, though, as FHSS changes frequency every few milliseconds, this doesn't matter. The reason I think that DSMX may be the best of both worlds is that it still uses the robust DSSS link, but hops as well (around 23 frequencies I believe)

Anyway, back on topic. I bought an FMS EDF f-18, which to my surprise came with 2.4 radio. I didn't like the look of it so fitted my normal gear to fly it. I later tried the FMS set in a small, fairly expendable model well away from everyone and it seemed fine. Only had a few flights with it, though. It's now sitting on the shed shelf and may end up in a boat or car. I would certainly stick to a proven brand for my models, and although all my stuff is Spektrum, I would happily use any of the other big brands. I still fly a couple of models on 35, with no issues, so as someone said above, if you're getting on with that fine, no need to rush to 2.4.

[Mark Powell 2]

Trevor,

I only changed a couple of months ago. Only then because my newly built F3A (ish) model piled in flat out on the concrete runway during a low pass, on it's fourth flight. Interfernce, for sure. I know that is usually just an excuse, but this time I distinctly heard the engine throttle back a split second before it hit, which was how the failsafe was set. Down it came, from about 4ft. I had put a little up trim in, but set the failsafe positions before I did this and had, of course, not bothered to take that out mechanically. So it behaved exactly as set. Old JR transmitter, used without fault for many years, and a Multiplex DS IPD receiver, which are about the most sophisticated of all, and which considerable experience with them shows them to be reliable.

I don't mind, omelettes and eggs etc, but I was in the 2.4 camp the next day.

[Peter Christy]

Posted by Mark Powell 2 on 16/08/2012 13:53:36:

DMS2 is a bit dodgy because it starts off by finding two clear frequencies and uses (i believe) just one of them. If it experinces interference it uses the other. If that becomes interfered with it tries to swap back, but that could still be subject to interference. So it is stuck.

That's not actually correct - it does use both frequencies, hopping between them in much the same way that other systems hop around several. The main difference is in the bandwidth. With the exception of FASST, all the frequency hoppers I've looked at use a bandwidth of 100-200 KHz. Spektrum (and FASST) use a bandwidth of 2 MHz. So to block a DSM-2 transmission, the interfering signal would have to completely blot out 4 MHz of bandwidth! The only thing I know that would do that would be a cheap video downlink!

When a wide-band spread spectrum signal is "spread", the peak energy of the signal is reduced, but it is spread over a wider area. The total energy in the signal is the same. However, when it is "de-spread" at the receiver, this has the effect of "spreading" any interfering signal on the same frequency. This allows the wanted signal to rise through the noise - a bit like Excalibur rising up out of the lake!

Because of the bandwidth of a DSM-2 signal, there is only room for 80 channels within the 2.4 GHz band, and since it uses two of them, that means that in practice you can only fly 40 models at once! Not really a problem in the UK or Europe, but potentially an issue at places like IRCHA in the US hence DSM-X - a modification of DSM-2 to use more channels.

A DSM-2 transmitter will refuse to power up unless it can detect two free channels. This is why it is important that the transmitter is switched on in "free air", and not whilst still in its metal carry case on the ground! This refusal to power up in the presence of no channels being available has, on occasions, led to pilots thinking their gear is faulty! It isn't! Its doing what its meant to do!

DSM-2 systems operate on a different philosophy from full frequency hopper like FASST (which I'm using here purely as an example). DSM-2 looks for a pair of empty channels. FASST like systems don't. They transmit blindly, not caring whether a channel is occupied or not. This is justified on the grounds that 1) because each channel is only occupied for 100mS or so, it won't cause significant interference to the other user, and 2) it can afford to lose packets of data because the repition rate is so high that it will be made good before the pilot notices.

In other words, DSM-2 tries to make sure that the signal path is clean. Frequency hoppers don't because the data rate is fast enough to make good any losses before they become noticeable.

Both approaches work well, as we demonstrate up and down the country every week-end!

I have no axe to grind here. My expensive models all currently fly with DSM-2 - and have almost since its inception. I've recently started using a FrSky system for fixed-wing, which has also proved to be utterly reliable to date. The FrSky has a better fail-safe system and recovers from signal loss quicker (on the bench, anyway! Not had it happen in the air, yet!). But for me, the killer application for Spektrum gear is "Model Match", which prevents the terrible mistake of flying with the wrong model memory selected! We've all done this at least once, and anything that prevents you from making this simple error is worth its weight in gold!

Just my 2p worth!

--

Pete

[Mark Powell 2]

I don't have any axe to grind either. I use Multiplex But only because I was happy with thie 35Mhz stuff (I moved to Multiplex 35 from a very old JR) which I have used for years.. Their flexibility is amazing, even when compared to the £2000 Futaba. You can even configure door sequencers. Not 'specialised', like the new Spektrum. but just by servo slow (all channels) and curves, (all channels) like heli pitch curves. But not limited to pitch and throttle, like a heli.. And, should you want, you can have an unlimited number of (say) ailerons, each configured so they match exactly, off the one aileron stick. Guess I am a MPX fan after all!

But I have thought of buying a Spektrum because of all these 'receiver ready' models available.

I did hear that Spektrum changed to DMSX because there were sometimes difficulties when a very large number (shows and things) were in use. But I have no personal experiance and the many I have seen in use have never had a problem to my knowledge.

Wish they were all compatible. Then we could swap makes around with impunity, like we did with 35 (except PCM of course).

[NigelH]

Spektrum have referred to SEFF and Joe Nall, where they have more than a thousand pilots. For regular flying, I'm not sure that DSMX make much difference.

[buster prop]

Thanks Peter, a useful and informative post. It explains one or two things about DSM2 that I didn't know. Interesting point about the spreading and de-spreading technique raising the S/N ratio, surely an advantage? As a DSM2 user one thing I'd noticed with my DX7 is that when I switch on the voltage reading goes up (by only 0.1V) after a second or two. That must be because it's not transmitting immediately, just scanning the band for 2 clear frequencies and the extra load pulls down the voltage slightly when it does transmit. I don't have a metal case, my Tx sits at the end of my flight box so is exposed anyway so I don't get the screening problem you referred to. I've been using DSM2 for about 3 years now with no problems, one day I may upgrade to a DX-7S or a DX8. DSMX does have better resolution of 2048 steps instead of 1024 but for my flying I don't think I'd notice much difference! I had a twiddle with a DX7S at W&W and liked it, nice smooth sticks and roller menu access, according to the blurb its easy to program so I'm tempted.

[Trevor Crook]

Mark

One of my flying buddies is a satisfied Multiplex user like yourself, but he's also bought a Spektrum Tx (DX8!) so he has access to the range of bind and fly models, specifically for indoors. You could do the same, although a DX6i would be a cheaper alternative that would do all you wanted.

Peter

Excellent summary of DSM2. What's really cool is the way that Spektrum have acheived full compatability beteween DSM2 and DSMX transmitters and receivers. Like Buster I've not had problems with DSM2, and can continue to use it with newer DSMX Rx's, but if I upgrade to, say, a DX8 Tx everything will still work.

[Mark Powell 2]

Trevor,

The DX6i is exactly what I had in mind. I will probably get one next time I am in the Spektrum dealing shop of my two nearby LMS's. Will probably use it for some of my smaller and simpler models anyway. With the incompatibility between different makes, but all, I suspect, equaly 'link robust' I am wary of putting all my eggs in the MPX basket.

Edited By Mark Powell 2 on 21/08/2012 14:26:26

[Simon Chambers]

A couple of points to correct some misinformation on this thread...

• DSM2 can be blocked out pretty easily with a high powered 2.4GHz transmitter (illegal, but are used). For the most part not a problem in the UK - but does occasionally happen.
• As DSM2 does not frequency hop, it is more susceptible to signal fading and blocking. Hence why you need satellite receivers and antenna installation is more critical
• Low power and short range??!! Using standard 3dB antennas, FPV'ers are frequently getting 1.5km+ range on FrSky and other gear. Plenty good enough range for normal line of sight flying.
• DSM2 is now inferior, technically, to most other big brand 2.4GHz systems. The lack of frequency hopping was due to the original RF chipsets used not being able to frequency hop over many channels quickly.
• Hence the slow lock-on at startup with the original Spektrum receivers. As to lock-on the receiver has to search over the complete 2.4GHz frequency band to find the DSM2 transmitter.
• However, for the most part in UK flying sites, DSM2 will work fine as there is little 2.4GHz interference in our flying fields.
• The chipset Spektrum now use is capable of frequency hopping but is also backwards compatible with the original chipset Spektrum used for the original DSM2 - hence why DSMX came about and is backwards compatible.
• The 40 transmitter limit on DSM2 is a theoritical limitation. It doesn't take into account other 2.4GHz traffic. Also the channel selection done by DSM2 is only done on startup. So, for example, if you have 20 DSM2 transmitters operating, and then another 10 frequency hopping transmitters are turned on in close proximity, it could increase the latency or loose signal on the DSM2 receivers.
• Multiplex use the same chipset as Spektrum does. Implementation different - otherwise they would be compatible. Certainly possible if Spektrum/Multiplex wanted to make them compatible - no doubt not due to commercial reasons.
• I can't remember how many channels DSMX hops over, but it's more than 5 channels, as FCC approval requires a minimum of (IIRC off the top of my head) 15 hop channels
• Not all 2.4GHz systems are created equally - even if they use the same RF chipset.
  For example, Radiolink uses the same chipset as Hitec/Futaba (SFHSS)/FrSky/Graupners and others.
  However Radiolink do not implement frequency hopping, so not very robust when interference.
• Futaba S-FHSS is not replacing FASST. Paraphrased points from Futaba marketing material:
  FASSTest is replacing FASST on high end systems (which supports telemetry).
  S-FHSS is for lower end systems.
  Both use different RF Chipsets, hence not compatible with each other. The Futaba 18MZ supports both, as it has both chipsets in the transmitter. Futaba have hinted that future high-end transmitter will also support both protocols.
• S-FHSS for all intents and purposes in use, is not inferior to FASST.
• Some transmitters are not full-range. This is down to if the transmitter has a Power Amplifier IC on its RF front-end. This increases the standard RF Chipset from 1 to 5mW (depending on the RF chipset) to 60-100mW (depending on the Power Amplifier used). I don't know if Simon's transmitter is full range or not.
• One other point not mentioned. The low-power test is as crucial, if not more so, on 2.4GHz than 35MHz. The components used on 2.4GHz receivers are just as easily damaged with shock, etc as on 35MHz. Also many 2.4GHz receivers use connectors for their antennas. These can come disconnected (through shock, strain and other reasons) which won't cause visible reception issues in the pit, but will cause significant range reduction. Many cheap 2.4GHz sets do not have a low power mode/range check mode.

Cheers,

Si.

Edited By Simon Chambers on 21/08/2012 21:28:33

Edited By Simon Chambers on 21/08/2012 21:28:58