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HomeRC & Power✈️Aircraft🚁HelicopterRadio - Spektrum DSM › Fasst with frequency jumping or Spectrum 2 ch.
09-03-2008 03:45 PM  12 years ago
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iedit4tv

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Santa Clarita, California

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Fasst with frequency jumping or Spectrum 2 ch.
Dependability Not Price!

I understand the difference between the Fasst spread spectrum and the Spectrum 2 channel system.

During my research, I was suprised to read that a second Fasst receiver can be added to the airplane or heli. Like the Spectrum remote antennas, I like that.

In fairness, I have a thread going in Fasst Radio section (http://runryder.com/t457250p1/). I didn't want to seem bias toward one system over another so I've posted here in the Specturm forum too. I am looking for the most dependable system where price is not an issue.

I recently lost a $4K camera ship due to a lockout with another 2.4 GHz system. I know it was a lockout because I have it on video! As you can imagine, the price of a system is not an issue; dependability is!

If you search Runryder for Spectrum lockouts, you'll find plenty of "Subjective" opinions on what caused their loss. However, if you search again for Fasst lockouts their pretty scarce.

Now in all fairness, the Spectrum system has been out longer than the Fasst system. The Spectrum system is more affordable than the Fasst system which would fuel its prevalence and market share. But again, I'm looking for dependability and not the lowest price.

So if price is not an issue and you're looking for the very best system, which of these two systems would you choose based on your experience and why!

Thanks
Mike
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09-03-2008 06:40 PM  12 years ago
pchristy

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Both systems have their pros and cons. Its a very complex system, and not many people understand how it works. My profession is electronics, and I must confess that there are aspects of it that I find puzzling!

Futaba fans will tell you that because of the frequency hopping, the signal never spends enough time on any given channel to suffer interference, whereas because Spektrum only uses two channels, the chances of being "splatted" are greater.

What they neglect to point out is that a Futaba channel is only 100KHz or so wide, whereas the Spektrum one is 1MHz wide! Wider bandwidth = better immunity (in crude terms!).

Also Spektrum have been very careful in their choice of spreading code, which again gives a significant improvement in the ability of the signal to cope with noise. In fact the Spektrum system is capable of recovering a signal that is BELOW the noise floor - something that I and many others have difficulty getting our heads round - but the maths speaks for itself!

The most important feature of a spread spectrum system is the "path budget" - in crude terms, the dynamic range of the system.

Most conventional PPM and PCM systems probably have a path budget in the region of 70 dB or so (every 3 db represents a doubling of the signal, so 70 dB corresponds to a figure of around 10 million times!).

In contrast, the "path budget" for a Spektrum system is of the order of 130dB! This represents a substantial improvement!

I appreciate your position with an expensive camera helicopter, but consider this: NO model technology is going to achieve the reliability of full size. If it did, it would cost the same as full size! You are trying to use a "domestic" appliance for an "industrial" application. Really, you have no choice, because nobody (except the military!) makes "industrial" strength RC systems!

However, for my money, I am backing the Spektrum system. I was skeptical at first because I have designed and built my own RC systems on 459 MHz (legal here in the UK) and I know how much trouble I had getting those to work in helicopters! I couldn't believe that 2.4 GHz would not suffer from even worse issues! However, I was applying "analogue" thinking - Spektrum are applying "digital" thinking, and the evidence speaks for itself - it works!

Furthermore, the design team seem remarkably accessible if you have problems or special requirements - something that a large industrial outfit like Futaba would have difficulty providing. I have also become convinced that the Spektrum system is technically superior. And I'm putting my money where my mouth is by slowly switching all my important machines to Spektrum 2.4 GHz!

--
Pete
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09-03-2008 08:26 PM  12 years ago
iedit4tv

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A very good explination of the Spectrum system Pete. Thanks for your expertise.

The industrial application of a consumer product is all that is out there right now. The only other option is to go ham bands, but they are not digital so you have the latency issue associated with analog systems.

At this point, my question is why are there so many lockouts with the Spectrum? The DX7 channel seems to be hit more times than the nine channel systems like the JR X9309 Heli. Is this a problem with the receivers or something more?

Why have I not heard of many lockouts with the Fasst systems? Am I not looking in the proper places or forums? Is the product too new or not enough of them out there?

I like both systems. I have used Futaba for twenty five years. I've had very few problems with them. JR is a very highly rated and respected system as well. I like them too.

I think bang for the buck they are about equal... my problem is the difference in technology.

I haven't made a decision yet...

Mike
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09-04-2008 02:37 AM  12 years ago
iedit4tv

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Thanks to all
After hours of reading multiple threads I must say I have gotten an education and I just wanted to thank everyone who lent their opinion and their experience with the Spectrum and Fasst systems.

It was a difficult decision, but I have just ordered the Futaba 10C radio. I've used Futaba for many years and while I've heard many good things about Spectrum, every servo I own is Futaba as well as many of their radios.

Many thanks to all who took the time to post to this thread and lend their help.

Mike
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09-04-2008 02:48 AM  12 years ago
Steff Giguere

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I just received my 10C today and I think it is an awesome Tx I'm pretty sure you will like it too.Team Synergy, Rail blades, Team Scorpion, V-Team
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09-04-2008 09:37 AM  12 years ago
pchristy

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Reading through the threads where people have had lockouts with Spektrum systems, most seem to be down to inadequate power supplies.

Because the receivers contain a microprocessor to decode the signal, anything that causes the microprocessor to reboot - like a sudden drop in voltage - will cause a momentary lockout. This will be followed by a period where the processor has to find its "parent" transmitter again. This could take a few seconds - not an ideal situation.

The latest Spektrums have improved code in the receiver (older sets can be updated) which make it look at the last channel in use before scanning for the Tx again. This considerably improves the recovery time.

However, the point is that the voltage should NEVER drop that low (below 3.5 volts) in the first place.

The problem seems to occur primarily with 3D pilots using a complete set of digital servos. Worked hard, these can draw a substantial amount of current, so it is VITAL that power supplies and switch harnesses are in tip-top condition!

I'm just changing the batteries in my aerobatic machine (I'm more into F3C style flying - not 3D) after I noticed the voltage display dropping into the red after two flights! Despite the voltage having dropped well into the red, I suffered no glitches or lockouts - and mine is an unmodified receiver!

I can only conclude that those suffering from this issue are using extremely below par power supplies, as my experience indicates the voltage has to fall a LONG way before problems arise!

--
Pete
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09-04-2008 10:22 AM  12 years ago
Climax

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Pete,

Unless I've gotten the wrong end of the stick, the "hopped" Futaba signal is actually about 1.6MHz in bandwidth. It's actually a DSSS signal in its own right. This does level the playing field in some respects.

I personally still prefer the Spektrum implementation, specifically the use of multiple receivers. As I understand in a FASST receiver the only one of the two antennae is in use at any one time (i.e. there is a single receiver) and when the receiver fails to decode the fixed frame pre-amble it switches antennae.

Also as the FASST signal is DSSS modulated, when it does collide with a Spektrum signal they will tend to co-exist and not interfere. Although there will be a slight reduction in the S/N ratio. Even if Spektrum and FASST used the same spreading code, you'd probably still get away with decoding a signal as it is highly likely that the "clocks" used in the modulators (i.e. whilst applying the spreading code) will be very out of phase. Spreading codes are chosen specifically such that they will work in this scenario, i.e. they chosen to have very good autocorrelation properties.

Please someone correct me if what I’ve written isn’t inaccurate...
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09-04-2008 02:19 PM  12 years ago
Havoc

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Ky.

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recently lost a $4K camera ship due to a lockout with another 2.4 GHz system. I know it was a lockout because I have it on video! As you can imagine, the price of a system is not an issue; dependability is!
Did you have a 2.4 downlink? Then you may have trouble with any 2.4 Rx. I don't follow that side of the hobby closely but did read where they seem to still use 72mhz for this reason.

You may find more lockout complaints with Spektrum because they have been around longer. Futaba users benefited as well in that brown outs were a known issue by the time their product came to market. One deciding factor I used in determining which system to go with was how long each had been on the market and how thoroughly tested by the end user. There are things I really like about my Futaba radio but still feel more comfortable with Spektrum/JR version of 2.4.
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09-04-2008 02:27 PM  12 years ago
Hotwings

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Florida, West Palm Beach.

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I have used the 9303 on 2.4 and my 14mz on 2.4 for a while now and i can say that both systems work, but, i have had lockouts on the 93-3 and none on the 14mz.

Futaba has been using their 2.4 tech in factories for almost 15 years, they are not new kids on the block, the gyros used in our heli's were developed for automotive (brakes) use. We are a very small part of the Futaba market.
Please cancel my clearance, I have the field in sight. Got my RW Turbine Waiver, need lottery!
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09-04-2008 03:23 PM  12 years ago
iedit4tv

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Santa Clarita, California

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My AP ship was a gasser with the receiver mounted up front between the frames with the 2.4 GHz antenna up above any nearby metal. The video TX is a 900 MHz mounted on the tale nearly 4ft away from the RX.

Please bare in mind, this ship had over 300 flights on it with no problems. But I was flying in a commercial environment while on an AP job where I'm sure there was a lot of RF noise (walkies; wifi; etc.).

I've been flying for over 25 years and I know a lockout when it happens. This was a lockout no doubt.

I know no system is completely bullet proof... I was simply collecting information so I could make an informed decision - and I have. I've purchased the Futaba 10C.

Thanks for your input!

Mike
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09-04-2008 03:45 PM  12 years ago
pchristy

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Climax: Looking at the documentation on the Ofcom website, if you use FHSS, you are only allowed 100mW per 100KHz of bandwidth.

The same documentation also says that for DSSS you are allowed 10mW per 1 MHz of bandwidth. Spektrum get around this because they only use about 8% of the available bandwidth, so even though they radiate 100mW, their average spectral density is only 10 mW! (Think of it as being a little bit like single-sideband as used by radio amateurs, where all the energy is concentrated into that part of the spectrum carrying the information. A crude analogy, but you get the idea!)

Because it hops (!) its actually very hard to examine the Futaba system on a Spectrum Analyzer, but from what I've seen when I have looked at one, the bandwidth is much less than the Spektrum, so the figure of 100KHz or so sounds reasonable. It also complies with the Ofcom figures.

But, as ever, I stand to be corrected!

--
Pete
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09-04-2008 04:06 PM  12 years ago
Climax

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Hi Pete,

That's interesting...

However, take a look at:

https://gullfoss2.fcc.gov/prod/oet/...tive_or_pdf=pdf

Specifically, pages 38-40 of the FFC certication report for the Futaba 6EX 2.4GHz transmitter. In brief it shows a bandwidth of 1.67MHz, I assume for the test the frequency hopping had been disabled.
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09-04-2008 06:24 PM  12 years ago
JKos

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Redondo Beach, CA

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The FCC report clearly indicates the FASST bandwidth is 1.6 MHz.

> During my research, I was suprised to read that a second Fasst
> receiver can be added to the airplane or heli.

You can also use mutliple Spektrum receivers (not just satellite receivers) if you wish.

- John
RR rules!
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09-04-2008 10:54 PM  12 years ago
Havoc

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Ky.

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Futaba has been using their 2.4 tech in factories for almost 15 years, they are not new kids on the block,
I've heard that and I'm sure it's true but they seemed slow to use it for our application. I know because I kept waiting on them. You would think the R/C guy at Futaba would walk across the hall and ask the spread spectrum guy for some circuits and a bit of solder at some point in those fifteen years before only deciding that it was a good idea after Paul Beard conjured one up in his garage. But at least its out now.
Please bare in mind, this ship had over 300 flights on it with no problems. But I was flying in a commercial environment while on an AP job where I'm sure there was a lot of RF noise (walkies; wifi; etc.).
The nice thing about Spektrum is they have a flight log for $20 and you can range check your equipment at a given location and see fades, frame loses, and holds. I'm not sure if Futaba has that. But with the risk of meeting equipment that may not be FCC compliant then it might offer peace of mind.
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09-05-2008 01:28 AM  12 years ago
flynbryan

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Long Island N.Y.

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I have had the futaba FASST system and I now have the spektrum DX7. I love both. I never get a glitch and I'm flying right next to a water tower with all kinds wires, cables and antenna's attached to it. I have always used Futaba and this year I decided to try the DX7. No interference with either radio. I also had the PPM 6ex futaba at the same field and got brief glitches. So fast it didn't affect anything. Just a little "twitch" and back to normal. Once I switched I never got hit again.
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09-05-2008 11:11 AM  12 years ago
pchristy

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Hmm! I'll have to see if I can get my hands on a Futaba set again for a closer look! All the guys in my area fly JR / Spektrum.

I'm pretty certain that when I looked at one last time, the channels were clearly narrower than a Spektrum - but it is difficult to see 'coz the darn thing won't stand still!

;-)

--
Pete
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09-05-2008 11:45 AM  12 years ago
Climax

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West London, United Kingdom

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Hi Pete,

There are some interesting spectrograms here, take a look at:

http://www.rcgroups.com/forums/showthread.php?t=789547

You can just about see the "3db peaks" of the individual DSSS components. I believe that FASST uses 36 channels, and you can just about distinguish 36 (ish) peaks... They appear to be about the right width, i.e. 1.67Mhz
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09-06-2008 09:23 PM  12 years ago
pchristy

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Devon, England

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Yes, I've had a look at those. They've been take with a Wi-Spy dongle. I've got one of those myself, and its an excellent little tool for monitoring purposes, but I'm not sure how accurately it reports the bandwidth.

It seems to scan along the X-axis taking samples and building up a "bigger picture" - which is the blue image.

I've got an old, but fully functional (and accurate, as far as I can tell!) HP Spectrum Analyzer and I trust that implicitly!

All I need to do is get my hands on a Futaba set again for an hour or so.....!

--
Pete
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09-06-2008 09:55 PM  12 years ago
JKos

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Redondo Beach, CA

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Unless you can disable the hopping as done for the FCC tests, you may find it quite difficult to accurately measure the bandwidth.

- John
RR rules!
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09-08-2008 01:49 PM  12 years ago
Climax

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Just for completeness here is the FCC test report for the Futaba 10 channel FASST module (TM10-2.4GHz). There is a photo of it here:

https://gullfoss2.fcc.gov/prod/oet/...tive_or_pdf=pdf

For the actual test report, see:

https://gullfoss2.fcc.gov/prod/oet/...tive_or_pdf=pdf

Page 10 section 3.4 describes the 3 of the possible 36 channels selected for testing, i.e. for testing whilst the frequency hopping aspect of the module is disabled.

Page 39 section 4.6 gives the DSSS bandwidths for the above static channels during the tests (these are between 1.67MHz and 1.7MHz).

There is a photo of the bandwidth measurement setup, although is a bit blurred! Take a look at (page 58 section 10.2):

https://gullfoss2.fcc.gov/prod/oet/...tive_or_pdf=pdf

The maximum peak output power measurements are shown on page 43 section 5.6. Here the largest value is 16.97dBm and this equates to about 50mW, less the gain of the antenna.

The antenna gain is shown as being 1.3dBi (see page 5), which would put the output power at 67mW - this seems quite low? Have I missed something here?

Even with an antenna gain of 2dBi - this is suggested by the separate antenna specification, see:

https://gullfoss2.fcc.gov/prod/oet/...tive_or_pdf=pdf

The output power would only about 80mW...
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