Futaba Fasst vs Spektrum radio link

[3Disaster]

When I used to race (r/c cars) we had one race where everyone was either using a Spektrum or a Fasst. No problems what so ever there were 5 Fassts and 6 Spektrums. I had a Spektrum
Mike

[NormS]

This thread was started by a question concerning spread spectrum (SS) technology, so I thought I would take a stab at answering the original post. Disclaimer: I’m not a spread spectrum expert but I am an engineer with an RF, networking and embedded system design background, and I’ve done some research on the entire SS thing. What follows is my own opinion and interpretation of the marketing information from the different companies and some info I’ve gleaned from various SS-dedicated sites and whitepapers. Also, I own a Futaba transmitter, but I'm not a Futaba bigot; JR, Hitec, etc. are all great transmitters.

As I understand it, Futaba’s FASST is a pure frequency hopping spread spectrum system (FHSS) with no feedback from the receiver to the transmitter. Spektrum’s DSM is a direct sequence spread spectrum system (DSSS) that uses receiver feedback at power up to select a pair of clean channels. I believe that DSSS is technically a better approach then FHSS because it has lower latencies and supports higher bandwidths, but it’s a lot more complicated and more prone to multipath problems. FHSS is easier and cheaper and usually transmits at higher power levels; it works well enough for RC requirements. FYI, I think XPS is a hybrid frequency agile DSSS system that uses one frequency until the receiver’s bit error rate hits a limit, at which point they both hop to another pre-determined frequency.

FHSS works more or less like this: The transmitter and receiver use the transmitter’s unique ID code to build a table of frequencies, sort of a hop order list. The transmitter and receiver build the same table, since the receiver uses the transmitter’s ID code once its bound. When it powers up, the receiver syncs with the transmitter by just monitoring one frequency in the table and waiting until the transmitter hops onto that frequency. The receiver knows that it’s the correct transmitter because every frame contains that unique ID code. This is important because over about a 200mS period every FASST transmitter will hop through all frequencies, and the receiver has got to pick the right transmitter; the hop order is a lot different from transmitter to transmitter due to a low cross-correlation between the ID codes and the hop-order algorithm, so there’s no long-term interference between different systems. When the receiver is looking for sync, if that first channel is noisy, the receiver will wait a little and if it can’t find sync it will go sit on another frequency. It will eventually find the right transmitter, and then they will hop together across the entire 2.4GHz range. An alternative to the pre-calculated stored table of hops is to use the ID code to generate the hop sequence on the fly; everything else works the same way. FHSS systems broadcast each hop at the same power. If FASST supports about 100 separate channels and assuming an 80MHz total frequency range and allowing for guardbands, their channel bandwidth may be around 500KHz and their data rate will be in the order of 100-250Kbps. That’s much lower then a DSSS system can potentially have, but certainly adequate for RC usage.

FHSS doesn’t know or care if it steps on someone else’s frequency (another hopper or a DSSS system); if it loses a frame, it will get another one in a couple of milliseconds. BTW, I’ve seen some posts about FASST’s pre-vision technology alluding to pre-vision changing the hop sequence. Its hard to get any real info from all of the marketing stuff, but I don’t believe that’s what it does because of the required complexity and because that’s just not what hoppers do; I think pre-vision is some form of forward error correction coding which will allow the protocol to correct a certain number of bit errors and also detect and discard badly errored frames. There are a ton of error checking/correcting codes out there that are easy to implement and inexpensive from a bitrate overhead perspective. BTW, my understanding is that most military battleground systems use FHSS.

DSSS (Spektrum’s DSM) is a lot different from FHSS. The real baseband data (servo data) is XOR’d with a long, high speed pseudorandom sequence (PRS) which effectively raises (spreads) its bandwidth to maybe a megahertz or two. A DSSS system’s data rate can be very high if it uses a long PRS and a very high speed xor’ing function, since this will increase the overall channel bandwidth. The transmitter and receiver use the same sequence, which is derived from the transmitter’s unique ID code which the receiver gets when its bound to the transmitter. The receiver knows the channel(s) its using (agreed upon during the power up sequencing) but it has to get in sync with the transmitter and then autocorrelate the real data from the high speed data (using the same sequence), which de-correlates any narrowband jamming signal such as a FASST system that hops through their channel. This correlation process introduces something called processing gain which allows a DSSS system to operate at lower power or to get better range at the same average power as compared to an FHSS system. Because of the different pseudorandom sequences in different transmitters, two or more DSSS systems can actually use the same channel at the same time with no interference (this is how CDMA cell phones work) but I don’t think Spektrum makes an attempt to do this because they seem to have a limit of 40 systems at a time. Spektrum uses two channels per link; if one is hit or goes down, its got another one running in parallel and it probably selects the “best” data from either receiver for each frame. If a DSSS system uses a long enough sequence, it is effectively unjamable and if the transmitted power is below the ambient noise its also totally undetectable with any kind of receiver (I bet the spook/delta force type guys love that one!). Speaking as an engineer this is all much cooler then frequency hopping, which is sort of a brute force solution.

DSM may be technically better then FASST (depending on your definition of better), but I think Futaba did a nicer job on the entire system by making their receiver work down to a lower voltage level and reboot/resync faster. Things like that are really important to me, much more important then how cool the approach is. Also, it’s a lot easier to deal with one receiver then several, and FHSS handles the multipath issues that are inherent with 2.4GHz systems better then DSSS. I believe this multipath business is why Futaba can get away with only using a two-antenna diversity system while Spektrum had to use two separate receivers and two different channels. DSSS’s (potentially) better range and higher speed (lower latency) data capability are not that important to me because you can’t fly out of sight anyway, and when you get down to it, we’re just flying little planes around with relatively low resolution servos and we really don’t need to send that much data.

I’m personally waiting for the TM-8 modules from Futaba for a couple of reasons: I think FHSS is good enough and I like their overall system design better then Spektrum’s. The main reason, however, is that I own a Futaba 9c super transmitter and if something goes wrong with the thing I don’t want to get into a pissing contest with two different companies to get it fixed; I’ll just pack the entire thing up and send it off to Futaba and let them worry about what broke. Also, I’m hoping that Futaba offers a software upgrade in the future to my transmitter and module to add features or maybe that ninth channel; this is something that Spektrum can never do in a mixed Futaba/Spektrum system. Notwithstanding the above, I think Spektrum has built some terrific stuff and that they’re a great choice if you like the technology or if you finally get tired of waiting for Futaba to ship their gear. A Spektrum module in a JR transmitter might also someday give you the software upgrade I hope to get from Futaba.

In my mind, a bigger issue concerning the Futaba stuff isn’t FASST vs. DSM2, its Futaba’s apparent lack of compatibility within its own system. You may know that the 6EX/2.4, 7C/2.4 and TM-14 module works with all of their old and new receivers while the TM-7 does not work with the original 6-channel receiver, the R606FS. This clearly indicates that in some sense there is a FASST-1 and FASST-2 system out there, and I would really like to find out what they changed (or fixed) when they came out with the newer stuff. The TM-8 module is listed on their webpage as NOT compatible with the R606FS, but its late shipment is hopefully caused by a redesign to fix this. At least I’m hoping that’s what’s causing it.

[somethnextra]

Good info. Maybe the ID sets have been changed in the new modules and radios?

[3ddd]

	Quote: Originally Posted by jonkoppisch
	I just mention that as it seems that is the excuse given by the companies for the loss of the aircraft... The test that Julez did, I believe shows that the receiver will still operate below the operating voltage of the servos so if you really loose a plane because of low batteries, it's actually because you deserve to

---------------------- the perfect response

[somethnextra]

Seriously. I haven't heard of any planes crashing because of interference on 2.4. Chances are the plane crashed because we screwed something up. Then we buy a new one with 2 2.4 receivers!

[d_wheel]

	Quote: Originally Posted by NormS
	DSSS (Spektrum’s DSM) is a lot different from FHSS.

Good information. However, there is one detail that I did not see. FASST is actually a DSSS system that also hops. The FCC data is here on page 5.

Later;

D.W.

[NormS]

	Quote: Originally Posted by d_wheel
	Good information. However, there is one detail that I did not see. FASST is actually a DSSS system that also hops. The FCC data is here on page 5. Later; D.W.

Hmmm
Well, I can’t argue with what it says, but here’s a link to a FAQ on Futaba’s FASST site: http://2.4gigahertz.com/faq/faq-6ex-2_4ghz-q888.html. Futaba seems to be trying to differentiate FASST from DSSS. Also, their talk about channel shifting and continuously changing frequencies sure sounds like hopping, and they mention somewhere else on their FASST page that they shift frequencies every 2mS.

When I was looking into the entire SS thing I did come across a few references to hybrid DS/FH systems in some of the stuff I found on the web, but wouldn’t the report have to label the system as such if that’s what its doing?

[d_wheel]

	Quote: Originally Posted by NormS
	Hmmm Well, I can’t argue with what it says, but here’s a link to a FAQ on Futaba’s FASST site: http://2.4gigahertz.com/faq/faq-6ex-2_4ghz-q888.html. Futaba seems to be trying to differentiate FASST from DSSS. Also, their talk about channel shifting and continuously changing frequencies sure sounds like hopping, and they mention somewhere else on their FASST page that they shift frequencies every 2mS. When I was looking into the entire SS thing I did come across a few references to hybrid DS/FH systems in some of the stuff I found on the web, but wouldn’t the report have to label the system as such if that’s what its doing?

It appears that Futaba ,or the FCC, would have to be the one to answer this question. Which leaves us to guess that it is indeed a hybrid type of SS. Most likely it is basically a DS system that uses some type of hopping algorithm to move the DS signal to a new frequency every 2ms.

Regardless of how they are doing it, we have several systems in use in this area and have had absolutely no problems. Everyone seems happy with it's performance and reliability.

Later;

D.W.

[NormS]

	Quote: Originally Posted by d_wheel
	It appears that Futaba ,or the FCC, would have to be the one to answer this question. Which leaves us to guess that it is indeed a hybrid type of SS. Most likely it is basically a DS system that uses some type of hopping algorithm to move the DS signal to a new frequency every 2ms. Regardless of how they are doing it, we have several systems in use in this area and have had absolutely no problems. Everyone seems happy with it's performance and reliability. Later; D.W.

Right.
I know about half a dozen people that have either the 6EX/2.4 or a DX6/7 and they're all very happy. No modules (Spektrum or Futaba) or XPS stuff yet, but I've seen several 2.4g users flying at the same time with no problems, both Futaba, Spektrum and mixed. However they've done it, their systems seem to work as advertised.

[Eagleburger]

Would be good if Futaba got a move on with their 12FG. But if futaba's are going to be module based and their is no latency maybe I should go the 12Z. I just want to see the two together before I buy. The 12Z is a beautiful radiio, but would be spoiled if it had a big ol clumsy module/antenna hanging of the back.

[thewrap]

I haven't heard much from the Futaba FASST systems. But I haven't heard anything really bad from Spektrum. I have the DX7 and have only a minor problem with a reciever that I bought. Sometimes it'll bind and sometimes it won't. Other than that the selection of radios coming from spektrum are great and priced affordable. Out of the 2 I would say that Spektrum is better. Few problems and a great rep.

[thewrap]

	Quote: Originally Posted by NormS
	Hmmm Well, I can’t argue with what it says, but here’s a link to a FAQ on Futaba’s FASST site: http://2.4gigahertz.com/faq/faq-6ex-2_4ghz-q888.html. Futaba seems to be trying to differentiate FASST from DSSS. Also, their talk about channel shifting and continuously changing frequencies sure sounds like hopping, and they mention somewhere else on their FASST page that they shift frequencies every 2mS. When I was looking into the entire SS thing I did come across a few references to hybrid DS/FH systems in some of the stuff I found on the web, but wouldn’t the report have to label the system as such if that’s what its doing?

And that doesn't sound very promising. What would happen if one shifted off just a single channel. That leaves me a little bit skeptical. Spektrum works by when you turn it on it grabs a slot or frequency and holds it. So nothing else will interfere on that slot. Makes a little more sense than channel hopping.