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HelicopterRadio - Spektrum DSM › My experience with the DX7 spread spectrum radio
11-11-2006 09:20 AM  10 years agoPost 1
AugustoElite Veteran - US - My Posts: All  Forum  Topic

I've been asked many times why I don't write articles anymore and the reason is that I've been very busy at work the last couple of years. I'm finally having some time to start writing articles again so here comes one of hopefully a series of articles related to our hobby.

DX7 Report

On a recent business trip to San Francisco I had the opportunity to have dinner with Paul Beard. Paul is the inventor and designer of the spread spectrum radios for RC applications. He also works now for Spektrum, the company that manufactures the spread spectrum RC systems.

Paul and I have had discussions in the past about spread spectrum technology although not related to RC systems and shared info on integrated circuits and RF design because we have both been involved in the design of spread spectrum systems so we are both very familiar with the technology used in those systems.

When we got together for dinner he surprised me by bringing a DX7 radio for me to try. Honestly I haven't been following the development of the spread spectrum systems for RC but I was obviously curious about the technology. If you had asked me before that dinner if I would risk my heli testing a new system I would have answered probably something like "not no but hell no". Despite that I was curious and excited but I had many questions about the transmission system's robustness, latency and all kind of electronic issues I would be concerned about if using a spread spectrum system. After all I didn't want to risk my brand-new Aurora on a system that I didn't know too well.

After several hours of conversations and at that point when you know the waiters are about to kick you out of the restaurant I was pretty comfortable with the technical side of the technology that he invented and put to use in this system. Once I realized the depth of care to the smallest detail in the security and reliability of the transmission link I knew I would have no problem giving it a try. I did warn him that I was very particular when it came to radio response because I do a lot of fast piro-flying and fast response is a must for that kind of flying to which he responded with a smile and a "don't worry this is the DX7" to which I answered "ok I hope you're right we'll see".

Here's a report on how the system worked and a few interesting things about how the technology works as well as the result of my experience with it last Sunday flying my Aurora.

Installation

It's important to understand that in order to maximize the performance given by the numerous diversity link robustness features the two receivers should be installed in such a way that optimizes the ability of one receiver to be in the most optimized receiving orientation when the other one is in the less optimized one. That way you get to have at least one optimally oriented receiving receiver no matter what the orientation or location of the aircraft is.

This is achieved by cross-polarizing the antennas so that one antenna is at its maximum receive gain when the other one is at its minimum.

Since the system uses two receivers each one with its own antenna what you do is install the receivers in such a way so that the antennas are pointed perpendicular to each other as you can see in the following picture.

When seen from the front the antennas are aligned in perpendicular planes (90 degrees)

The main receiver is located on the radio tray and the secondary receiver is servo-taped to the front side of the GY-611.

Since I fly Futaba all the servos have tabs so what I did was to cut some slots in the case because I don't like cutting the tabs off the Futaba connectors. Besides I didn’t know if I was going to like the radio enough to leave it there and I didn’t want to end up with a bunch of tab-less connectors.

As you can see in the following picture the connectors go inside the receiver with the tabs still in place and can be connected by making a small slot on the receiver's plastic case.

The radio uses 7 channels so it's able to control everything as well as the governor speed and the gyro gain. Notice how the battery is connected in a horizontally aligned slot below the 7 channel ones. That connector is also used to install the binding plug used when binding a receiver to the transmitter. By the way, the binding binds not only the transmitter to the receiver but also to the selected model program in the transmitter to the particular receiver in the model so no more flying heli #2 with the Heli #1 program because it simply won't work with it.

One of the most important features this radio has is the additional receiver for the secondary receive path.
The small receiver is connected to the main one by a 6 inch 3-wire cable.

This is not an additional antenna. This is a complete fully working receiver that receives, de-spreads and demodulates the signal into its data components and sends the information to the central processing unit for processing.

Programming

I found the programming pretty simple and straightforward.

Pitch Curves are adjusted on each screen for the four flight modes Normal, ST1, ST2 and Hold:

The Throttle curves are also pretty straightforward and simple to program.
I like the 5-point curve because with the EXP function it pretty much takes the round V-Shape pretty easily. Not that it matter that much anymore because in most cases it’s only needed when the governor stops working.

Programmable mixes can be used to compensate for extra load from cyclics or rudder inputs. A nice feature is that they can be activated based on the flight mode. In this case they are only on when you're on stunt 1 or 2 (IDLE UP 1 or 2)

Miscellaneous functions like ATV or travel adjust, swashplate selection and swash mix programming, timer and servo monitor are also pretty easy and straightforward.

Failsafe on the throttle is programmed by simply setting the throttle to the level you want and re-binding the receiver and transmitter.

Technical Aspects

Diversity a.k.a. Redundancy

For people that work on spread spectrum systems there is a concept related to the robustness of the link called diversity.

Diversity or redundancy is a pretty much a way to describe any spread spectrum system that has some mechanism that allows it to receive several copies of the same information in several ways to insure that somehow at least one or multiple copies of that information successfully reach the receiver.

During my conversation with Paul I was able to learn that in the case of the DX7 redundancy was taken to an unusual level to assure that the possibility of a lost link was pretty much nonexistent.

There are three kinds of redundant schemes that were applied to the DX7 system.

• Frequency Redundancy:

Every DX7 system transmits in two channels simultaneously to both receivers at the same time. This means that the transmitter is continually transmitting in two channels so even if one channel got jammed somehow, which is by itself almost impossible to do since it's an anti-jamming spread spectrum signal, there is a second channel that still transmits and both receivers receive that channel.

• Path Redundancy

Path Redundancy means that the receiving end gets information from two different transmission paths. This is achieved in the DX7 because having two different and separate receivers in different physical locations assures that each one "sees" the information coming from two different paths. If you add to this that the installation of the antennas is perpendicular then the Redundancy gets strengthened by the fact that they will also see different polarizations that will change as the aircraft changes flight attitude respect to the transmitter.
Again this brings two copies of the information through two different paths making it almost impossible to have both fail at the same time. It might not seem a lot to have a receiver a few inches apart from the other but we need to remember that in terms of wavelengths a few inches is a world away at 2.4GHz.
At 2.4 GHz a half wavelength (180 degrees) is only 2.45 inches. This is the distance that matters because it means the second receiver is getting a completely opposite signal 180 degrees apart in phase.

• Time Redundancy:

Additional to the ones above there is a third Redundancy namely Time Redundancy. It means that the information packets are sent twice in different time slots so that if the package doesn't somehow reach the receiver end the first time it will very likely receive it the second time.

Additional to the ones mentioned above, there are some other aspects of the demodulation process built in into the silicon chips used that have to do with multi-path demodulation that add something called phase diversity to it that enhances the robustness of the link even more.

After listening to the technology involved and knowing from my own experience in spread spectrum I was pretty sure that the risk of losing my Aurora because of a link failure was pretty much non-existent.

Channel assignment

One of the interesting aspects of the technology is that the transmitter will listen to all the 2.4 GHz radios in the vicinity before selecting what channels it will use to transmit. Once it knows what channels are available it assigns the new channels and tells the receiver what channels are going to be used. Keep in mind that there can be up to 40 aircraft at the same time in the air.

In the very unlikely event that there are more than 40 transmitters transmitting in the air the transmitter simply won’t start. Despite that even if for some reason two transmitters happened to end up in the same channel it won’t be a problem simply because being spread spectrum the two transmitters are using different spread sequences and they are also differently synchronized. In such a case the receiver’s de-spreading process will convert the other transmitter’s signal into background noise and recover only its own signal preventing interference.

As a matter of fact that’s exactly how spread spectrum works in our cell phones where all the phones share the same channel and use different spreading codes to prevent information collisions.

It’s interesting to note that in reality when you turn a DX7 one you’re actually joining a very organized “many one-to-one” network with a very robust fail-proof protocol.

Flying

It all sounded pretty good and I knew from my own design experience that such a transmission system was solid as a rock so I had no qualms picking up my Aurora and installing the equipment in place for a test. I just wanted to do the test I was interested on which was the response in flight.

First thing I did once I had the radio installed was to do a range check.

The DX7 has a nice feature for range testing. Since there is no antenna to collapse the system uses a button in the back of the transmitter that will cut the power of the transmitted signal down 40 dB while it’s being pressed. For those not familiar with dB numbers that mean that the signal’s power will be ten thousand times lower than normal.

I walked 30 paces away from the heli and while pressing the green button I did my range check and everything kept working perfectly.

I checked the throttle hold and the failsafe by turning the transmitter off to check the throttle and everything worked perfect so I went to the helipad to give it a test flight.

Before I took off I set the radio on the ATV adjust to be able to adjust the governor’s speed by clicking up and down on the switch.

I took off and adjusted the speed of the governor until it sounded right. After that I tested the pirouetting rate and adjusted it to my liking.

After that I simply went off and started flying it hard. I wanted to see how it would performs and the initial feeling was pretty good. The Futaba 9351 servos were working perfect with the DX7’s receiver. Then it came the moment for the real test. Fast pirouetting maneuvers and that’s where I was very surprised.

Usually I can tell how fast the response of a radio system is by simply doing very fast pirouetting maneuvers like for instance real fast piroflips. When you do very fast piroflips you have to advance your commands ahead of time so that by the time the commands reach the servos they end up at the spot you intended them to be at.

For instance I know that with a 9Z I need to advance them about a quarter of a pirouette. With a stylus or a 14MZ I know I got to be only about 30 degrees ahead which is a pretty fast response. This is one of the reason people say that it’s a lot easier to do fast piroflips with a 14MZ. What surprised me is that with the DX7 I needed to be even less ahead. Probably only about 20 degrees ahead. This surprised me because I had heard some reports that the DX7 had a latency similar to the 14MZ and the Stylus but this real life result in flight, which is the only one I really care about, told me it was actually faster somehow.

Suffice it to say I was really happy and flew the heli all day with the DX7 tank after tank and was flying it as hard as I could. When I got home I called Paul and told him about my experience and asked him how was it that I was getting faster than expected response. He explained to me that the transmission protocol was more complicated than what the tests were designed for and that the CCPM servos were in a much faster response path than the other ones resulting in the better response.

Conclusion

CONS:
• Call me shallow but I like more bling hehehe
• I wish it had an additional channel so I could use a fuel mixture servo. I know, I know I’m spoiled.
• Lack of a kill switch (easily done with the throttle trimmer though)
• I think they need to use some dummy weight in the top portion of the radio to mimic the antenna weight or move the hook a bit lower in the front face of the radio. It's not like it's bothersome or anything like that it's just that I'm used to a lifetime of flying with the weight of the antenna up front.

PROS:
• Fantastic response when paired to good servos like the S9351
• Bulletproof transmission system.
• No glitches
• Simple and straightforward to program
• Radio $350.00… no waiting in line for frequencies at the IRCHA Jamboree priceless

I hope you guys find this information useful.

Augusto.

Avant Aurora Ultimate

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11-11-2006 09:32 AM  10 years agoPost 2
rroback

Elite Veteran

Irvine (UCI), Ca

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Great to hear. Does this mean I have to sell my 14mz :P ?

Rhett..... I can't fly, but the Profi sure can.

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11-11-2006 09:35 AM  10 years agoPost 3
Augusto

Elite Veteran

US

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nope I'm not selling mine either. I'm too much of a hardcore Futaba guy but it's pretty damn good to have one of this next time you go to a funfly
I'm keeping mine installed in my Aurora for the time being though. The feeling from the response is pretty addictive.

Augusto.

Avant Aurora Ultimate

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11-11-2006 11:56 AM  10 years agoPost 4
dialarotor

Elite Veteran

Traverse City, Michigan

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Thank you Augusto,
A very nice, clear concise report for us lesser pilots and you even backed it up by putting your AURORA on the line to prove the technology.

Think I will hold off on a new 72mhz radio system.

Dialarotor

RapRex Pilot

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11-11-2006 12:36 PM  10 years agoPost 5
tchavei

rrProfessor

Portugal

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Very nice indeed. I just wish it had the bling of the 14Mz. How is stick feel compared to the 14mz?

Now stop playing around and go back to work to finish that batch of Auroras we are waiting for!

TOny


--------------------
"Perfection and patience usually walk side by side..."

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11-11-2006 12:53 PM  10 years agoPost 6
THX1138

Elite Veteran

Camp Bondsteel, Kosovo

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Thank you for the wonderfull write up.

So Paul, how about modules for the 10x nad 14mz?

Lou

Just give me five dudes and two flashbangs...

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11-11-2006 01:38 PM  10 years agoPost 7
andres.c

Key Veteran

Florida , Dade County 33173

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Great Work as always - Thank you for taking the time to write this article up for us here intersted in this radio system

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11-11-2006 01:40 PM  10 years agoPost 8
andres.c

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Florida , Dade County 33173

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Paul

Also thanks for letting Augusto play with the new toys

maybe he will let you play with the Aurora , LOL

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11-11-2006 01:56 PM  10 years agoPost 9
HUTCH964

Key Veteran

Taylor,Texas 76574 local airport T74

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Very Cool!

But Hummmm I will wait for the 9ch or a module for the 9303..

Thanks for all the hard work Augusto and well done report!

*****

Tim

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11-11-2006 01:59 PM  10 years agoPost 10
Professorwiz

Veteran

Livonia, Michigan - USA

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This sounds awesome for the ap guys. Much less chance to get shot down. Is there any info on when a normal guy can buy this system and check it out?

Russ
AP wanna be..

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11-11-2006 02:05 PM  10 years agoPost 11
Ralphw

Key Veteran

Spring, TX

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According to the Horizon website it's should be available in early December.

Ralph W.
"Life's Short, Fly Fast"

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11-11-2006 02:19 PM  10 years agoPost 12
JKos

rrProfessor

Redondo Beach, CA

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Great write up Augusto! I definitely look forward to seeing more articles from you.

> I had heard some reports that the DX-7 had a latency similar to the
> 14MZ and the Stylus but this real life result in flight, which is
> the only one I really care about, told me it was actually faster
> somehow.

> He explained to me that the transmission protocol was more
> complicated than what the tests were designed for and that the CCPM
> servos were in a much faster response path

Yeah, there's just a little more to it then straight numbers although they don't lie much. Paul was VERY conscientious about helicopter applications of the DX7 and took great care to make sure the latency of the swash channels and eCCPM performance are world class. He definitely hit the proverbial nail on the head!

Thanks,
John

RR rules!

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11-11-2006 02:34 PM  10 years agoPost 13
greg

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Yorkville, IL

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Waiting for the 9 channel version as well. I don't think the modules that some are waiting for will have the same response time that this baby has. But I could be wrong. Paul Beard responded to one of my pm's some time back and said they had no intentions of producing modules. So if modules do come out it will be from other manufacturers.

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11-11-2006 02:46 PM  10 years agoPost 14
Professorwiz

Veteran

Livonia, Michigan - USA

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Is there any more info reguarding range? I know that the 2.4 will give better range just because of the freq, but is there any word on what can reliably be expected in a "noisy" near city type environment?
Russ

Go that way really fast.. If something gets in your way.. TURN

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11-11-2006 03:17 PM  10 years agoPost 15
BJames111

Elite Veteran

San Diego, California

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sounds great! After talking with Mike Sanereki (sp) from Horizon, he assured me that they were using this system in Jets, which you can imagine get far away really fast, with no problem at all! Do not be concerned with range with the DX7.

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11-11-2006 03:43 PM  10 years agoPost 16
Trace

Senior Heliman

Wildwood, MO

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I too am waiting for the 9 Channel version..(this thing looks darn good though).... Not because I need the additional channels, but more because I like the additional programming options.

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11-11-2006 03:59 PM  10 years agoPost 17
stickyfox

Key Veteran

Rochester, NY - US

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Thanks for the heads up. Maybe you know the answer to this:

I really like the DX6 but there is a delay in calculating the third CCPM servo (from AIL to AUX). As a result it can't beat the stock transmitter on the BCP. Has the DX7 taken care of this? My DX6 is sitting in a box for when I take up flying planks some day.

I can show you pictures but it sounds like you'll know what I'm talking about. The guys in the hobby shop don't, and I'm not spending the money until I know it's fixed.

-fox

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HelicopterRadio - Spektrum DSM › My experience with the DX7 spread spectrum radio
11-11-2006 05:17 PM  10 years ago •• Post 18 ••
drdot

Elite Veteran

So. California, Orange County.

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fwiw...

Once again, an unsolicited testimonial from one of the best...SS is here, it works, and conventional fm/pcm systems are officially obsolete.
Thanks, Augusto, great job: and Many thanks to Paul Beard!

John.

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11-11-2006 05:52 PM  10 years agoPost 19
AirWolfRC

rrProfessor

42½ N, 83½ W

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FWIW,
To really have an omni-directional receive strength, you need 3 antennas perpendicular to each other. With only two, you still have a 3Db differential to the missing 3rd antenna due to polarization being 90º out of position. Two antennas are better than one but three are better yet but,here apparently, deemed not worth the effort or cost.

I'm glad that Paul is no longer calling the second receiver just an "antenna". http://runryder.com/helicopter/p2322977/

StickyFox,
I really like the DX6 but there is a delay in calculating the third CCPM servo (from AIL to AUX). As a result it can't beat the stock transmitter on the BCP. Has the DX7 taken care of this?
Spektrum tells me that this question has been addressed in the DX7 by moving the AUX channel up next to the ELE and AIL when in CCPM mode.

drdot,
Once again, an unsolicited testimonial from one of the best
You can hardly call being given a FREE DX7 "unsolicited".

Wolfgang

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11-11-2006 06:13 PM  10 years agoPost 20
RCfan

Key Veteran

Longwood, FL USA

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Augusto, very good info and explanation of the SS technology, e.g. diversity. Looking forward to seeing this being used by folks and how other vendors will try to "compete" with the tech.

Oh, and looking forward to seeing more articles from you!!

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