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 AspectsDiversity 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
• 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.
• 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.
Avant Aurora Ultimate