Gregor99
Key Veteran
Location: Western Wa
| Funny how we all bring our RC questions here even though many of them are not Esky related. I do it too. Mostly because there are some many helpful members in this little sub forum.
Anyway on to the question. The DX6 is based on the last Spektrum protocol DSM v1. When I was shopping for one, I read a lot of forum threads like "Hacking the DX6 to work with more than 2 models" and "I don't care what they say I'm using a DX6 on a larger heli". Kind of made it sound like it was for small park fliers and pilots with a small fleet. The DX6 also is very limited in the range of adjustments for things like pitch and throttle curves. It’s been superceded by the DX6i which uses the new DSM2 protocol.
I know how little enhancements and bug fixes can quietly get implemented in the latest release of, well you name it. Software, cars, anything were code is running. The DX7 (and DX6i) are based on Spektrum's latest rev of their protocol DSM2. DSM2 adds new functionality addresses the Spektrum nay-sayers reliability concerns by simultanuiously transmitting on two channels. All their latest stuff, including their dual receivers use the new protocol.
You didn't mention Futaba, so I will. Until Spektrum came out Futabu was all I used. I still have a number of old Futaba radios for my cars. Love them and believe in the brand. Spektrum was new comer to the RC industry and rocked the status quo their crystal free, no interference paradigm. The big names are trying to catch up but many like JR (a Specktrum partner) simply license the technology and build the radio around it.
The magic of Specktrum is that it scans the current frequencies and automatically selects two open channels a primary and a backup . This helps avoid interference in "steady state". But if some 2.4ghz device were to come on and stamp on both of your channels, then you are toast and would have to power cycle the Tx and Rx to get onto a clear channel. The second channel was added in the DMS2 protocol and is implemented in all DSM2 receivers as well. Since the radio isn’t changing frequencies, the second channel is pretty much a requirement to address the weaknesses in the first implementation.
The more common implementation is to rapidly change channels on both Tx and Rx. This way if there is interference it’s only for a millisecond. This is referred to as frequency hopping. Or in its full name "frequency hopping spread speckrum".
As I recall, the folks at Spektrum tried FHSS but couldn't get the bandwidth they were after as well as the speed required to implement some of their advanced features . There is some latency (however small) in switching channels. Most implementations do the switching many many times a second. So the delays add up. Also frequency hopping uses a narrow band vs Spekrum's method that uses a wider band, and is called direct sequencing. As there is also some bandwidth overhead for channel switching, the already smaller data path is further constricted. Specktrum gains a fair bit of bandwidth by staying on the same channels, and gets it redundancy via running the two channels.
Specktrum has this feature (don't remember what's called) where they can sequence simultaneous commands, send them out through the serialized DSM2 protocol, then re-assemble them on the receiver preserving the time alignment. For eCCMP heli, 3 servos are constantly being controlled. Without this feature, what is actually happening is that the Tx does the mixing for the 3 swash servos, then sends out a the single servo move command to one servo, then the next then the next. There is no mixing on the Rx so when the signals arrive, they are sent to one servo at a time. It all happens quite fast so it’s not obvious. What the DSM2 protocol does is send out the each command with timing or "sequence info". When the Rx receives it assembles the matching commands and operates all three servos simultaneously.
All that said to give you the background for the Futaba implementation called "FAAST". They use the more resilient frequency hopping variant. Its constantly switching channels so it virtually impervious to narrow band interference. This method comes at the cost of bandwidth so there is less room to incorporate advanced features that consume precious time in the data stream. I couldn't find details of the FAAST protocol but as I recall they don’t have the sequencing reassembly feature. However the protocol is lightning fast so it may not be critical. Futaba is keen to point out the speed of their protocol is faster than Spektrum’s. So it’s likely that what they lack in bandwidth and inherent protocol speed, they make up for in processing speed in the Rx and Tx.
In summary, I would avoid the DX6 and if budget is an issue consider the DX6i. If you are going to be in the hobby for while consider going upmarket to the DX7, the much more expensive 9 channel Spektrum version from JR, the X9303. Or look look at some of the FAAST offerings from Futaba. |
