3DHS --FLY LOW-- Always OFF TOPIC

[njswede]

My field has about 20-30 people on a Saturday, 40 on a good day. Kinda hard to find 2 clear channels with 6 people up at a time, all using 2.4.

First of all, it's not that hard. Second of all, DSM2 uses a technique called CDMA (Code Division Multiple Access) which allows multiple transmitters to used the same frequency without knocking eachother out. I'd love to geek out and get into the details, but you'd fall asleep...

 

[RCAddict16]

Hit me with your best shot. I'm a sucker for details. Most people don't get half of what I say.

 

[losifanatic]

It's not that you need more of the bells and whistle when upgrading from the dx6i its just that radio has low resolution, cheap sticks, high latency and other low end stuff. I just feel that it's not s very good radio and if 3ding or precision flying a better radio makes all the difference in the world.

 

[RCAddict16]

Any other suggestion? Does futaba make a decent radio for around $200? I couldn't go over that...........

EDIT- just checked out the 6EX. $220 I could do. Any major advantages of the 7C over the 6EX? If its a lot better I could swing $60.

 

[njswede]

Hit me with your best shot. I'm a sucker for details. Most people don't get half of what I say.

You asked for it...

Ok, here's the simplified version:

With CDMA each transmitter sends out a signal that's been "encrypted" with a unique key so that no transmitters in the world would never have the same key(1). When you bind to the receiver, the transmitter and receiver agree on the same unique key, so they can understand eachother. However, no other receiver in the world would understand it. It would just look like backgrund noise to them. Now, this encoding has another interesting property. You see, the codes are worked out such that even if two transmitters happen to be on the same frequency, the receivers can still recreate the signal from their transmitter by applying their encryption key. Why this is requires a bit of pretty deep math to explain, but it has to do with the fact that the signal just looks like noise unless you have the right key.

Another interesting thing about CDMA is that you spread out the signal over a much larger frequency band, which is great for resiliency against interference. Let's say your signal occupies 10MHz and some nasty bastard transmits another strong signal that occupies 5MHz in the middle of that spectrum. At that point, you've lost 50% of your signal and you're probably toast. Now, let's assume instead you spread it out over 100MHz. A 5MHz wide interference would only destroy 5% of your signal, and you'd have 95% left. And that's most likely enough for the clever software in the receiver to "guess" the remaining 5% and perfectly recreate this signal. Coincidentally, this spreading out over a larger frequency band is exactly what frequency hopping does. Signal processing nerds would say that CDMA (or DSSS) does it in the "time domain" and frequency hopping (FHSS) does it in the frequency domain. But the end result it EXACTLY the same (although FHSS is much easier to understand).

So, in summary, CDMA does two things for you:

1) Give your signal a unique "signature" that is only understood by the receiver and distinguisable even if someone else is transmitting on the same frequency.
2) Spread out the energy of the signal over a large frequency band, thus making it more resilient to interference.

Remarks
(1) - At least it's extremely unlikely, you'd have to have trillions of trillions of transmitters for it to be even remotely likely.

 

[RCAddict16]

So how many mghz does your signal occupy? And if in the case of an idiot hopping your signal, can the rx recreate half of it?

 

[njswede]

So how many mghz does your signal occupy? And if in the case of an idiot hopping your signal, can the rx recreate half of it?

I'm not sure exactly what the frequency bands look like with DSM2, to be honest. And yes, if another TX happens to be on the same channel, your RX is able to recreate it. Obviously, there's a limit to how many transmitters can use the same frequency before it becomes difficult for the RX to decode it. And I don't know exacly what that limit is either. But I know that 10-20 transmitters on at the same time is no match for DSM2. You can probably easily have 50 transmitters on without problems, but when you get to events like Joe Nall or SEFF with hundreds of transmitters, you may run into problems. And that's why they created DSMX, which is essentially just DSM2 with a layer of frequency hopping on top of it.

So the bottom line is that I wouldn't worry about DSM2, unless you plan to attend huge events. The DX7 seems to be a much better transmitter than the DX6i, and if you can get a DX7 for about the same price as a DX6i, I would jump on it. Unless, of course, you plan to spend a lot of time on huge events like Joe Nall, IRCHA or SEFF. I can personally attest that DSM2 was not a problem when we were 13 planes flying the "total destruction, no rules, full contact combat" at FLIE. All crashed were caused by malicious attacks by fellow 3D enthusiasts...

 

[njswede]

And another interesting fact is that the more people switch to DSMX, the less likely they are to interfere with your DSM2-link. This is actually easier to understand, since DSMX changes frequency all the time and is not going to spend more than a few milliseconds on the two channels you're hogging with DSM2.

 

[gyro]

I'll reiterate my initial advice. Find a used DX7 and you'll be set for a while.

 

[SteveT.]

No way gyro.......not using 2 channel selection DSM2........... I like the channel hopping much better.

You only need DSMX if you are flying where there are more than 40 2.4gZH radios in use at one time. DSM2 is fine up to that point....

SteveT