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The difference between DSM2 vs DSMX

Mik3

Mr. Gee Bee
#1
DSMX is basically two layers of multi-access techiques.

DSM2 is based on something called CDMA (Code Division Multiple Access). In simple term, this means two things: 1) The signal is spread out over a wider frequency band and 2) each transmitter/receiver pair uses it's own coding scheme to "scramble" the signal. The coding schemes are designed in such a way that even if two stations are transmitting on the same frequency, the respective signals can still be isolated by the receivers, thanks to the coding schemes.

CDMA is great, because it's resilient to static, interference and other transmitters transmitting on the same frequency. However, there's a limit to how many stations can transmit on the same frequency before the signal gets too diluted to be successfully isolated by the receiver.

So DSM2 employs a simple scheme to try to avoid this. When starting up, the transmitter tries to find two free frequencies and starts transmitting. The receiver is scanning the frequencies for the unique signature of its paired transmitter. Once found, the receiver locks in and you have a working link. DSM2 uses two separate channels, so if one gets knocked out by interference, the other channel may still be usable. The problem is that the channel allocation happens on transmitter startup, so if both channels become unusable at some later point in the flight, you may still lose the link.

And that's why DSMX was designed. DSMX essentially uses the same encoding scheme as DSM2, but the frequency changes thousands of times per second according to a pseudo-random sequence negotiated between the transmitter and receiver. Even if some channels used become completely saturated and unusable, it would normally only result in cut-outs a couple of milliseconds, which would be too short to even notice (in theory).
I found this while searching/scratching my head about the differences between the 2.

If I understand this correctly, and please correct me, for 99.9% of RC fliers there will never be a need for DSMX and I shouldn't fret too much over going with a DSMX Tx over a DSM2 Tx?
 

Foam Addict

Squirrel member
#2
Yes and no.
I've been blanketed several times with DSM2, and did lose one plane because I couldn't recover it after being blanketed.
That being said, unless you are flying a souped up stryker or Pylon racer, you probably won't notice it.

Yes, DSM2 is fine for most areas and aircraft, especially indoors.
 
#3
If you are flying alone, and you are not flying near any sources of radio interference, there will likely be no difference between DSM2 and DSMX.

If you are just flying a cheap foamy, I really wouldn't worry about using DSM2.

However, if you have a TX that does DSMX, and you are flying an expensive plane, it is just silly *not* to use a DSMX RX.
 

El_AMPo

Junior Member
#4
Easy terms: DSM2 uses 2 channel hopping on the 2.4G band and DSMX uses the full 2,4G spectrum.

If you are away from wireless security cameras, microwaves and saturated wifi spots, both work equally well.
 

Cyberdactyl

Misfit Multirotor Monkey
#5
I have flown at Dorothea Dix field in Raleigh NC many times with DSM2 with many other DSM2s, where there was well over a dozen or so planes, helis and MRs in the air at once with no problems at all. I have no idea what would "saturate" DSM2, but it must be a LOT.