Review: Spektrum's DSMX

I've been a long-term critic of the Spektrum DSM2 2.4GHz system and that has earned me much scorn from Horizon and Spektrum themselves.

For almost two years, I have tried to tell the world that DSM2 was past its "best-by" date and that in an increasingly noisy environment, it simply was not as good as the growing number of FHSS systems being sold by other manufacturers. In fact, before DSMX was introduced, Spektrum's DSM2 was just about the only DSSS system left on the market.

Despite this, Spektrum continued to argue that DSSS was better than FHSS and that there were no intrinsic weaknesses to DSM2 (despite my proof to the contrary).

But then, in a rather abrupt about-face, Spektrum launched its new 2.4GHz system, DSMX and the first radio to offer this truly agile system, the DX8.

So how do the DX8 and DSMX stand up to close scrutiny?

Does it hop?

DOES IT USE THE WHOLE BAND?

Yes, at long last, Spektrum hops!

So, does this mean they have forsaken DSSS for FHSS?

No, just like JR with its DMSS system, Spektrum has opted to combine the strengths of DSSS and the strengths of FHSS to produce a hybrid that not only constantly hops about the band but also uses DSSS to spread each of its chosen channels far wider than simple FHSS alone could manage.

Spektrum claim this is the most advanced system on the market and while I have no doubt there are others who'd challenge that claim, I have to admit that it is a very good strategy for creating the most resilient link between radio and model.

Powering up the spectrum-analyzer (SA) I found that yes indeed, the DSMX system does hop and produces truly excellent spreading right across the band

Is it resilient?

WHY DSMX IS GOOD

Most of the FHSS systems on the market rely on what is called temporal displacement and constant agility to prevent collisions between their signal and that from other users of the band.

In essence, this means transmitting for a very short period of time on one frequency and then switching to another frequency for the next short transmission. Since the radio is transmitting about 10%-20% of the time, this means the chances of two radios actually hitting the same spot on the band at exactly the same time are very minimal and, even if this happens, the pseudo-random nature of the hopping means that any data loss will be minimal.

However, if two FHSS systems do collide, the signals may interfere with each other and the data contained in those transmissions will be lost.

DSMX however, combines DSSS and constant hopping to deliver what should be a greater level of resilience.

Even if two DSMX systems find themselves transmitting on exactly the same part of the band at exactly the same time, the fact that they're also DSSS means that if each is using a separate spreading code, data will probably not be lost.

So does it work?

Well I tested the DSMX system under heavy interference at intensities several times greater than those which would knock a DSM2 system out of action -- and it didn't even blink. DSMX is indeed many times more resilient than DSM2.

Is it more resilient than an FHSS system like the Futaba FASST, FrSky ACSST or Hitec AFHSS system?

Well to be honest, it was hard to tell. These systems all have such good interference rejection capabilities that it's virtually impossible to knock them out, even in an artificially contrived test situation. Suffice to say that although DSMX does have a theoretical advantage, they're all pretty damned good.

Receiver Performance

LINK-UP, BROWNOUT ETC

Spektrum have made huge advances in their receiver technologies since those awful early days of high-reboot voltages and extraordinarily long reboot times.

The AR8000 receiver that came with the DX8 was as good as any other I've tested in terms of its reboot voltage and time.

The receiver continued to work just fine, right down to 3.2V -- sometimes a little lower and when the normal voltage was restored, it rebooted in under a second.

The reboot time for a total power loss was a bit slower but only slightly longer than one second. All totally acceptable.

Perhaps my only complaint about the Spektrum receivers is the need for satellite units.

As all the other major players have shown, modern 2.4GHz systems can perform perfectly without the need for multiple receivers. However, if you're one of the belt and braces brigade and can afford it then there's no harm in using satellites.

Summary

WHAT A DIFFERENCE

Top marks to Spektrum for bringing their 2.4GHz systems into the second decade of the 21st century.

In one fell swoop, they've gone from being very much at the back of the pack to now running right up there with the rest -- perhaps even better than the rest.

DSMX is a system that provides the combined strengths of DSMX and FHSS to deliver the most resilient link possible between pilot and plane.

If you're a DSM2 user who flies anything larger than a parkflier and wish to remain brand-loyal, thus retaining compatibility with your existing equipment then I really do suggest that you upgrade to DSMX as soon as you can.

Footnote

TO THE GUYS AT SPEKTRUM/HORIZON

It has been reported to me by many people that Spektrum/Horizon were bad-mouthing myself and my critiques of DSM2. Apparently, I was not someone whose opinion could be trusted and I "know nothing". If you can't disprove the message then kill the messanger I suppose.

Well I'm sorry to disappoint but I simply call a spade a spade. DSM2 *is* past it's best-by date and is no longer suitable for noisy environments but DSMX is an excellent replacement.

Surely, the very fact that DSMX even exists is proof that there was some truth to my criticisms. If DSM2 was without fault, why replace it?

Despite what has been claimed, I have no agenda, I'm not on the payroll of any other manufacturer and I don't have a grudge against Spektrum.

I'm just following the policy I've always held: When something is good, I'll tell people and, when it's crap, I'll tell them that too. Plain and simple!

DSM2 bad, DSMX excellent.