Review: JR's DSMJ Frequency Hopping 2.4GHz RC System

IS IT ANY BETTER THAN DSM2?

Dated: 17 Dec 2010
See the video of this review

Since I wrote this article on JR's DSMJ system I've been trying hard to get my hands on a DSMJ transmitter module and receiver in order to see whether the frequency-hopping alternative to DSM2 was actually any better.

Fortunately, a fellow modeler (Steve Engle) lent me his JR11X Zero, along with a DSMJ module and receiver for review purposes -- and here's what I found.

Does it hop?

DOES IT USE THE WHOLE BAND?

There are basically two types of 2.4GHz RC systems on the market today: the "pick and stick" fixed-frequency DSSS systems like Assan, Corona and DSM2; and the "constantly agile" (sometimes called FHSS) systems such as FASST, Hitec, FrSky, FlySky V2 and most others.

In the USA, JR has been relying solely on the DSM2 system for its 2.4GHz capabilities but elsewhere in the world it has been using an FHSS version it calls DSMJ which should, at least in theory, be better able to cope with high levels of interference.

Of course a poorly implemented FHSS system can actually perform worse than a well-implemented DSSS one so I was eager to see if JR had "gotten it right" with DSMJ.

Powering up the spectrum-analyzer (SA) I found that yes indeed, the DSMJ system does hop.

It transmits across the entire band - but that's only half the story. I've seen FHSS systems that could still be knocked out even by interference that only affected a small portion of the band so I decided to produce some "hostile conditions"

Is it resilient?

DODGING INTERFERENCE

I reduced the power output of the transmitter by switching it to "range test" mode and fired up my very powerful 600mW video transmitter. The result was that the transmitter's signal was totally dwarfed by the level of the interfering signal and around a full third of the band was effectively obliterated.

When I tested the DSM2 system, I found that this level of interference was enough to kill the link between transmitter and receiver, if the interfering signal happened to coincide with the frequencies chosen by the DSM2 at start-up time. This therefore, would be a great comparison between DSM2 and DSMJ.

No matter how hard I tried, nor where I positioned the frequency of the video sender, I could not get it to lock-out the DSMJ system. Throughout the testing, the DSMJ system delivered a faultless solid, reliable link.

I have to admit that this did surprise me. I had expected the DSMJ to outperform DSM2 but, given the circuitry involved, I had not expected it to perform quite this well.

DSMJ would appear to be as resilient as any of the other FHSS systems on the market, albeit without the Hitec's ability to adapt to the noise-profile of the band if required.

What else?

LINK-UP, BROWNOUT ETC

Since the DSMJ system appears to be using pretty much the same hardware as DSM2, I wasn't expecting much difference in terms of brownout performance and that proved to be the case. While improved over the early systems, the DSMJ receiver still has a relatively high brownout voltage but the good news is that it reacquires the signal quite quickly, (albeit not as quickly as the latest versions of DSM2) when voltage is restored. The average time to relink after a reboot was around 0.5 to 0.75 seconds. Again, this is not as good as some of the other systems but still acceptable.

The initial link-up between transmitter and receiver however, seems a little faster than with DSM2.

Servo movement was smooth and without the jitter sometimes seen on other FHSS systems and resolution was good.

Although no measurement was made, the level of latency seems the same as with most other module-based 2.4GHz solutions.

Of course nothing is perfect, but the weaknesses of DSMJ seem to be more related to marketing choices than technical issues. I believe that ModelSelect does not work with DSMJ and you can't use it with the large array of Bind and Fly models from Horizon. DSMJ receivers also seem to be significantly more expensive and there is a much smaller range than with DSM2.

I also note that the receiver does not flash its LED in the way that DSM2 does if there has been a brownout or interruption of power since the transmitter was turned on. This could make it more difficult to track down any power-system issues with a DSMJ setup.

Summary

WHAT A DIFFERENCE

I didn't want to go ripping the borrowed system apart to check but I'm told that the DSMJ system simply represents new software running on the standard DSM2 hardware. If that is the case then I'm left aghast that JR and Spektrum even bother continuing with DSM2's "pick and stick" configuration.

The DSMJ system is vastly superior to DSM2 in terms of interference rejection and, with the 2.4GHz band growing more cluttered and hostile every day, that has to be a big benefit to any RC flier.

I can't imagine for one second why Horizon hasn't offered DSMJ to its US customers since it would go a long way to addressing my concerns about the shortcomings of the current JR offerings in that country -- which are all centered around DSM2. The only reasons I can imagine are ones of commercial expediency and promoting/protecting the Spektrum product.

So, would I recommend you buy DSMJ?

Well if you're a died in the wool JR user who has concerns over the ability of DSM2 to cope with today's harsh 2.4GHz environment then DSMJ will offer you far greater headroom and peace of mind. However...

JR has announced that DSMJ will be phased out and replaced by its newer DMSS system so investing heavily in DSMJ equipment right now might leave you out on a limb in terms of getting new receivers at a later date.

DMSS also has the option of telemetry which will make it more attractive to some fliers.

So, if you need a reliable JR-branded 2.4GHz frequency-hopping solution right now for your upper-end JR radio then DSMJ will probably be your only option and it looks like a fairly good one by comparison with DSM2.. If you can wait then, if the way they've implemented DSMJ is anything to go by, I suspect that DMSS will also be a bullet-proof answer.

Of course I'll try to get my hands on a DMSS system as soon as I can but both JR and Horizon seem unwilling to submit product for review or even answer my requests for information. That doesn't stop me delivering honest, objective reviews however.

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