Spektrum/JR DSM2

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Spektrum/JR DSM2

Postby RCModelReviews » Tue May 04, 2010 4:56 am

This review can be found on the RCModelReviews YouTube channel in two parts as follows:

Spektrum/JR DSM2 (part 1)
Spektrum/JR DSM2 (part 2)

Comments, questions and feedback on this review can be posted to this topic
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Re: Spektrum/JR DSM2

Postby dougclind » Tue Aug 03, 2010 9:19 pm

I've been debating back and forth on an RCU forum about JR lockups and we brought up your YouTube videos. I have some questions about the tests.

1. You weren't able to show really close channels on the JR. You only cycled the JR 3 or 4 times. How close have you seen the channels get in your testing? I have to think that there's SOMETHING in the firmware which keeps the separation to some kind of minimum.
2. The DSM2 signal is a spread spectrum signal which has a certain immunity to interference just because it is SS. It kind of looks like the bandwidth of the JR signal is as much as 50 MHz. Have you tried clobbering both channels to see at what point the rx loses the signal?
3. You used a 600 mW video transmitter which looked like it put out a 100MHz wide signal. An NTSC TV channel only has a bandwidth of 6 MHz. So was that any kind of realistic test? Are there things in the 2.4 band that generate that kind of broadband signal?
4. Do you have any idea of what kind of power our rc transmitters put out? Maybe we need kilowatt amplifiers. (just kidding)

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Re: Spektrum/JR DSM2

Postby RCModelReviews » Tue Aug 03, 2010 10:51 pm

dougclind wrote:1. You weren't able to show really close channels on the JR. You only cycled the JR 3 or 4 times. How close have you seen the channels get in your testing? I have to think that there's SOMETHING in the firmware which keeps the separation to some kind of minimum.


The example shown in the article was typical of the "overlapping channels" scenario. because this only happens very occasionally I didn't spend too much time trying to observe any closer spacing than this (for fear of wearing out the transmitter's power switch :-)

With channel spacing as close as shown in that article, a video-sender can almost completely envelop the two channels and thus cause a loss of the link. I tested with a 600mW video sender but another engineer in Australia was able to kill the link with just a 50mW transmitter in the same circumstances.

2. The DSM2 signal is a spread spectrum signal which has a certain immunity to interference just because it is SS. It kind of looks like the bandwidth of the JR signal is as much as 50 MHz. Have you tried clobbering both channels to see at what point the rx loses the signal?

DSSS Spread-spectrum signals do have a high immunity to noise generated by other spread-spectrum sources but video-senders are not SS sources. They don't transmit in short bursts with gaps, as other SS equipment does (thus allowing some degree of interleaving), they have a 100% duty-cycle which means their effect on a SS signal is more pronounced.

You'll also notice that although the DSM2 signal does spread quite a bit near the base of the spike, the bulk of the power is spread over a range of only about 1-1.5MHz which is not very much when trying to cope with a non-SS source of interference.

3. You used a 600 mW video transmitter which looked like it put out a 100MHz wide signal. An NTSC TV channel only has a bandwidth of 6 MHz. So was that any kind of realistic test? Are there things in the 2.4 band that generate that kind of broadband signal?

The video sender uses about 15MHz of the band, the entire scan of the analyzer is only 100MHz. The DSM2 channels were spaced at just under 10MHz apart.

4. Do you have any idea of what kind of power our rc transmitters put out? Maybe we need kilowatt amplifiers. (just kidding)

Most 2.4GHz RC transmitters have an effective radiated power (ERP) of about 100mW. This is usually obtained by using a 60mW transmitter and an antenna with a gain of almost 2. The DSM2 transmitters seem to have an ERP of little more than 100mW (about 125mW by my measurements).

The reality is that, under good conditions, the DSM2 system is as good as any other on the market -- it's just that when you use DSM2 you've got less headroom (insurance against unfavorable conditions).

Couple the low headroom from an RF perspective with the low headroom from an operating-voltage perspective and it's easy to see why DSM2 equipment is over-represented in the "unexpected loss of control" statistics.

When DSM2 users have a perfect power-setup (good batteries/regulators) and fly in relatively benign RF environments then they will have no problems at all. This explains why the vast majority of DSM2 users have flown for years without a single lockout or glitch.

However, the reality is that not everyone's batteries are 100% and not all regulators can deliver the required currents all the time. What's more, as we see a massive increase in the use of the 2.4GHz band for RC and other applications, the RF environment we find ourselves flying in is sometimes far from benign -- downright hostile in fact!

Users of other RC systems such as FASST and even many of the cheap Chinese options have fewer problems because they have more headroom. Those systems are better able to handle the effects of less than perfect batteries and a hostile RF environment.

When you buy DSM2 you buy a system that will work well under ideal conditions but you get far less insurance against the unexpected than is the case with almost all other brands.

DSM2 was developed in a different era -- a time when the 2.4GHz band was much quieter than it is now. To be totally honest, it's really past its "best-by" date. I had hoped that they'd use the launch of the DX8 to either switch to DSMJ or at least something like DSM3 with an extra RF channel or two that would help mitigate a noisy RF environment. Both of those changes could have been made 100% backwards compatible.

Why they didn't take the opportunity to enhance their firmware and provide customers with much greater headroom is a question only they can answer -- and they refuse to even acknowledge my emails on the subject :-(
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Re: Spektrum/JR DSM2

Postby dougclind » Wed Aug 04, 2010 1:37 am

Thanks for the great response. Try as I might, I can't find any flaws in your reasoning. And I'm definitely on the same page with you on them needing to update their technology. DSM3 sounds good except that if they're static then that will really cut down on the number of simultaneous transmitters that can be on at one time. They have 80 "channels" they work with, which means a max of 40 tx's on at one time, which means in practice it should be a lot less than that. At Joe Nall this year they had something like 20-25 pilot stations plus 1000+ registered pilots. People were told to keep their tx's off when not on the flight line AND people were limited to 2.4 radios only. So everyone was on 2.4 and how many guys would "cheat" and turn on their tx's at their bench location on the sidelines for testing? In my opinion they shouldn't have limited it to 2.4. I guess they didn't want the hassle of impounding 72 MHz boxes.

Let me bounce one more thing off you. I lost a plane a couple of months back, at our flying field. It's got maybe a dozen houses in proximity to the field. My rx locked up when the plane was pretty much in the middle of our area, as far as it could get from any houses. I had just taken off, made a broad 180 turn and the plane was only about 300' away from me when it the rx locked up and the plane went down. It was an EDF jet, a hobby lobby F15 with a new battery, 4S fully charged. When I got to the jet the battery was still connected and everything worked!

Interference causing the lockup? I have trouble believing that. What, in a residential neighborhood, would do that? And why did it wait until the plane had returned from it's takeoff pattern and was closest to me? It just doesn't make any sense to me.
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Re: Spektrum/JR DSM2

Postby RCModelReviews » Wed Aug 04, 2010 3:19 am

What receiver were you using Doug?

There seems to be some very real questions as to bugs in some of the Spektrum offerings (AR500 and AR6250 in particular).

One thing with electrics is the ever-present risk of BEC shutdown, even just temporarily.

Most of the Chinese-made ESCs have really poor quality BECs and most will shutdown very quickly when overloaded (even for a moment), taking quite some time to restore normal operating voltage to the receiver/servos. If/when this happens you get the "uh-oh" lockout but everything seems fine when you get to the scene of the crash (unless you've got a version of the receiver firmware that flashes the LED after a brownout).

I'm going to be doing a review of a range of ESCs and UBECs in the near future and one aspect I'll be looking at is the current (transient and continuous) required to produce a shutdown.

The other thing that can produce "Bermuda Triangles" at a field is where you get a narrow beam of strong 2.4GHz signal. These are sometimes used for data-links and are created by systems with directional antennas that may be several miles away.

Because 2.4GHz is very much line-of-sight, it's possible for a strong point-to-point transmission to pass directly across your flying field without you knowing it's there. Flying through such a beam will usually have little effect, since the model will be in and out of the highest area of intensity quite quickly. However, flying along the beam can cause a protracted period of signal loss, resulting in a lockout -- especially if your system isn't a full-time frequency hopper and the interfering frequency coincides with the DSSS frequencies used by your radio at the time.

At ground level you probably see no sign of the high-intensity signal because it is blocked by the ground clutter but, once you get 100 feet in the air, the signal can be very strong indeed. The only way to check for this is to use a data-logging spectrum analyzer that you place in a model and fly through the "danger zone".

The other problem is that, like much 2.4GHz traffic, such signals can be relatively transient in nature and only appear when data is being sent -- making it even harder to detect their presence.
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Re: Spektrum/JR DSM2

Postby dougclind » Wed Aug 04, 2010 3:56 am

My receiver was a 6100e.

The plane was somewhere around 2/3 throttle. It's really hard for me to believe that the BEC was overloaded because the plane only has 4 analog servos and I was in a medium turn using only ailerons. The 4S battery was new and fully charged. The plane had flown successfully before.

Also, the field is not in a clear, level area. It's in somewhat of a depressed area with trees surrounding one side. I don't know for sure if any microwave relay links pass through the area but it seems a bit dubious.

I thought at the time that it was like someone had hit the plane with a jammer. I just can't give any explanation credibility. They all seem so unlikely. An ESC whose BEC circuit temporarily failed and then worked OK? I can't buy that one. Interference? Well, almost had to be but from what? Especially since our club flies from that field and people are in that area all the time with Spektrum/JR 2.4.

I just wonder if there is some kind of weird failure mode that the designers didn't anticipate. That happens ALL THE TIME with software. I just don't get why JR would let this condition fester without addressing it. They have to know it will hurt them in the long run. I love model match and that's why I won't switch from JR but that's only going to get them so far.

I feel like I'm talking about Microsoft!
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Re: Spektrum/JR DSM2

Postby RCModelReviews » Wed Aug 04, 2010 4:06 am

Sometimes shirt happens, it's an unfortunate fact of life.

It is always possible that there may be a very obscure fault in some models of DSM2 receivers that only appears under very specific and unusual circumstances. The thing about any non-trivial software is that you can only prove conclusively that it has bugs, you can't prove it doesn't :-)

When people encounter otherwise unexplained issues with Spektrum I generally suggest they try flying with another system and see if the problems go away.

I've yet to encounter a case when switching to another brand didn't "fix" the problem -- or at least the symptoms thereof.

I'm not anti-Spektrum -- in fact I have zero brand-loyalty, I just fly whatever seems to work.

My hangar has JR, Hitec, Futaba and Chinese radios in it. When I buy another radio I won't be looking at the badge, only the way it performs and whether the features it offers suits my needs.

I do find it worrying however, that Horizon refuse to be drawn on the various issues that have been raised in respect to DSM2. Perhaps they think that even acknowledging those who ask will be seen as an admission of fault. Hopefully most people can see through that and draw their own conclusions based on the evidence.
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Re: Spektrum/JR DSM2

Postby jeffie8696 » Wed Aug 04, 2010 7:28 am

"The only way to check for this is to use a data-logging spectrum analyzer that you place in a model and fly through the "danger zone"."
Hey I need one of those!! our field has the "crop circle of death" at the north end. Luckily this year it is planted in soybeans and the planes are easy to find. :lol:
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Re: Spektrum/JR DSM2

Postby pwy » Wed Aug 04, 2010 9:38 am

jeffie8696 wrote:"The only way to check for this is to use a data-logging spectrum analyzer that you place in a model and fly through the "danger zone"."
Hey I need one of those!! our field has the "crop circle of death" at the north end. Luckily this year it is planted in soybeans and the planes are easy to find. :lol:


As it happens I have one of these "data-logging spectrum analyzer that you place in a model and fly". Ok, not quite but close enough ;-)

I have recently acquired a Weatronic module which is a German made 2.4Ghz module often used by the big jet guys and has some unique features. One of these feature is that you put a micro SD card in the transmitter module and it constantly records signal strength of transmitter and receiver (has 2-way communication and dual sender/receivers so you get 4 separate values) plus voltage, temp, all servo positions and heaps of other information.
The software can then analyse the log files and it is simply amazing what you can read out of this. On the test flights I have done so far the worst I have seen was 5% frame loss on one receiver (and 0 loss on the other at the same time) but what these cheap BECs deliver can be shocking. Even so I never noticed an issue with some BECs, I have started replacing them with better quality stuff.

On a side note, the Weatronic software turns the transmitter module into a realtime spectrum analyser (just need to have the laptop connected to the module via USB) and I have used that to check my JR12x. During the short 10 minute "playing around" I caught the JR three times with 2 channels very close together! It may happen more often than you think...
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Re: Spektrum/JR DSM2

Postby RCModelReviews » Wed Aug 04, 2010 10:06 pm

Yes, my tests were done in a benign RF environment, it may well be that the adjacent-spacing of DSM2 channels occurs more frequently when there's more noise about. Unfortunately I only had the DSX9 available for a short period of time so didn't get to test for that.

I would also like to review the Weatronics. I'll contact them and see if they're interested in submitting a system for close scrutiny.
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