How do the various RC radio systems sold by JR, Spektrum, and Horizon Hobby implement the spread spectrum technology?
Horizon Hobby, Spektrum, and JR
I have always thought of Spektrum and JR as the same company. Up until a short time ago all of their products were completely compatible with each other. Now I know that the situation is a little bit more complicated than that.
JR (Japan Radio) is a leading manufacturer of radio control systems. As their name implies, they are based in Japan. Horizon Hobby is a large distributor and manufacturer of radio controlled products. They are based in Illinois here in the United States. Spektrum, on the other hand, looks to me like just a division of Horizon Hobby (HH). It is not a separate company.
JR is a long time partner of HH. But when it came time to develop spread spectrum radio systems, HH took a leadership role. That is what led to the creation of Spektrum.
And yes, do not get the two names confused. Spektrum is the brand name, spread spectrum is the technology.
All of the products that I am about to describe are commercial proprietary products that belong to private companies. Being in a competitive industry, many of the technical details are closely guarded secrets.
It took a lot of digging to find the few technical details mentioned below. I was very careful to consider the trustworthiness of my sources. Nevertheless, I would be pleasantly surprised if I managed to get every single technical detail right.
2.4 GHz ISM Band
All of the spread spectrum radio systems operate in the 2.4 GHz ISM band. It is called this because it was set aside for industrial, scientific, and medical users. It is about 80 MHz wide, and with the required minimum spacing of 1MHz between channels that I mentioned in my last article, it can handle about 80 usable channels.
You might think that the limit on the number of simultaneous operating radio systems would be 80, too. In practice it is more like 100. One reason is the very fast multiplexing and switching of frequencies that goes on. The other reason is the unique identifying numbers associated with each transmitter. It keeps receivers from getting confused as to which signal they are listening to.
DSM (Digital Spectrum Modulation) is their original spread spectrum system that was sold under the Spektrum brand name. This is a DSSS (Direct Sequence Spread Spectrum) system.
It is very similar to the far more popular DSM2 system that superseded it. But why was this system limited to park flyers only? The most plausible explanation I was able to find is that the receivers only listened to one of the two channels in use by the transmitter. Presumably, they picked the one with the strongest signal and stuck with it for the entire flight.
Easily the most popular spread spectrum technology sold by these companies. It is full-range and uses the DSSS protocol. It uses two channels at a time, giving it a limit of about 40 simultaneous transmitters operating at the same time.
The AR6100 DSM2 park flyer receiver has only one physical receiver that rapidly switches from listening between the two frequencies.
This is where things start to get interesting. I do not believe that this technology was ever introduced into the United States. I also have reason to believe that is is the brainchild of JR and not HH.
This is the first frequency hopping spread spectrum (FHSS) system sold by these companies. It does not support telemetry, but does support plugin modules for older transmitters.
DSMX is very popular right now. It is fully backward compatible with the DSM2 hardware. In other words, all DSMX transmitters and receivers also support DSM2. This is a frequency hopping system and has been shown to support 100 simultaneous operating transmitters. This was also the first technology to support the downlinking of telemetry data.
It is actually an interesting hybrid between DSSS and FHSS. Based on its unique id, each transmitter will only ever use 23 of the available 80 channels. Like a DSSS system, it transmits over this subset of channels. Like a FHSS system, it does so in turn.
I do not have any solid evidence to support this, but my hunch is that the DSMX development was spearheaded by HH and DSMJ was created by JR (hence the J in the name?).
The interesting new kid on the block. It stands for Dual Modulation Spectrum System. To my knowledge, this is the first RC radio system that truly incorporates the best features of DSSS and FHSS. It is used in the new JR XG series of radio systems.
It is only available from JR. HH/Spektrum is sticking with DSMX, at least for now.
It uses a 3 MHz wide signal on each channel. Every other system uses 1 MHz or less. Note that the minimum required spacing is 1 MHz, but the signal itself could be wider. For extra robustness, it also uses the pseudo-random encryption of DSSS.
But this is also a full-blown FHSS system. Apparently, it is based on the DSMJ technology developed earlier by JR. Based on the specs alone, I would expect DMSS to be far more robust against radio interference than anything else out on the market today.
An excellent telemetry system is fully integrated into the transmitters. You can have vibrating alarms go off depending on various conditions in the aircraft, for example. On the receiver end, a separate telemetry module is required. JR already sells sensors for pressure altitude, temperature, RPM, and monitoring electrical power (voltage, amps, and watts).
Many have reported that this is the first Spektrum/JR radio system that does not support satellite receivers because it does not need them. That is not true. The telemetry transmitter module has a satellite receiver built-in.
All of the fancy manipulations of the signal come at a cost. Where an 8-bit processor used to be powerful enough to encode the signal, now a 16-bit processor is needed. The radio frequency hardware is also different from the systems that came before it. I believe these are the primary reasons that led to the big downside of the technology.
You see, DMSS systems are not backward compatible. The transmitters do not inter-operate with DSM2 and DSMX receivers, for example.
A lot has been said about the cost of the new receivers, but I do not see it. A 6 channel DSMX receiver from Spektrum sells for about $60. A 6 channel DMSS receiver starts at $100. However, it comes with the telemetry module that costs $43 all by itself. Remember, this also has a built-in satellite receiver. The electrical system monitoring telemetry sensor, arguably the most useful one, costs an extra $100.
In other words, the cost of the receiver itself is about the same as what a brand-name JR/Spektrum receiver has cost all along. DMSS compatible receivers manufactured by other companies have already started to hit the market. Telemetry is an immensely useful feature. Whether it is worth an extra $143 per aircraft ($100+$43) is entirely up to you.
Despite all the complaints from others about the cost, I think the new XG systems are fairly priced. My X9303 radio system cost less than an XG8 sells for now, but not by much.
You could argue that HH/JR are just following an old tradition in the RC business. After all, we went through AM, FM, narrow band FM, PCM, and S-PCM before spread spectrum was introduced. All of these technologies are incompatible with one another and more expensive than what came before.
A good argument can be made that DSMX is good enough. Many say that the interference problems today, even in worst case scenarios such as large meets, do not demand bulletproof technologies such as DMSS.
But guess what? It is nice having the choice.