Spread Spectrum RC
The introduction of spread spectrum technology into the world of radio control is without doubt one of the most important advances to occur in the history of the hobby.
The last major shake up of radio control systems was the introduction of Pulse Code Modulation (PCM), some when back in the 1980s, which allows the analogue signals being sent out from a standard FM transmitter to be converted to digital signals. This results in a stronger, more positive signal with less chance of interference either from other rc transmitters or 'metallic noise', whereby two metal surfaces inside the model rub together, generating a weak electronic signal that gets picked up by the receiver, causing unwanted servo movement.
But spread spectrum rc technology uses a completely different method of signal transmission and operates within the 2.4GHz frequency band, which is well clear of any of the other bands used for rc and other common radio signal transmissions.
This new technology for rc use has been developed from Direct Sequencing Spread Spectrum, a form of secure radio signal transmission that has been in use by top government bodies (military, NASA etc.) for a while. The rc version is fundamentally the same, but it's been tweaked a bit for the hobby industry.
At the time of writing this page (2008), there are two brand rc spread spectrum technologies available; DSM / DSM2 was the first to appear and is used in the Spektrum and JR systems, while FASST is Futaba's equivalent. They stand for Digital Spectrum Modulation and Futaba Advanced Spread Spectrum Technology respectively.
As previously mentioned, spread spectrum rc technology is completely closed to any form of interference whatsoever. A Globally Unique Identification Code is assigned to every radio transmitter during manufacture, and this is just one unique code out of millions of possible codes.
The receiver is programmed to identify that unique code in what is known as the 'binding process', and so the Tx and rx lock together with the same GUIC, blocking out all other codes in the process. This process only takes a couple of seconds every time the system is powered up.
Once locked together, the transmitted signal is spread out over a wide band before being identified, collected and 're-assembled' by the receiver, further increasing security.
Futaba's FASST systems also rely on high-speed frequency hopping to further increase signal security; the signal changes frequency every two milliseconds to ensure that signal conflicts don't occur.
Another big difference with spread spectrum rc is the response time (latency) - everything happens within a few milliseconds, so servo movement and the associated model response is instantaneous and direct, a major bonus for demanding 3D flying.
What does Spread Spectrum mean at the flying field?
In a nutshell, DSM and FASST radio control systems let you fly your aircraft with absolute peace of mind. Once you turn on your system, your flying frequency is chosen for you and becomes unique to that model - no other radio signals or interference can penetrate your channel.
Spread spectrum rc systems don't use crystals, so you don't need to carry a spare set of those around and you also won't be needing to put a peg in the club frequency board before you fly!
Another safety aspect of a 2.4GHz transmitter is the antenna - the signal can be sent through a much shorter antenna, one which doesn't have to be extended several feet in front of you.
Spektrum are the pioneers of the 2.4GHz technology for the radio control hobby industry, and for a while were the sole manufacturers of spread spectrum rc systems. Their DX6 was an immediate success as soon as it hit the hobby shop shelves, and has since been upgraded to the DX6i.
Spektrum also manufacture DSM modules, designed to fit into certain JR and Futaba radio systems. This is an excellent idea, as this upgrade option means that many rc pilots won't have to change to a complete new system but can continue flying on the one they are familiar with.
Since writing this page, Futaba have released more 2.4GHz spread spectrum systems, to add to their first one, the 6 channel 6EX.
Where does it go from here?
DSM, DSM2 and FASST rc systems are very new to the hobby but are already proving to be a huge success, unsurprisingly. With such secure signal transmission and instant model response, it's hard to imagine how radio control systems can be improved now - but think about what's happened with technology in the last few years!
With time it's probable that more Ready To Fly rc airplane and helicopters will be supplied with a spread spectrum set as standard. Currently there is only a handful of RTF models being sold as such, the Blade CX2 coaxial electric rc helicopter from E-flite was the very first.
Wherever it does go from here, spread spectrum rc technology is certainly going to be seen more and more on the flying field, and club frequency boards might even become a thing of the past!
Update: this page was written in 2008, when 2.4GHz spread spectrum radio control systems were in their infancy. Several years on and the inevitable transition was obvious - 2.4GHz systems have all but replaced traditional MHz ones, and every RTF rc aircraft is now sold with a spread spectrum radio. That's progress!