Spektrum DSM

The Little Gray Box that will change RC forever

Words: Steve Pond

There have been quite a few defining moments throughout the history of RC, but the impact of any product or technology isn't always immediately apparent. Vehicles such as the original RC10 and the T-Maxx didn't achieve cult status until some time after their introduction. Electronic speed controls (ESCs), Ni-Cd batteries, peak-chargers, RTRs and dozens of other developments had been around for years before everyone realized that each represented a pivotal moment in the advancement of RC. We don't need the benefit of history to tell us that the introduction of the new Spektrum DSM system will profoundly change the future of RC. The days of buying and changing crystals, waiting for a frequency clip during practice, random interference and almost all the other shortcomings of current radio technology, will be history. The new Spektrum system also promises to usher in technology that has never been available in a radio system. There's a lot to look forward to; you just have to open your eyes and watch it unfold.

The Spektrum DSM replaces your existing transmitter module and receiver, and plugging it into your radio advances it light-years ahead of current technology. Current systems in the U.S. operate on 27 and 75MHz; the Spektrum DSM operates on the ultrahigh 2.4GHz band (2400MHz), and there are 79 channels available. RC cars now have 79 channels on a band that is inherently more stable and universally accepted throughout the world. If this article stopped right here, that alone would be huge news. You could stop reading right now and know that running and racing RC cars just became exponentially better. But the aforementioned barely scratches the surface of this new system's benefits. We've tested this new system and talked extensively with the experts who developed it to give you the complete scoop on this fantastic new technology.

The FCC requires that any device that operates on the 2.4GHz spectrum is required to have some type of "collision avoidance" technology, which is a fancy way of saying that it absolutely can't interfere with other devices operating on the same frequency. It's technology that's never been available in the RC car market, but now it's the basis of what makes the Spektrum such a significant leap in technology. There are two standards of this so-called collision avoidance, each with a different approach to guaranteeing reliable operation. Although it's a little techy, it helps to understand the differences between the two methods that provide incredible signal stability and security against interference–so here's the scoop on "FHSS" and "DSSS."

> Frequency band: 2.400 to 2.4835GHz
> Channels: 79
> Channel spacing: 1MHz
> Bit rate: 15.626kbp
> Range: 3,000 ft.
> Latency (response time): 5.6ms
> Resolution/channel: 4,096 steps

> Weight: 0.40 oz. (11.3g)
> Dimensions: 1.62x1.06x.57 in. (41.2 x 27.0 x 15.3mm)
> Input voltage: 3.2 to 9.6V
> Channels: 3
> Current requirements: 40mA @ 4.8V
> Antenna length: 3.6/8.5 in.
The Spektrum DSM system costs about $160 and consists of a transmitter module and receiver. Extra receivers can be programmed to work with the same module and are available for $80 each.

FREQUENCY-HOPPING SPREAD SPECTRUM (FHSS). The FHSS standard means that the radio system rapidly jumps from one channel to another using a random sequence of channels. Here's how it works: a code at the end of every string of data sent from the transmitter to the receiver tells the receiver the next channel in the sequence. For example, the transmitter might "tell" the receiver, "left, left, left, left, switch to channel 51." The signal jumps randomly from channel to channel hundreds of times per second. Other systems using this standard use different random sequences, and although there's a remote possibility of crossing channels, the resulting interference would last only milliseconds, so you wouldn't be able to detect it.

According to Spektrum's designers, there are two significant elements of the current FHSS standard that make it less than ideal for RC applications. First, the latency time (response time) with the FHSS standard is typically about 50 milliseconds. For reference, the Airtronics M8 out of the box has a response time of about 12 milliseconds, and the ultra-fast Futaba HRS system clocks in at about 8 milliseconds. The fastest FHSS-based systems the Spektrum crew tested had response times of about 25 milliseconds, which is more than twice the response time of a standard M8. The second drawback is the system's response to interference. In the event of signal interruption, it can take 2 to 4 seconds for the transmitter and receiver to resynchronize. The inability to control a moving vehicle for up to 4 seconds would obviously be unacceptable.

DIRECT SEQUENCING SPREAD SPECTRUM (DSSS). This is the system used by Spektrum. The DSSS standard is unique in that it hunts for and locks into a single channel; there isn't any channel-hopping. When the transmitter is turned on, the module scans and locks on to any channel that is not in use. Because of the 2.4GHz spectrum's collision avoidance requirements, locking into one channel requires different anti-interference measures than those implemented in FHSS systems. A DSSS-based system spreads the RC data over a wide bandwidth using a pseudo-random noise code. The receiver knows its transmitter's spreading code and can distinguish its intended radio signal. Spreading has the additional benefit of digitally increasing the range of the system for the same power. A further security method particular to the DSM is the use of a globally unique identifier code (GUID). This code is encoded into every RC data packet. The GUID is one of 4.2 billion potential codes; there aren't even 7 billion people on the planet.

Any radio system that processes signals digitally has a defined number of steps (servo positions) throughout its range of travel. The 1024 standard was made popular by Futaba's PCM radios, they have 1024 steps across the range of control movement. For example, if a servo connected to a Futaba PCM system were to start from the full left position and sweep all the way to full right, the movement of the servo would be broken down into 1,024 steps. It usually happens so fast that it's virtually seamless. The Spektrum has 4,096 steps of resolution, a little less than four times that of any other popular system.

The Spektrum DSM module is simply installed in place of the factory module in selected radio systems. Our test system is installed in the Airtronics M8, but it will be available for most popular radio systems that include a factory module. "Binding" the receiver to the module is a quick one-step process, and it's the only setup required to make the Spektrum fully functional. The high 2.4GHz frequency means the short antenna seen on the module is all that's required. The collapsible standard antenna isn't functional as long as the Spektrum module is installed, so it can be removed if desired.

The Spektrum receiver is installed and outwardly functions in much the same way as a standard receiver. The magic that happens inside the housing is transparent and rock-solid in terms of reliability. Like the transmitter module, the receiver needs only a short 3.6-inch antenna for maximum radio range.

The Spektrum receiver includes a single setup button, which is used to "bind" it to the transmitter during initial setup. Two holes molded into the housing permit different antenna routing depending on the installation.

The inside of the Spektrum receiver features a double-deck circuit board (see below) and high-tech surface-mounted processors. All this horsepower is used to process the signals sent back and forth between the module and receiver.


Latency (response time) is a measure of how long it takes from the time you move the steering wheel or throttle to the time the servos or the speed control react in the car. Even in sport radios, it's often so fast that the delays are undetectable, but for the people who race their radio stats, the Spektrum has the fastest response time ever—5.6 milliseconds.

The Spektrum's ability to seek an open channel and its other robust anti-interference features mean that Spektrum-equipped radios don't have to be impounded. They can't interfere with conventional radios, and they can't be turned on within their own band unless there's an open channel, so only paranoia and faulty logic would land your radio in the impound, but some tracks may still require that you hand over your transmitter.

The technology of the Spektrum will eventually eliminate the need to monitor frequency usage. You don't have to worry about frequency clips on the drivers' stand, and race officials won't have to scan the system to see if it's on a legal band because it's global, and it's fixed. Nobody will need the "funnies" anymore. Can I bind extra receivers for use with my other cars? You can bind an unlimited number of receivers to the same transmitter. Extra receivers are available for a retail price of about $80 each.

At the time of our testing, the Spektrum for the Airtronics M8 was the only one available, but by the time you read this, modules should be available for the KO EX-10 Helios and EX1 Mars, the Futaba 3PJ and 3PK, the JR 756 and R1 and the Hitec Lynx 3D, CRX and SRX.

International acceptance of the 2.4GHz band means you can use the radio system that you use at home overseas. Current radio technology requires a module and receiver tuned for approved frequencies that are often unique to one country or one part of the world. In addition, all 2.4GHz devices (RC and everything else) are limited to a maximum of 1 watt of transmitting power, and that eliminates the potential of your RC signal being crushed by powerful 100,000W signals from industrial users as it could on the 27 and 75MHz bands.

Interference with the Spektrum is highly unlikely. The 2.4GHz band is very far from the spectrum in which most model-generated interference occurs. Motor arcing, metal parts contacting each other, noise from a speed control and the vast majority of interference that comes from within your RC car happens mostly below 300MHz. At 2.4GHz, the Spektrum is immune from the nuisances that plague our current radio systems, and that eliminates the need for anti-interference measures such as capacitors and other devices. There are also numerous defenses against other potential interference sources. As previously stated, all devices using the DSSS standard must have the ability to seek an unused channel. In the unlikely event that defense is bypassed, the GUID would reject any signal that didn't have the proper ID. If a rogue signal was on the same channel and somehow managed to be using the same ID, it would have to be on the same part of the band as the Spektrum unit. The band has the equivalent of 100 data paths (think of a 100-lane freeway). The interfering signal would have to be in the same "lane" or on one of the adjacent lanes for it to have a chance to break in (and even then, the built-in fail-safe would save you). Basically, interference is just this side of impossible.

The 27 and 75MHz bands currently in use in the U.S. have 6 and 30 channels, respectively. Although that's technically enough channels to do what we have to do, the burden has always been having enough spare sets of crystals to be reasonably certain that you'll have an open one when it comes time to race with 10 or more other cars. The Spektrum has more than twice the number of channels, so there's no need to spend extra moola for channel changes. The number of available channels is determined by the Spektrum developers, and there's plenty of room on the band to add more channels down the road, but it's inconceivable that more would ever be needed. The distance between the channels allows many simultaneous streams of information from optional telemetry devices.

All wireless devices on the 2.4GHz have to include collision-avoidance technology—no exceptions. There could be 20 cordless phones, as many wireless routers and a multitude of other devices, and the result would be the same: no interference.

The high-frequency 2.4 GHz band requires a short antenna. The Spektrum receiver comes with an 8.5-inch antenna, and it's every bit as effective as the really long antennas included with the 27 and 75MHz receivers. In fact, the included antenna is 8.5 inches long so that it can make its way out of a receiver box. According to the instructions, it can be cut to 3.6 inches for touring cars and other installations without any loss of performance. The antenna should be installed so it stands vertically, but it doesn't have to come through the body at all.

The positions of the steering and throttle/brake channels are stored, so in the highly unlikely event of signal loss, the servos will return to the preset fail-safe positions. Here's the bonus: even if your radio gets stuck in impound, when you flip on the receiver switch, the servos will still go to their present fail-safe position. That will be especially helpful when warming up gas cars; you won't have to worry about the engine revving too high when you look away.

The Spektrum receiver draws about 40mA. This is higher than the 12 to 15mA drawn by standard radio receivers, but when the servos are drawing upwards of 1000mA, this is insignificant. On the other hand, the module draws only about 120mA, which is about 40 percent less than a standard radio. That means you'll get long life out of the transmitter batteries.

Ordinarily, operating your transmitter with the antenna removed or collapsed can damage it, but the Spektrum module disables the original antenna, so it can be run collapsed or removed entirely.

Horizon promises that telemetry options will be available for the Spektrum system, and the first is described on the Spektrum website spektrumrc.com: "The first optional telemetry module will include the ability to monitor real-time engine temperature, engine rpm, speed in mph or kph, battery voltage, signal strength and individual lap times. You'll even be able to record the data and review it later on your PC. We hope to have this telemetry module available in early spring." It also indicates that the information will be read on a hand-held display that you or your pit man can read from anywhere on the track to monitor engine performance.

During a quick "binding" process, a button on the receiver and one on the transmitter are pressed so the two components can be synchronized, and that ensures that the receiver will recognize only that transmitter. It only takes a moment. It's wise to be aware of other people binding their receivers and transmitters in the immediate vicinity, but the chances of a mix-up are slim. The binding range is only 5 to 6 feet, and someone would have to be in exactly the same stage of the process in order for your receiver to "listen" to the wrong transmitter. If there were a problem, you would just have to rebind your system.

I installed the Spektrum in an XRAY T1 Factory kit and spent quite a bit of time racing the car at Hobby Chamber in West Haven, CT. The track's tall drivers' stand places transmitter antennas inches from the roof's steel trusswork, and glitching isn't uncommon even with high-end radio systems. The Spektrum performed and continues to perform flawlessly. It simply operates glitch-free every time the car is on the track.

Track interference should never be a concern, but we have discovered certain hot spots where even the best radios have been known to glitch or shut down due to interference. One of the worst is the upstairs hallway of our building. It's about 200 feet long and notorious for trashing cars. It's surrounded with fluorescent lights, copiers and printers that combine to produce the toughest environment we've ever seen to test a radio system. It's virtually impossible to make it end to end for without hard glitches that can re-kit a car in the blink of an eye. Even here, the Spektrum performed transparently—no hits, no glitches.

A more recent discovery is the area under my desk. My computer, combined with an electrical outlet and a gaggle of networking plugs, causes fierce glitching even at close range when I set up radio gear on my bench. This became apparent when we used the standard M8 to pilot the cars in the new Virtual RC simulator. After I had installed the Spektrum, the interference disappeared.

The only function we could not test was the Spektrum's channel-seeking capability, since we were not testing it alongside other Spektrum users. But we can cite the experience of factory team drivers who have been racing with the system and say that it does perform as promised—just switch it on, and the Spektrum does all the channel-surfing.

The Spektrum system is an incredible leap forward in radio technology that will reshape the future of RC. Yes, buying a top-end radio and the Spektrum system is expensive. That's the cost of advancing technology, but things like this tend to become more affordable over time, and it shouldn't be long before you see this technology built into an off-the-shelf radio system. Until then, the Spektrum brings the technology to us today, and it works. No matter what happens in the years to come, we can all look back on this moment and know that we have witnessed history in the making.







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