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| Issue 35 (May 2009) |
Words: Chuck Bassani |
The DX7 has been with us for around two years now. During this tenure it has become one of the most prolific transmitters ever offered, and the proverbial "tough act to follow." Spektrum, however, has stepped up to the challenge of taking the beloved DX7 to the next level, and claims to have made it faster and more precise. The result of that effort is the DX7se (Special Edition). So sit back while RC Heli takes a look under its hood to see exactly what they did.
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The "Special" part of the DX7se
First and foremost, the DX7se boasts an 11 mSec frame rate. For those that don't know what this means - it's essentially the rate at which the transmitter sends channel position information to the receiver. Previous to the DX7se, the industry standard frame rate has been around 22 mSec (or a little over 45d updates per second). The DX7se has doubled that to over 90 updates per second, and that should improve latency. The other major update was to increase the resolution from 1024 steps to 2048 steps, thus doubling the number of discrete control positions that can be encoded. To top it off, the case graphics have been updated, mainly the "Spektrum" badge on the top and the bezel that surrounds the display. They've even included a "Special Edition" neck strap.
Features
Increased frame rate and higher resolution aside, the features, programming, and hardware are identical to the standard DX7. As a quick re-cap, they include:
• 20-Model Memory
• Model Select
• Model Name
• Model Type Select
• Model Reset
• Servo Reverse
• Sub-Trims
• Travel Adjustments
• Swashplate Type (1-Servo Normal, 2-Servo 180°, 3-Servo 90°, 3-Servo 120°)
• Three 5-Point Throttle Curves (with Expo)
• Four 5-Point Pitch Curves (with Expo)
• Revolution Mix
• Gyro Sense
• Three Programmable Mixes
• Throttle Recovery
• Input Select (for what controls the Aux2 and Gear Channels)
• Dual Rate & Expo (Aileron, Elevator, and Rudder Channels)
• Dual Rate Switch Select
• Auto Dual Rates and Expo
• Timer
• Servo Monitor
• ServoSync (channel grouping)
• ModelMatch
I'm not going to go into these features in detail, as they have been covered in numerous reviews already published. I am asked a lot about the gimbals; they remain unchanged - leaving each control axis supported by one ball bearing and one bronze bushing. Fortunately, the sticks on this radio do work quite well. However, I must point out that notably missing is the "Trainer" function. By dropping this feature, it renders the DX7se incapable of being used for buddy box training or as a flight simulator controller.
The Nitty Gritty ...
Moving on to the DX7se particulars, there's been much speculation circulating on how exactly the DX7se really operates. I'm constantly asked questions such as, "Do I get the faster frame rate with 1024-step resolution receivers?" and "Am I really going to notice the difference between the DX7se and the standard DX7?"
To answer these questions, we obviously had to test the DX7se with all three classes of receiver; the DSM2 2048-step resolution capable receivers (AR7100, AR7100R, AR9000, etc.), the DSM2 1024-step resolution receivers (AR6100, AR7000, etc.), and the DSM 1024-step resolution AR6000 receiver.
2048-step Resolution Capable DSM2 Receivers
This is where the DX7se breaks away from the pack. It does indeed transmit at an 11 mSec frame rate with these class receivers. Interestingly, though, only channel 2 (Aileron), 3 (Elevator), and 6 (Aux1) are updated at this rate. All other channels are updated at a 22 mSec rate.
Channel grouping and output sequencing varied among receivers. For example, with the AR7100 / AR7100R we saw the receiver output channel 1 (Throttle), then channels 2 (Aileron) & 6 (Aux1) simultaneously, then channels 3 (Elevator) & 7 (Aux2) simultaneously, and finally channel 4 (Rudder). 11 mSec after that frame began, channels 2 & 6 were again output simultaneously and then channels 3 & 5 (Gear) simultaneously. It takes two frames to get all channel information across, however the swash control channels are updated every frame. You can see this visualized in the following signal traces measured at the receiver.
With the AR9000, we witnessed the following:
Again, only the three swash channels are updated at the 11 mSec frame rate. However, you can clearly see the channel grouping and sequencing is different from that observed on the AR7100.
Performance wise, the DX7se can't be beat when using this class of receiver (see Sidebar for performance and latency measurements). CCPM performance is excellent.
1024-step Resolution DSM2 Receivers
When using the DX7se with this class of receivers, all channels are updated at a 22 mSec frame rate. Here are the signal traces captured when using the AR7000 receiver:
As you can see, channels are output sequentially in the order 2, 6, 3, 4, 1, 5, & 7. Importantly, the three swash channels (2, 6, & 3) are output one after the other, minimizing interaction and maintaining excellent CCPM performance.
For those (like me) who may be wondering if the transmitter might still be operating at an 11 mSec frame rate with the outputs possibly being throttled by the receiver, we turned our focus over to the transmitter's RF stage. In the following figure you will see the output of the transmitter's RF module (blue trace) imposed over the receiver's channel 2 output (red trace).
Notice that the RF module's power is cycled twice (once for each of the two RF channels it's transmitting on) every 22 mSec. This confirms that the transmitter is in fact operating at a 22 mSec frame rate when working with the 1024-step resolution receivers. The latency measurements also jive with these findings. Conclusion - there is essentially no difference in performance over the standard DX7 when using this class of receiver.
1024-step Resolution DSM Receiver
We already knew that ServoSync is not supported with the DSM protocol. So what we would expect to see are channels output sequentially, and at a 22 mSec frame rate. Here are the signal traces we observed when using the DX7se with an AR6000:
[sidebar]Note - The DX7se continues to suffer from the same issue that has plagued the standard DX7 and the JR X/XP9303, and that is - when Swash Expo is enabled and Sub-Trim is used on any of the swash servo channels, the swash servo throws on those channels with Sub-Trim dialed in do not "track" correctly. Because of this, it's best to use as little Sub-Trim as possible when Swash Expo is enabled.[/sidebar]
Latency & Performance
Measurements performed by John Kos
As you will see here, the DX7se is the fastest system currently on the market when used with a 2048-step resolution capable DSM2 receiver. With that being said, latency does vary depending on the receiver class used. For that reason, latency measurements are broken down by receiver class.
2048-step Resolution Capable DSM2 Receivers:
Tested with Spektrum AR7100R and AR9000 receivers
• Collective Input Latency (channels 2, 3, & 6):
o Minimum: 12.0 mSec
o Average: 18.0 mSec.
o Maximum: 26.0 mSec
CCPM performance is excellent. Aileron and Elevator input latencies also fall within this range. The minimum and maximum latencies also apply to normal swash mode.
• Frame Rate:
o 11.0 mSec (channels 2, 3, & 6)
o 22.0 mSec (channels 1, 4, 5, & 7)
1024-step Resolution DSM2 Receivers:
Tested with Spektrum AR6200 & AR7000 receivers
• Collective Input Latency (channels 2, 3, & 6)
o Minimum: 11.0 mSec
o Average: 23.5 mSec.
o Maximum: 36.5 mSec
CCPM performance is excellent. Aileron and Elevator input latencies also fall within this range. The minimum and maximum latencies also apply to normal swash mode.
• Frame Rate:
o 22.0 mSec (all channels)
1024-step Resolution DSM Receiver:
Tested with a Spektrum AR6000 receiver
• Collective Input Latency (channels 2, 3, & 6)
o Minimum: 26.0 mSec
o Average: 40.0 mSec.
o Maximum: 55.5 mSec
CCPM performance is good. Swash channel outputs are not grouped, but this does not really impact performance since all channels are updated every frame.
• Frame Rate:
o 22.0 mSec (all channels)
At the Field
When evaluating a radio such as the DX7se, one can measure (as we did) and post the performance numbers. But how those numbers translate into what the pilot "feels" when using this system is best communicated by pilots possessing advanced skills; pilots who can appreciate its speed and precision so to speak.
I put the test sample in the hands of Tommy Heffel, Tommy works at Elwood Hobbies in East Northport and is one of the most talented local pilots in my area. I gave the DX7se to him to try for a month or so. Tommy says "The response of the model to stick movement is noticeably faster with the DX7se. The faster response time and 2048 resolution give you a more connected feel with your model" He went on to say "I have flown the Futaba 14MZ, standard DX7, and am currently flying the JR X9303 2.4. To be honest, the DX7se's response time felt like that of the 14MZ, which I feel is slightly faster than my current JR X9303". Asked to sum up his experience with the radio, he said, "It provides the response time and the "in-the-air" feel of radios costing four times as much. The performance of this radio for the price is unbeatable".
Conclusion
Here are the facts. The DX7se is the fastest radio currently available, period. Add to this 2048-step resolution, comprehensive programming, a price point slightly over $300.00, and backward compatibility with all Spektrum and JR 2.4 GHz air receivers - and you have yourself one hell of a bargain. The DX7se should be a serious consideration if you're in the market for a sport (and possibly even pro) class radio.
Happy flying ...
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HITS
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• Comprehensive programming
• 2048-step resolution (with Spektrum AR7100/AR7100R and all Spektrum/JR 9 & 12 channel receivers) and excellent CCPM performance
• Outstanding value
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MISSES
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• No "trainer" function
• Inconsistent swash servo throws when "Swash Expo" is enabled and "Sub-Trims" are used
MANUFACTURER: Spektrum RC
PART NUMBER(S): SMP2731 - Mode 2, SMP27311 - Mode1
STREET PRICE: $319.99
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