Spektrum DSM latency measurements

[rsilvers]

I measured the data rate and latency of a Spektrum DX3 and Pro receiver system using a Tektronix TDS-220 100 Mhz bandwidth digital oscilloscope.

http://www.photomosaic.com/albums/Hobby/IMG_4155_800.jpg

The data rate being sent to the analog servo from the Spektrum receiver is 20ms per frame, or 50 updates per second.

For comparison, JR PCM radios are approx 46 updates per second for Z mode, and 47 for S mode per channel. Standard FM PPM systems are about the same.


Latency of the Spektrum DX3 when using the channel-3 switch is 7-8ms.

Here you can see that the latency is over 6ms:

http://www.photomosaic.com/albums/Hobby/IMG_4158_800.jpg

Here you can see that the latency is less than or equal to 8ms:

http://www.photomosaic.com/albums/Hobby/IMG_4157_800.jpg


The Nomadio salesman said that the Spektrum system had about 15ms of latency inherent in the transmission, and another 30ms of latency provided by the transmitter encoding the controls. That would be 45ms. I was not able to duplicate those numbers.

Spektrum's claim of '3ms' is quoted as 'the figure added to your radio's processing time' so I have not confirmed or denied that specific claim because I included the radio processing time in my calculations. If the radio processing time was 4 or 5 ms, then their figures seem accurate.

Nomadio also claims 100 updates per second for their system, and does not specify a latency figure. I would like to verify if Nomadio means this for ALL channels combined or for EACH channel. Then I would like to specifically test to see if they update any given servo at 100 hz. I currently have no Nomadio system to test.

So while I have not tested the Nomadio as of yet, the Spektrum has very little latency (just 1/125 of a second).

[GordonFreeman]

Dude, you rock. Nice, keep going on this RS-dog. I am so disapointed with Nomadio. Just as I thought, an inexperienced RC company producing an immature product. I think I'll keep my 3PK. Post your results on the Nomadio site, see what they say.

If you're near Wisconsin, you can test my Nomadio.

[rsilvers]

For comparison, I tested the standard radio that came with my E-Maxx. Since there is no digital processing, it has slightly less latency.

http://www.photomosaic.com/albums/Hobby/IMG_4159_800.jpg

http://www.photomosaic.com/albums/Hobby/IMG_4162_800.jpg

I measured 6ms, or about 2ms less than the Spektrum (1/500 of a second less).

http://www.photomosaic.com/albums/Hobby/IMG_4161_800.jpg

According to my measurements, the '3ms' that a 'Pro' Spektrum 'adds to your radio's processing time' (their marketing claim) is likely a correct / conservative number. I saw 'likely' because I did not use the same radio but it seems to be adding up.

[rsilvers]

If you factor in the latency of having to wait for the next 20ms frame if you just miss a frame, then latency can run from 7 to 24ms. That is about 15ms average. Nomadio is sending me one of their systems to test. It will not surprise me if it has less latency.

However, the Spektrum is evidently about identical to an FM or JR PCM system (within 2 or 3ms).

[InspGadgt]

8ms for a fully digital system with auto frequency detection vs 6ms for an analog system...I'd say that's worth the extra 2ms to go with the Spektrum.

[rsilvers]

There is also the Futaba PCM G3 2048. It has a 56hz update and 14 channels (you can expand the channels with a multiplexer).

How many bits of precision (steps on the servo) does the Spektrum and the Nomadio have?

[rezenclowd3]

rsilvers: thanks for these tests!:-) As far as the latency between FM, Spektrum and whatnot, my personal opinion is that it has hit a point that the driver will no longer feel a difference and that it wont make the car drive any better, BUT it is the accuracy and how likely it is for the unit to "glitch" that gives the higher end FM, and Spread Spectrum units the confidence and accuracy that racers need. It isnt like only Spektrum/ Nomadio users are the only ones winning the races. Although personally I love the confidence my M11/ Spektrum unit gives me knowing that I theoretically should not have any interference and feel it while driving. Again though, these tests are excellent and you are out to prove or disprove claims, not the feel of a system.

[Ball Racing]

What does this mean in layman's terms?
What is latency?
Is what you found with the spektrum good or bad?

[rsilvers]

Some people said that the Spektrum had a 'delay' (compared to normal radios) and I have proven that is not the case -- at least not on the recent one I bought. The Spektrum is fine compared to previous technologies (though not faster responding).

Next I will find out if the Nomadio is a leap forward over that, or about the same.