What is latency?

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What is latency?

Postby RCModelReviews » Wed Jun 30, 2010 12:41 am

If you've read the feature article titled What is latency? and have any comments or questions, you can post them here.
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Re: What is latency?

Postby ransideous » Wed Jun 30, 2010 3:30 am

dont believe 99.9% of the rc community would even notice much. visually, latency is barely noticed by the brain at those minute distances of time. same times changed to audio and the brain significantly notices the difference more easily.
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Re: What is latency?

Postby RCModelReviews » Wed Jun 30, 2010 3:47 am

I can definitely feel the difference between a DSP receiver (35mS avg latency) and straight FM (15mS average latency).

The difference when shifting from straight FM to DSP and back was very noticeable and required quite a bit of "relearning" timing-sensitive maneuvers such as hovering and rolling harriers.
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Re: What is latency?

Postby skyguy04 » Wed Jun 30, 2010 4:46 pm

Excellent article Bruce!

I didn't realize the DX7 still used PPM. So would it be fair to say that my new setup of the JR 9303 + FrSky would be equal latency to my Spektrum DX7 setup? If so, that's great! I'm perfectly happy with the latency on my DX7 and thought i may notice a delay with the new system. Haven't tried it yet. Still sitting on the bench.
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Re: What is latency?

Postby nardoticus » Wed Jun 30, 2010 5:52 pm

I agree that this is an excellent article.
I have only one bone to pick with it. Your analysis that 8ms is only 8% of the stick to servo time
is exaggerated. The servo movement time in your analysis is 62% of your average time and really what a pilot
senses is the time at which he detects the model starting to react, the real delay he will sense is
the 30ms to the start of the servo movement and odd change as his eyes and brain process that.
So the 8ms reduction in the time is closer to a 26% improvement in perceived latency. Much more likely to be
significant than the 8% you claim.
Does this mean that we need to run out and get this. I don't really think so, at least not yet. One of
the primary processing miracles in our brains is the ability to adapt to latency, but we are not all
created equal here. Some people have a much better ability to adapt to latency. Interestingly these are
usually the top performers in activities like stunt flying. So ironically, low latency might be a bigger help
to below average performers than the elite fliers. Where low latency might help the elite flier is not in
doing practiced routine stunts but in reacting to the unexpected, such as me flying my PZ supercub into his airshow. :D
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Re: What is latency?

Postby Captain351 » Mon Jul 05, 2010 1:59 am

I hate to be the conspiracy theorist in the room but could the radio manufactures be touting these lower latency figures for an entirely different reason. Bruce has pondered on the issue that radio's with removable frequency modules seem to be a dying breed. They all have one thing in common they use PPM to communicate with the module. Could it be that Lower Latency is an excuse to get rid of PPM which also gets rid of Frequency Modules and has the side affect of locking us into proprietary manufactures receivers.

Just Saying!
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Re: What is latency?

Postby RCModelReviews » Mon Jul 05, 2010 2:37 am

Reducing latency is one of the easiest (and cheapest) ways to improve the (perceived) performance of an RC system.

It's simply a software change so the marketing folk probably said "why not? We can sell it as an essential feature"

And to a degree I'd have to also say "why not?"

The only downside of lower latency is increased temporal density of the 2.4GHz signal (which is to say that the RC system has to spend a little more time transmitting because it's sending data more often). In theory, this means you would reduce the number of systems that could operate concurrently but in practice this really isn't going to be an issue.
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Re: What is latency?

Postby Captain351 » Mon Jul 05, 2010 2:57 am

I know that I'm not even in the top 90% of fliers but I have flown my share of planes and my opinion is that servo speed is way more likely to be noticed than a few micro seconds of latency. If your not using Digital Servos then there is no point in even worrying about it. If you have a balls to the wall Acrobat, Pattern Plane or Jet with half a dozen or more really fast Digital Servo's you might notice a difference between a high and low latency radio (maybe I should say Low or Lower) but most of us don't have those kinds of planes.

I myself fly with 2 Futaba 9C radios one has a Spectrum Module and one on FM or FrSky. I think my radios still have a lot of life left in them and to be quite frank nobody in my club has better radio equipment than I do. So why would I spend several hundred of my hard earned dollars on a radio that has lower latency and lock myself into buying very expensive receivers when it's not likely to make one bit of difference in my flying. If I needed a new radio right now I would buy the Futaba 10C that still has a frequency module because that is more important to me than the low latency radios that are on the market with no module.

If a manufacturer were to build a quality Low Latency computer radio on 2.4 and were willing to make their digital communication protocols available available to the world that is the radio I would buy. How cool it would be to be able to buy a 2.4 gig receiver from Castle Creations that had all of the features of their Berg FM Receivers and cost about the same price.

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Re: What is latency?

Postby RCModelReviews » Mon Jul 05, 2010 4:10 am

Captain351 wrote:How cool it would be to be able to buy a 2.4 gig receiver from Castle Creations that had all of the features of their Berg FM Receivers and cost about the same price.

Which features in particular?
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Re: What is latency?

Postby Captain351 » Mon Jul 05, 2010 8:55 pm

Which features in particular?


The Berg 7 Receivers are compatible with their Castle Link System that lets you program them from your computer. I'm not even sure what all they do now but the two features that I have used the most often are reversing the servo direction and the ability to assign any channel to any channel number on the receiver.

A Futaba radio used with a CC speed control will always need the throttle reversed. When I get a new Castle Speed Control I usually hook it up to the Castle Link just to make sure it has the newest firmware (That in itself is a feature). While I'm there I usually reverse the throttle channel regardless of what model the receiver will go in I just prefer to have that as a baseline for all my receivers. My Futaba 9c requires the use of channel 8 when using the built in mix for dual elevator servos most of my receivers are 7 channel so I usually set channel 8 to physical port 7 problem solved. I know they do a lot more than that but that's just what I can remember having used.

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