Recalculate after sensor-delay change

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jwmelvin
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Recalculate after sensor-delay change

Post by jwmelvin »

I see in my charts that GPS and OBD speed are off and it looks like GPS is the better candidate to shift (OBD seems to line up with acceleration better). So I made a small change in the advanced settings but is that only going to affect new laps or is there a way to apply it to already-recorded laps?
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Harry
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Re: Recalculate after sensor-delay change

Post by Harry »

All delays are applied during recording. The only exception is the video delay, which is applied when overlaying.

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jwmelvin
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Re: Recalculate after sensor-delay change

Post by jwmelvin »

To adjust the delay for existing laps, would it work to export, manipulate the data, and then import?
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Re: Recalculate after sensor-delay change

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Yes, but this is not trivial. To export / import, you need to use the .hlptrl format.

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jwmelvin
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Re: Recalculate after sensor-delay change

Post by jwmelvin »

Ah, I see, not trivial indeed. I will spend some time on the street working out the delays.

Do you have advice on what to move? I see that my OBD speed is currently 3 seconds ahead of GPS speed. Lineal acceleration basically lines up with OBD. Current delays are Internal GPS = 0.7, OBD = 1.0, Internal Acceleration = 0.5. This is iPhone 5 using GoPoint BT1. The video seems to line up with the timing info when I use Video Sync of 0.5.

This is such a helpful tool, it's great. Thank you. I can see that I need external GPS for better data, but I'd like to get the current setup properly tuned.
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Re: Recalculate after sensor-delay change

Post by Harry »

Thanks for the feedback. :-) The speed delivered by GPS sensors is not derived from position changes but the so called Doppler effect. From my experience, GPS speed is "behind" the rest of GPS data and should not be used to find the correct synchronization. Wheel speed (OBD) will be quite a bit ahead of GPS speed for your configuration as latency from wheel speed sensors is faster than the satellites based method and higher update rates for OBD compared to GPS adds a bit too.

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jwmelvin
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Recalculate after sensor-delay change

Post by jwmelvin »

That's helpful, thank you. Do you have a recommendation on a procedure to sort out the best acceleration, gps, OBD, and video timings?

Also, if OBD is available, is that used for the video overlay speed?
Last edited by jwmelvin on Mon Mar 24, 2014 4:15 pm, edited 1 time in total.
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Harry
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Re: Recalculate after sensor-delay change

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No, not really. The most obvious mismatches one recognizes when watching videos are inconsistencies between audio / sound and rpm development (for OBD), and acceleration and left / right / brake / acceleration seen in the video. I'm still looking for a math class at University to generate a set of formulas and algorithms to auto calibrate all that stuff. This is definitively possible but pretty complex too. I have a pretty big collection on what can be done but haven't found the time to work on it so far.

In LapTimer development, things sometimes take long, but always get always available some day. :-)

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jwmelvin
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Re: Recalculate after sensor-delay change

Post by jwmelvin »

Harry wrote:The speed delivered by GPS sensors is not derived from position changes but the so called Doppler effect.
Wouldn't it make sense to calculate GPS speed from the position fixes, if that is more accurate?

Related: it appears that certain laps have a slower lap time but a faster reported average speed. This makes no sense to me. Can you explain what I am missing? Perhaps it's related to the use of GPS speed? My BT1 connects but frequently showed a zero update rate at the last event. Quite sad.
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Re: Recalculate after sensor-delay change

Post by Harry »

Speed calculated from position changes is extremely noisy and requires a lot of smoothing to show something meaningful. Especially when using this approach in real time adds an additional delay to the already late speed value. So no, it is not an option and has been replaced by Doppler in most modern GPS chips.

Average speed is derived from lap time and lap length. LapTimer uses a measured lap length (in this case derived from the fix distances - same problem as above) and this introduces the avg speed variation. I have an item on my list to use the standard track set length (if defined) in a future version.

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