Overlaying GPS speed rather than OBD?

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jp.fox
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Overlaying GPS speed rather than OBD?

Postby jp.fox » Sat Apr 15, 2017 8:57 am

Firstly, big thanks for helping sort out the issues I was having on track yesterday! I managed to get a good few recordings in the afternoon.

One other thing I've noticed is that the latest update seems to have lost the option to use the GPS speed rather than OBD speed in the video overlay? Is there a way to achieve this from a different menu rather than where it used to be on the final overlay options screen?

I'm using the OBDLink MX on a 2004 Renaultsport Clio and it connects at 2hz. I believe this is the fastest I can achieve with the car and as such, the engine data overlay is a bit jumpy but still useful. I used to use the GPS speed as using my Quartz GPS receiver made it a lot smoother and more accurate, is there a way to still do this? I've attached a screenshot showing the OBD speed v GPS speed from my track day yesterday so you can hopefully see what I mean.

Thanks,
James
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Harry
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Re: Overlaying GPS speed rather than OBD?

Postby Harry » Sat Apr 15, 2017 12:12 pm

You can find that option in Expert Settings now. I have removed it from the main user interface because it is not relevant for the majority of users.

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Re: Overlaying GPS speed rather than OBD?

Postby jp.fox » Sat Apr 15, 2017 5:47 pm

Thanks Harry, that's great. I've uploaded a clip and I'm not sure if this is as much as I can expect from the OBDLink MX with my car or if I can do something to improve it? It seems a lot more jumpy than other footage I've seen... https://youtu.be/8Fhvi1jsxU4


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Re: Overlaying GPS speed rather than OBD?

Postby Harry » Sun Apr 16, 2017 8:40 am

From the snapshot you sent in your initial post you can see there are quite some spikes in OBD speed. I assume you will see the same for RPM. This is either due to spiked data coming in from the bus, or due to some special combination of GPS and OBD update rates. To address the later, please play with the Expert Setting "OBD Tweaks / OBD Extrapolation". Start with "Conservative" and try "Aggressive" in case thing top not get better. You need to record something new, existing recordings are not affected by the setting.

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Re: Overlaying GPS speed rather than OBD?

Postby gplracerx » Mon Apr 17, 2017 3:02 pm

You can get noisy OBD speed when the time offset is wrong. In the snapshot, the OBD speed is delayed compared to the GPS speed. I think you need to reduce the OBD time offset or increase the GPS time offset. I may have that backwards, though.


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Re: Overlaying GPS speed rather than OBD?

Postby jp.fox » Sat Apr 22, 2017 1:02 pm

Thanks Harry, I've been having a play with the OBD tweaks settings this morning and when only connected to the OBD reader, it was much smoother. However when it's also connected to the Qstarz GPS at the same time, the rpm becomes very jumpy again as if it's not being updated frequently enough and the app isn't smoothing it when also connected to the GPS. This happens on both conservative and aggressive settings.

Any other ideas on what to try next? It seems a shame to only be able to use one accessory at a time now that I've bought both... I'm using a Sony Xperia Z3 Compact if that makes any difference.

I'd like to get the jumpy rpm sorted before fine tuning the delay for each accessory but thanks for the heads up on how to.

Any help is much appreciated!


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Re: Overlaying GPS speed rather than OBD?

Postby Harry » Sat Apr 22, 2017 5:30 pm

I wonder why using "conservative" doesn't work with this rate combination from 10 Hz GPS and 2 Hz OBD... You have recorded something new? The setting has no effect for existing recordings...

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Re: Overlaying GPS speed rather than OBD?

Postby jp.fox » Sat Apr 22, 2017 7:05 pm

Yes, I tried a few different combinations of new recordings and then played back the overlay...
Conservative/OBD/no external GPS - rpm smooth
Aggressive/OBD/ no external GPS - rpm smooth
Conservative/OBD/ external GPS - rpm jumpy
Aggressive/OBD/ external GPS - rpm jumpy

Is there anything else I can try?


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Re: Overlaying GPS speed rather than OBD?

Postby jp.fox » Mon Apr 24, 2017 9:53 am

I've had another play around with it and I'm wondering if it may shed some more light on the problem... All of the previous tests with external GPS had been with it set to 10Hz so I've updated the list and added a few more...

Conservative/OBD/no external GPS - rpm smooth
Aggressive/OBD/ no external GPS - rpm smooth
Conservative/OBD/ external GPS set to 10Hz - rpm jumpy
Aggressive/OBD/ external GPS set to 10Hz - rpm jumpy

Conservative/OBD/ external GPS set to 1Hz - rpm smooth except odd moment
Aggressive/OBD/ external GPS set to 1Hz - rpm smooth

So it appears to be a problem between the OBD reader and the GPS when set to 10Hz, what does the OBD tweaks setting do? To my simple mind, it seems like I could use an additional 'very aggressive' setting but presume this may cause problems elsewhere? Are there any other settings that I could try?

Am I right in thinking that using the external GPS set to 1Hz will be the same accuracy as using the 1Hz GPS built into the phone? Just with more faff so not really worth using?


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Re: Overlaying GPS speed rather than OBD?

Postby Harry » Mon Apr 24, 2017 4:30 pm

A 1 Hz rate from an external devices is usually a lot better than the internal GPS. These are specialized devices, not a compromise built into some other chips.

For timing accuracy, the update rate is not significant. Absolute accuracy is a lot more important. More fixes are certainly beneficial to understand the line driven or to do advance analysis. This is the reason I recommend a 5 Hz setting for Qstarz devices instead of 10. Qstarz devices can not use satellite based augmentation (SBAS) when operated at 10 Hz. This feature is available for 5 Hz and below only. That's by thew way the only reason they are rated with four starts instead of five on www.gps-laptimer.de/compatibility/android.

Back to the issue: I can't give a better workaround (although 5 may help already) currently. It is strictly about rates, not the actual GPS source. I know how to solve it in general, but a bigger refactoring is required that I can do for a major release only.

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