Speed delay in video overlay

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my2011wrx
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Speed delay in video overlay

Post by my2011wrx » Wed Feb 08, 2012 12:16 pm

Hi Harry,

I use an iPhone 4 and a Kiwi Wi-Fi. The only two issues I've come across are iPhone autofocus and the occasional delay in GPS speed. I see that you have a work around for the autofocus coming out soon so that leaves me with the speed delay.

I exported the data to Excel and had a quick look. The fact that I'm using a kiwi means I can also detect the speed directly from the ODBII port. The two readings in OBDSPEED[km/h] and SPEED[km/h] are sometime very different. Mostly they are similar but sometimes can be out +- 35kph.

Is there a way that I force the overlay to read the OBDSPEED[km/h] data that was logged as it’s more accurate that the GPS data?

If I accelerate or brake hard the GPS seems to lag behind. I've pasted a few of the results below with correct ones highlighted.

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Thanks,
Pat
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Harry
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Re: Speed delay in video overlay

Post by Harry » Thu Feb 09, 2012 8:47 pm

Hi Pat,

First of all, GPS speed is more accurate than measured wheel speed, at least for higher speeds. This is actually the reason I'm not using OBD speed everywhere. On the other hand, you are right, GPS speed has a higher latency than wheel speed. In addition, I believe Apple is using some smoothing technics - leading to further delays. You can partially work around this setting one of the GPS Tweaks: use 'calculated' for speeds here.

Maybe you can test that and post an update here. :-)

I will reconsider using OBD speed in the video overlays again.

- Harry
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my2011wrx
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Re: Speed delay in video overlay

Post by my2011wrx » Fri Feb 10, 2012 8:49 am

Thanks Harry. I agree that GPS would be accurate more often than not but in my case it seems to lag and I sometimes get 0 speed readings so having the option to use OBD speed is great. Now that I know how to change it - no need to consider using in for future updates as it would provide incorrect information for cars that frequently lockup wheels or like to drift.

I tracked down a lap and re-synced the video then exported the lap to my iPhone 4S to re-generate the lap but I received the same occasional 0 or out of sync speeds so I guess I will just need to go back to the track with the new setting and test.

Is there any reason why I can't regenerate the existing lap with the OBDSPEED[km/h] data since it's actually logged the data?
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Harry
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Re: Speed delay in video overlay

Post by Harry » Fri Feb 10, 2012 8:55 am

Hi,

Recorded data is never changed within LapTimer (only exception is the recalculation of lateral / lineal forces), changing the speed calculation will have no influence on existing laps. In case you really want to change existing data, you need to export the recording to .hlptrl, edit and send it back to your iPhone for import (long press the attachment in Mail).

- Harry
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geefin
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Re: Speed delay in video overlay

Post by geefin » Wed Mar 14, 2012 6:05 am

Is the 'calculated' speed calculated from GPS and accelerometer data or from OBD II data (RPM/gear ratios/etc)?
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Re: Speed delay in video overlay

Post by Harry » Wed Mar 14, 2012 8:08 am

Hi,

After setting speed to 'calculated' in the Expert Settings (instead of 'measured'), LapTimer will calculate the actual speed from GPS fixes. There are two ways to get speed out of GPS: either you use the speed delivered from GPS directly, using the so called "Doppler Effect" (measured), or derive speed by calculating the speed required to get from one GPS position to the next one within the time gap between these fixes.

Using the Doppler effect is the default and should be more accurate. However, there are several reports that the speed delivered here is constantly to low. In case you perceive this is the case for you, try the calculated option.

In case you have an OBD sensor attached, LapTimer will switch to wheel speed in most displays.

- Harry
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