obd and vid slightly off timing

[MrBlah]

Couple issues with my vids, not sure how to fix.

Hardware is a racecapture pro MK2 50hz gps & canbus using bluetooth to phone, gopro 4 and a xiaomi redmi 3 phone

sample video

https://www.youtube.com/watch?v=XYL8-AqRLcU

First issue: it seems to be dropping the accelerometer data, the g force drops to zero repeatedly, I think it's using the phone data not the mk2 data, I can set the canbus update speed, could that be affecting things?

second issue: the canbus data is slightly off on it's timing, or the gopro video timing is off should I try adjusting the video timing first? is there any way to adjust the canbus timing?

[Harry]

Accelerator sampling rate needs to be raised for 50 Hz RCP recordings. I do not recommend to use 50 Hz sampling by the way, 20 Hz is more than you actually need for analysis. Video: for footage added manually, you need to readjust the video data sync: you can readjust the sync by selecting the respective Lap in the bottom of the raw video's details view.

- Harry

[MrBlah]

20 is enough for autocross? is it best to set the accellerometer to 60 so it's divisible by 20 by 3?

i'm not adding video manually, the video I linked used the built in phone cam and the master was the gopro, auto imported

[gplracerx]

A 20Hz GPS is enough for autocross. If you were going at 330kph or nearly 100m/s, like the top speed of F1 cars at the USGP, you might need a higher frequency. I set my accelerometer update rate to the maximum. It says 100Hz, but in practice it's more like 90Hz. The lower the update rate, the lower the cutoff frequency of the accelerometer data filter. I find a setting of 30Hz completely unacceptable whether the GPS update rate is 5Hz, 10Hz or 20Hz.

[MrBlah]

I had my accelerometer at the default, and look what it did in the video, it's dropping data

the howto documents say to set it to 3 times the rate of the GPS, is this incorrect?

[gplracerx]

I had my accelerometer at the default, and look what it did in the video, it's dropping data

the howto documents say to set it to 3 times the rate of the GPS, is this incorrect?

I wouldn't say it's incorrect. It probably works just fine for race tracks where the yaw rate is much lower than in autocrossing. What I'm saying is that in my experience with data collection while autocrossing, setting the accelerometer update rate to the maximum makes the data look more realistic regardless of the GPS update rate. In particular, 30Hz is too slow. It significantly reduces peak height and introduces a phase shift. That's exactly what you would expect from the way the accelerometer data is filtered (exponentially weighted moving average with a fixed alpha) before being sampled. The filter time constant is directly proportional to the time between samples. That means that the filter time constant is three times longer and the cutoff frequency is lowered by a factor of three for 30Hz compared to 90Hz.

[MrBlah]

so if I raise it, now do I stop the data drops like the video shows

[gplracerx]

so if I raise it, now do I stop the data drops like the video shows

Dunno. I doubt it, though. I don't think that's related to data rate.

[MrBlah]

I need to convince harry to use the mk2 accelerometer data

[gplracerx]

The vast majority of people don't have accelerometer data dropout, so good luck with that. I also have serious doubts that there's enough bandwidth to do that in real time. I'd like to see full sensor fusion, but I'm not holding my breath.