Data is out of sync with video

[shadowpuck]

I've been trying to run down this problem for a long time now, with no success and i'm curious what others have done to fix this.

i'm using an iphone 8+, and now with a racebox mini external gps.
i'm using the internal camera on the phone only.

what i've noticed is through several versions of harry's now, and whether or not i use a bad elf gps, racebox - the speed and g force data are out of sync with the video - sometimes ahead of and sometimes after the video. i've tried messing with the end point of the lap - but, i've also had videos where the end point of the lap is right but the data is still out of sync. i've also had times where g force is close enough to be acceptable, but speed is always off.

as an example - if i'm traveling straight at a given speed, say, 100mph, and then i brake - the speed overlay may start changing before or after the actual event on video and sometimes its significantly out of sync. and, sometimes it's really close.

i use the 8+ only for laptimer.

[Harry]

Video, GPS, engine, and IMU data cannot be synced automatically. Video, engine, and IMU data is coming in with arbitrary sensor delays, while GPS comes with a defined and precise timestamp. That's the reason LapTimer uses configurable delays set in Expert Settings. These delays give LapTimer a hint how old a measurement delivered is. LapTimer subtracts this delay by sensor type before merging all the channels together. Video delays are handled different, they are applied during overlay.

The real delays will never match the assumption set in Expert Settings. They will vary depending in wireless disturbances (BT / WiFi), processor load etc. So in case you are very sensible about "out of sync", you will need to fine adjust the sync point on top of a proper delay setting in Expert Settings.

I do not recommend tweaking the sensor delays, this can get a nightmare. Especially, because they are applied for new laps only. Opposed to this, adjusting the video delay for a representative lap makes sense and will usually result in "acceptable" videos. In case you got (over-) sensible on this topic you will need to manually adjust the sync for each session. When doing this - ignore the video frame. Make a guess how much data is behind or ahead (e.g. 0.2s) and adjust the sync point by using the numeric display. "Play" (not Export) the video afterwards and repeat if necessary.

- Harry

[shadowpuck]

thank you harry for the information. i admittedly did not completely understand how the delays impact things here. i'll work on that.

i'm willing to accept some out of sync - i recognize the limitations of the systems and hardware involved here - it's just become very noticeable over the last year or so for me...

[shadowpuck]

doing some testing - dropping the overlay synch to 0 seems to have helped some.

now the g force seems more accurate compared to video.
speed sync is still off, but closer to correct.
what happens now is the speed display starts dropping before the video. so, if i'm doing 70mph and i hit the brakes in the video the speed has already dropped before i start hitting the brakes.

[Harry]

You can’t change the relation of GPS, OBD, IMU after the fact. It is only the video / all data relation that can be changed.

[shadowpuck]

so is there anything i can do using an iphone and a racebox to get better correlation between the gps data and the video?

[Harry]

The Racebox Mini has a high update rate, so the delay is low. LapTimer will use Sensor Delays / External GPS Sensor for it. The default value is 0.7. I can't say more because the startup time of videos has a high variance depending on system load. So you really need to select a reference lap and tune the parameter for your phone. That's the best can do.

- Harry

[shadowpuck]

The Racebox Mini has a high update rate, so the delay is low. LapTimer will use Sensor Delays / External GPS Sensor for it. The default value is 0.7. I can't say more because the startup time of videos has a high variance depending on system load. So you really need to select a reference lap and tune the parameter for your phone. That's the best can do.

- Harry

harry - thank you.

i apologize for my lack of understanding - but which parameter are you talking about? are you talking about the sensor delay/external gps sensor parameter? if so, just to confirm - any changes made to that parameter are only seen on future recordings, right?

when you say a reference lap - do you mean a fastest lap or just start with any completed lap and determine what delay in sync i'm seeing?

[Harry]

Yes, future recordings only.
With reference lap I meant one which can serve as a typical example of laps you record.

[shadowpuck]

thank you harry for the info!