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Video lag while in video mode but not in photo mode

masist

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Hi guys, I have a M2Z for a few months now, and I am a bit disappointed by the video latency. I decided to do a "measurement" of the latency of the video feed.
I use a regular RC with a Samsung A7 2018. App version 4.3.37
While doing this measurement I noticed that in photo mode the latency is much better than in video mode. While in video mode, it does not make any difference if the AC is recording or not, if the sd card caching is enabled or not, or if the video feed is set to HD or normal mode. I only did measurements in 1080 video res.
As seen on the attached images, in photo mode I have around 0.1s latency, which is great, as DJI states 120-130ms latency on their website.
In video mode I get around 0.3s. Maybe this does not seem much but for me it is quite a big lag, even while taking videos from a distance. For close up filming is even worse.
The big mistery for me is why in video mode the latency is so much higher.
Do you have any ideas?

Thanks.
Silviu
 

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old man mavic

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i believe it is due to the amount ,of information that a video has compared to a picture ,that has to be processed ,before it can be displayed on the screen device
 
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masist

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Yes, but the live video feed is still a video even in photo mode. The only apparent difference would be that in photo mode the aspect ratio is 4:3 which means a slightly lower resolution when displayed on a 16:9 screen, but this does not justify the 3 fold increase in the live video feed lag.
The question is if this is the situation for everyone, or there is a problem with my setup.
 

Arnold LeVine

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Try it with 4K Video, and 60 fps, etc. and see if the lag gets worse. Then try a different RC, like a Smart Controller (which is probably much less powerful than the A7), or a new 2021 phone with faster processor, and compare the results.
Also, maybe the speed of the SD card? Lots of variables might have effects. (or maybe I don't have a clue!)
 
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tlswift58

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Plz explain more about the latency you find? Is it during flight that you see it on your screen or does it look that way from the SD Card? Hard to tell what issue you are having.

If it's a lag on your screen while flying - you'd see that if you can truly tell a difference in 0.1 - 0.3 seconds. I surely wouldn't be able to. It does take time for the radio signal to travel from RC to drone and back - thus some inherent latency - as sound travels slower than light. Obstacles and other things create lag as well and even signal loss.

As stated above - if it's on the SD Card, then I'd say you don't have the proper one installed. DJI states what SD Cards work best and if you're not using one of those or some no name brand - that may be the culprit.
 

masist

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Thanks for the feedback.

I will try higher fps video modes to see if anything changes. I will also try another phone on my rc, but it will pe hard to find another rc or even a SC. I have a friend with a Mini 2 but that is another type of RC, I doubt it would pair with the Mavic 2. Anyway, visually it seems to have the same lag on video mod as I have. Didn't try the photo mode on the Mini 2.
Also tried the DJI Pilot app on my phone, the situation looks the same. Responsive video feed on photo mode, bigger lag on video mode.

I will explain exactly how I made the measurements on the first post, with the photos attached.
I turned the phone on airplane mode and closed all the running apps.
Connected the phone to the RC, powered on the RC, opened the DJI Go 4 app.
I unfolded the arms of the AC, propellers and gimbal cover removed and powered on the AC.
Then I put the camera in photo mode, in the mode that you use to take pictures with the drone.
I put the phone near the screen of my laptop and started the stopwatch app on the laptop.
I placed the AC so that the screen of the laptop is in view, and the counter is displayed on the screen of the phone.
Then, using another phone, I took pictures of both screens and uploaded them in the first post.
foto1.jpeg and foto2.jpeg are pictures taken with another phone, while in photo mode.
In foto1.jpeg in the upper part of the image you can see the counter on the laptop screen showing 03:43.76.
On the lower part of the image you can see the phone screen connected to the RC showing the counter value of 03:43.66 changing to 03:43.69.
If you subtract these two values you get around 0.1s latency in the video feed.
Next I changed into video mode with 1080P/24, not recording, and took with the other phone another picture, video 1 nr.jpeg. here the lag is a bit under 0.3s.
Then started recording on the SD card in the AC and took another 2 pictures, video 2 rec.jpeg and video 3 rec jpeg.
Again around 0.3s.

This lag, even if seems low, it has quite an effect while flying low, close to obstacles, like on a narrow alley with trees on both sides.
While flying high above obstacles it does not have such a great impact unless you are trying to shoot a manual orbit while zoomed in.
The speed of sound has nothing to do with this, radio waves, as light waves, are both electromagnetic radiation, which, of course, travels at the speed of light.
Indeed in air it is a bit smaller than in vacuum, but nonetheless it is still around 300.000 km/s.
Obstacles can create a lag, especially on longer distances as the signal gets weak, but the measurement I made was with the AC next to the RC.
Also this lag induced by poor signal should affect the photo mode video feed as well, right?

As the lag is almost the same in video mode while recording or not, I don't think the SD card has anything to do with it.

My main question here is that if this is how DJI drones, especially the Mavic 2 zoom with the standard RC handles the video feed, or there is a problem with my AC / RC / phone.
 

masist

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I made several other measurements in the same conditions as above, for different resolutions and framerates, multiple measurements for each mode, and made an average for each mode.

The results are:
Photo mode: 0.16s
Video:
1080p24 0.3s
1080p30 0.3s
1080p48 0.18s
1080p60 0.177s
1080p120 0.23s
2.7k24 0.3s
4k24 0.245s
4k30 0.24s

In video modes with 48fps and higher, the image gets cropped a bit when compared to 24-30fps.
I guess that the higher lag for lower frame rates is caused by a higher computational power used by the AC in order to interpolate the sensor resolution to the video resolution.
Probably in higher frame rates a simpler algorithm is used where the extra pixels from the sensor are just dismissed or something like this, and thus the AC is able to encode faster the live video feed.
Also this is the case fo photo mode, where you see in the video feed the whole sensor area.
 

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