Geebax
 Captain
Australia
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Mabou2 Posted at 2017-8-12 06:36
Hiya Geebax.... Interesting conversation with you. I'm open to learning at all times, so I'm not digging my heels in and did the research you suggested.
I can't speak for MPG because I usually shoot .mov.
I will explain a little more, at the risk of boring everyone. The DJI cameras record MPEG streams, either to the H.264 or H.265 standard. They then put those files in a 'container' to make handling them easier. The two main forms are MP4 or MOV. The containers are like having washing powder sold to you in a tin can or a cardboard box. The contents are the same, the MPEG stream, just the container that varies, and the container type is not important. Most of the world uses MP4, but Apple just had to have their own format, hence MOV. But the pictures in each are identical. I laugh when people say they found the video was better when they set up the camera to record MOV, because it is quite untrue.
There are various ways of recording the images from the camera, one method CinemaDNG, records RAW images with each frame occupying its own seperate file. In this method, if the camera cannot keep up with the recording process for some reason, it will drop frames. A good camera also lets you know if it has dropped frames. In other types of cameras, it is a single file, but the frames are organised in individual packets, and also can be dropped if necessary.
The MPEG stream was not designed to be used for recording images into memory in cameras, it was primarily designed as a transport medium, so it has no seperate packets of frames, because apart from the I frames that contain full information of a single frame, all the other frames are nothing more than information about what has changed between one frame and the next. But the high compression available with MPEG made it very attractive for camera manufacturers trying for high quality combined with small storage requirements. So now it has become more or less a standard. Personally, I would like to see a switch to CinemaDNG, as it is RAW video, but it requires a high performance card to record onto, and is at present out of the range of SD cards or vastly too expensive. Note that the Inspire 2 uses small SSDs instead of SD cards.
When the recording sub-system in the camera is recording individual frames, it can drop them, but with a continuous MPEG stream, there is no provision for doing so, therefore if the recording system finds that the card is not keeping up, it simply stops the recording process. This is why from time to time you will see a post here from someone saying their Phantom is only making short recordings and stops all the time. This indicates the SD card is borderline, and should be replaced.
One problem is when you have a card that is borderline on speed. Remember, there are write speeds, read speeds and continuous, or sustained, read and write speeds. Some cards can be barely OK on both, and when played back, the replay system cannot read the file fast enough, and will skip frames. It can do this because the original purpose of the MPEG format was to transport the information to the viewer, primarily over TV channels. If the transmission process loses some of the data, the reception system must be able to discard the frame and move on to the next.
So skipping frames can be the fault of the replay system. When replaying, it is important NOT to play back from the SD card, either the card, the adapter or the USB connection may be too slow. Instead, copy the file to a fast HDD, or preferably an SSD, then play it.
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