Geebax
Captain
Australia
Offline
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'There is no need to reformat the card every time. I have had several drones by different manufacturers and the process is the same for every one of them.'
You are quite correct, there is no need to re-format the card every time, however if you make a habit of cutting/deleting files off an SD card you will soon arrive at the DJI error 'Index Full' or get to the point where the system simply stops writing to the card when recording video.
How an SD card is treated or handled inside the aircraft bears no relation to what happens to it in a computer, because the aircraft system does not implement all the functions you would have available to you in a full blown computer like the TRIM function.
The whole process of reading the sensor, encoding the files in H.265 and writing the file to the SD card is handled in the Phantom by a single large FPGA chip, and while it is very clever and fast, it does not have all the niceties available, particularly buffer memory. If you are shooting 4K video, the system is trying to write data at around 60 Megabits per second to the SD card. It can sustain that rate as long as the SD card is rated fast enough to do the job, and also as long as the File Allocation Table on the card is making it easy to do so by arranging the memory blocks to be in consecutive locations.
But if the card has been used for some time with you cutting the files or deleting them, the memory blocks are now located all over the card, and this slows down the writing process to a small degree, as the FPGA chip needs to enquire the location of each block to write it, and it takes a bit longer each time if the blocks are not contigous or one after the other. SD cards have a feature that allows large blocks of memory to be written at a time, providing the blocks are contiguous. This will not work on a fragmented card, and yes, cards can become fragmented just like spinning hard disks do.
If this process slows the process too much, and the card is not fast enough, eventually the FPGA over-runs its very small buffer and cannot sustain the process. In some high-end cameras, it would solve that problem by skipping frames, but it cannot do that easily once the frames are encoded in H.265, so it simply stops recording.
A further point about formatting a card using up write cycles, a so-called 'quick-format' simply writes a series of nulls to the File Allocation Table, it does not write the entire memory space of the card, so the impact is very small. And this has already been thought of by providing two copies of the Fille Allocation Table.
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