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Selecting a memory card is way more confusing than it has any right to be because these cards started out as storage media for images, not video, and the different uses require much different behavior from the memory card. For images you want a fast burst speed but you don't care about the minimum sustained speed. In the worst case, it just takes a little longer to write your image(s) to the card. On the other hand, video requires the card to have a guaranteed minimum write speed. If the card is any slower than that, in the best case the camera will drop frames and in the worst case it will generate an error and stop recording. In the past manufacturers would specify only the maximum write speed, which is only useful when recording bursts of images. When SDxx cards came into use for video recording the industry developed speed classes. These specify minumum (continuous) rather tham maximum (burst) write speeds. The classes relevant to HD and 4K recording are SDHC/XC Class 10, UHS-i U1, UHS-i U3, and UHS-ii. Each of these standards (SDHC, SDXC, UHS-i , UHS-ii) uses a different electrical and software interface to the card, each of which provides enhanced performance and increased capacity. In the case of UHS-ii there are six additional connector contacts on the back of the card, while the others use the original set of 9 contacts. The newer cards are backward compatible with the older interfaces, that is, you can use a UHS-ii card in a SDHC or UHS-i camera, but it will still only operate at SDHC or UHS-i speeds. The phantom 4 pro has a UHS-i interface and so it supports Class 10 or UHS-i U1 or UHS-i U3 cards. If you spend the extra money on a higher speed UHS-ii card it will work but it just falls back to UHS-i mode and speed. So don't waste your money. The fastest video compressor on the P4 pro runs at 100 Mb/s (12.5/MB/s). DJI conservatively suggests a card with minimum speed rating of 15 MB/s. That suggests that you need a UHS-i U3 card which guarantees a minimum of 30MB/s. While Class 10 and UHS-i U1 (both guarantred @ 10MB/s) will most likely work, for the few extra dollars it's worth buying the U3 card.
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