Many users report that the write speed of UHS-1 is worse than SDHC CLASS 10 for hacked cameras. The reason is that UHS-1 uses 3 extra pins to transfer data, totally 4 bits. So, if your camera does not implement UHS-1 mode, it cannot use 3 extra pins and will use the traditional SD 1 bit mode. The data transfer rate will drop obviously.
Check this wiki : http://en.wikipedia.org/wiki/Secure_Digital
It depends on the card. Basically if you will be using a card in HS mode, you need to judge its performance in HS mode. The UHS-I mode performance is not necessarily predictive of the performance in HS mode.
But it is wrong to say that you shouldn't use a UHS-I card in a camera that doesn't support UHS-I mode. Some of the best cards we have for HS mode happen to be UHS-I cards.
Don't confuse bus modes, speed classes, and capacity standards. A UHS-I card can be Class 10, and it can be SDHC or SDXC.
I am not to say you cannot use UHS-1 SD card as Class 10, but it will degrade the card speed. I simplify the hardware, UHS-1 uses 2 clock cycles to transfer 1 byte, Class 10 uses 8 clock cycles to transfer 1 byte. A 95 MB/s UHS-1 SD card design will use all its 4-bit capacity to reflect its transfer rate. If it steps down to 1-bit mode and can still transfer something like 40MB/s. The manufacturer will mark it as 1xx MB/s SD card instead of 95 MB/s. Theoretically, the speed of 4-bit mode is four times of 1-bit mode.
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