You’ve all seen SD memory cards. Do you know they aren’t all the same? No, it isn’t just about their storage capacities. It’s not just about SD, SDHC and SDXC (which is primarily related to maximum storage capacities). They are different in their read/write performance. The read/write performance becomes critical when you need to move large amounts of data very quickly, as you might with digital still cameras and video recorders these days.
Some digital cameras could “run” faster had they not been slowed down by a slow memory card. Capturing high-resolution RAW images in continuous drive could, for example, require the camera to write tremendous amounts of data to a memory card very quickly. If the memory card is slow, the camera has to buffer data internally. Eventually, the continuous drive speed will slow down if the buffer gets filled and the camera cannot flush it out fast enough.
A high speed SD memory card would be very useful for many of today’s digital imaging products. Of course, that’s assuming that the digital imaging product isn’t itself the bottleneck, and is actually capable of keeping up with the performance capabilities of the higher speed SD memory card.
The speed class rating is one of the primary means of categorising the performance of a SD card. Speed Class 2, 4, and 6 specify a sustained read/write performance on a card in a fragmented state at that number of MB/s. Class 10 is slightly different, asking only for 10MB/s minimum non-fragmented sequential write speed.
Things have just gotten slightly more complicated with UHS Speed Class. UHS stands for Ultra-High Speed. There is UHS-I and UHS-II, and they specify the bus speed interface speeds of up to 104MB/s and 312MB/s respectively. That doesn’t necessarily mean the memory card itself supports read/write at that speed. The numeric class is still specified to indicate the read/write performance. I.e. UHS Speed Class 1 specifies a minimum read/write speed of 10MB/s.
Since UHS is a bus interface, a compatible UHS host device is required to use the faster interface on a UHS SD memory card. A UHS SD memory card inserted into a non-UHS host device will operate in non-UHS mode.
The SD memory card pictured above is a UHS-I card (indicated by capital “I” next to the SDHC logo). It supports standard Speed Class 10 when connected to a non-UHS device, and UHS Speed Class 1 when connected to a UHS device.
Not all devices a UHS compatible. So it might help to check to see if the device you want to use the SD memory card with has support for UHS. Of course, even if not, you might still want to buy a UHS capable memory card now for future needs.