Secure Digital (SD) memory cards are stamp-sized storage devices used in digital cameras, cellphones and other portable devices. They contain built-in copyright protection that prevents devices that do not follow the standards set by the SD Association from reading or writing to the memory cards. There are several SD card formats that differ in capacity, purpose and bus-interface speed, but all follow the same physical dimension standards.
Physical Dimensions
SD memory cards are available in three sizes: standard (32 mm by 24 mm by 2.1mm), miniSD (21.5 mm by 20 mm by 1.4 mm) and microSD (15 mm by 11 mm by 1.0mm). The standard and miniSD sizes are meant for digital cameras and camcorders while microSD is meant for cellphones. A device built for standard-size SD cards can still read minSD and microSD cards with an adapter. However, there are no adapters for larger cards into small slots.
Bus-Interface Speed
The bus-interface speed of a SD card defines the minimum speed at which data is transferred between the device and the card. The SD Association has set two transfer speed categories: Speed Class and Ultra High Speed (UHS). Speed Class is further defined into Class 2 for SD video recording, Class 4 and 6 for HD recording and Class 10 for HD recording using a high speed bus-interface. UHS Class 1 allows for greater performance in capturing large-sized HD video.
SD Cards by Capacity and Bus-Interface Speed
The standard SD card format supports capacities of up to 2 GB and bus-interface speeds of Classes 2, 4, 6 and 10. SD High Capacity (SDHC) cards have capacities of between 4 GB and 32 GB and are available in Classes 4, 6, 10 and UHS Class 1 bus-interface speeds. The SD Extended Capacity (SDXC) standard allows for capacities of between 32 GB and 2 TB and has bus-interface speeds of Class 10 and UHS Class 1.
Backward-Compatibility
A standard SD card can be read by a device rated for SDXC, but a SDXC card cannot be used by a device rated for standard SD. Only devices rated for the higher-capacity cards can use them. On the other hand, bus-interface speed has little bearing on compatibility. A Class 8 SDHC card and a Class 10 SDHC card will work in the same device. For the best performance, use the most advanced SD card a device is rated for.
Tags: bus-interface speeds, device rated, memory cards, bus-interface speed, capacities between, Class recording