Secure Digital (SD) Flash Memory cards come in two varieties: SD and SD High Capacity (SDHC). Both cards can be used in portable electronic devices such as digital cameras, video game systems and mobile phones, but the newer SDHC format has a wider scope of functionality than the original SD format. SDHC cards are faster than SD cards and have a greater storage capacity. SDHC cards also use a more flexible file system than SD cards, allowing them to be used in a greater variety of devices.
Physically, SD and SDHC cards are identical. Standard cards measure approximately 32 by 24 by 2.1 mm and weigh roughly 2 grams. Both SD and SDHC cards have a physical write-protection switch on the side of the card that can be used to prevent accidental deletion or corruption of information stored on the card. Both SD and SDHC cards are available in the smaller, more fragile "Mini" and "Micro" sizes.
The largest SD cards have a memory capacity of 2GB. SD cards can typically be purchased in 1GB and 2GB formats, though older cards are available in smaller increments of 128MB, 256MB and 512MB. SDHC cards have a memory capacity between 4GB and 32GB and are typically available in increments of 4GB, 8GB, 16GB and 32GB.
SDHC memory cards are faster than SD memory cards. SD transfer rates are based on optimal performance estimates and reflect the maximum possible data transfer speed. SD memory cards can transfer data at a maximum speed of 2Mb (megabits) per second. SDHC cards are rated in three different speed classes. A "Class 2" SDHC card can transfer data at a rate of 2Mb per second. A "Class 4" SDHC card can transfer data at a rate of 4Mb per second, and a "Class 6" SDHC card can transfer data at a rate of 6Mb per second.
SD cards can be used in devices that have a standard SD slot or an SDHC slot. SDHC cards can be used in devices with an SDHC slot. Some electronic devices with a standard SD slot can receive a firmware upgrade that enables them to use an SDHC card for greater storage capacity.
SDHC cards are more versatile than standard SD cards because of their higher memory capacity and transfer speeds. SD and SDHC cards are most commonly used to store digital photographs on digital cameras, but SDHC cards can also be used in some digital video recorders to store high-quality digital video. SD and SDHC cards can also be used as storage devices in video game systems, some MP3 players and printers with an SD or SDHC slot.
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