SDHC (Secure Digital High Capacity) was established to meet the growing demand for HD (High Definition) video and high resolution image recording now used in many SD-enabled devices.

SDHC is the same physical size and shape as standard SD but meets the new SD specification of version 2.0. If the SD card is version 2.0 and 4GB or above it is classed as a SDHC card.

Currently, SDHC specifications allow for memory cards of a capacity between 4GB and 32GB.

SDA (SD Card Association) has also established a Speed Class rating (Class 2, 4 and 6) for the standardisation of data transfer speeds (compulsory for SDHC cards):

CLASS 2 – a guaranteed minimum write speed of 2MB/s or Higher*

CLASS 4 – a guaranteed minimum write speed of 4MB/s or Higher*

CLASS 6 – a guaranteed minimum write speed of 6MB/s or Higher*

* The minimum speed capability depends on the manufacturer of the host device and model.

All Class 2, Class 4 and Class 6 SDHC cards are compatible with SDHC devices. Please note that if the device manual suggests a SDHC card of speed Class 4 (minimum), the user should install a Class 4 or Class 6 (faster) SDHC, NOT a Class 2 card as this may reduce performance of the device.

SDHC cards are not backwards compatible with standard SD devices, so they may not work in digital cameras or USB Memory Card Readers manufactured before Summer 2007.

Most new devices that require the new SDHC format to operate will display the SDHC logo on the product (such as printed on the side of a camera casing) and/or user guide.

Standard SD Cards will work in SDHC compatible devices such as Memory Card Readers. In a SDHC camera a standard SD may be of limited use due to the slower read and write speed of this older technology.

The information contained in the article also applies to microSD and new microSDHC plus miniSD and new miniSDHC formats. The only difference will be the physical size of the memory cards, the speed class information and compatibilty issues are the same.