SD vs. SDHC vs. SDXC
Secure digital (SD) non-volatile memory cards are used in devices all over the world for storing data such as photographs, videos, and documents for transfer to other devices later on. According to the SD Card Association, “As the de-facto industry standard, SD technology is used by more than 400 brands across dozens of product categories and in more than 8,000 models.”
If you own a recently-made digital camera or pocket camcorder, there is a good chance it uses SD technology. There are two main types of SD cards out on the market to date. Standard (SD) and high-capacity (SDHC) are currently the frontrunners of the standard with mini and micro variations used more frequently in smartphones and other smaller devices.
Compatibility If you have a device that is SDHC capable, it will accept both SDHC and SD cards. SDXC capable devices accept SDHC and standard SD cards as well. Some devices that support the older format have received firmware updates to accept SDHC cards as well. The SDHC standard was introduced in 2006 and was quickly adopted by hardware manufacturers for its higher transfer speeds and larger capacity, and for this reason most if not all SD-friendly devices made today accept the higher-capacity cards. Existing SDHC hosts will only support SDXC cards at up to UHS104 speeds.
Prior to SDHC’s release, some card manufacturers forced standard SD cards to a 4 GB capacity by changing the memory block sizes outside of the standard. This created a card that was rarely accepted by most devices and was quickly replaced when SD 2.0 standards were released.
SDXC cards are currently emerging and gaining ground as new devices come out. Because they use an exFAT file system, devices and computers need to have built-in support for the format. At the present time, operating systems supporting exFAT include:
Windows Vista SP1+
Windows XP SP2 and SP3 with update KB955704
Windows Server 2008 SP1+
Windows Server 2003 SP2 or SP3 with KB955704
Windows CE 6+
Mac OS X Snow Leopard (Intel-based) 10.6.5+
Mac systems released in 2010
Capacity Currently, the maximum supported capacity on a standard SD card is 2 GB. This is due in part to memory block size limitations and industry-set standards.
SDHC cards can theoretically reach 2 TB in capacity, though industry standards currently set an artificial limit of 32 GB. It is believed that the standard will be revised at some point in the future to include larger capacity. One of the reasons SDHC cards have such a higher capacity is the decision to use sector-based memory addressing rather than bytes.
In 2009, SDXC was introduced that works with a newer standard allowing a maximum capacity of 2 TB. As of March 2011, the largest capacity SDXC card on the market weighs in at 128 GB.
Transfer Speed Thanks to the SD standards, cards are given ratings in order to express their general transfer speeds. With standard SD cards, these ratings represent a maximum speed. SDHC and SDXC cards are rated by their average sustained transfer rate. Here are the current card ratings via Wikipedia:
Class 0 cards do not specify performance, which includes all legacy cards prior to class specifications.
Class 2, 2 MB/s, slowest for SDHC cards.
Class 4, 4 MB/s.
Class 6, 6 MB/s.
Class 10, 10 MB/s.
You can find these class numbers located on the front of the cards surrounded by a broken circle resembling a thin C. Class 0 cards are rarely sold currently. A class 2 card is great for standard-definition video and non-raw photography. Class 4 and 6 cards are capable of full HD video recording in addition to RAW format photography. Class 10 is geared for full HD and HD still consecutive recording.
More recently, a new speed class called UHS has emerged which boasts transfer speeds high enough to record real-time broadcasts and capture large-size HD videos. This classification is available on some SDHC and SDXC cards and is indicated by a U surrounding a 1 where a numbered classification would appear.