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Tuesday, February 9, 2010

Gadget Bag: SDHC Memory Cards

Secure Digital memory cards are becoming more popular in all sorts of cameras, and as maximum limits approach terabyte capacities, the format is poised to be the mainstay for still and HD video shooting in the future

Despite strong competition from CompactFlash, the Memory Stick family and xD Picture Cards, the Secure Digital High Capacity memory card format—known as SDHC, for short—leads today’s popularity parade for digital image storage. Recent advancements in performance, the development of compatible derivatives and the industry-wide adoption of speed standards are working together to assure SD’s continued reign as the top-selling card.

First-generation SD cards offered storage capacities up to 2 GB, but that barrier was shattered with the introduction of SDHC, which pushes the upper limit to 32 GB. SDXC (see the sidebar) promises internal architecture that will support up to 2 TB—that equals 1,000X greater density than the original 2 GB limit.

In addition to higher capacities, SDHC offers other advantages. Before SDHC, cameras couldn’t determine the storage conditions inside the card when writing data. Writing throughput is heavily dependent on card fragmentation—the availability and arrangement of empty storage space greatly influences performance. With SDHC, devices designed to take full advantage of the standard can check the fragmented state in the card and calculate the write speed at every storage location. This means they can determine where to write the data according to its speed requirement.

SDHC is similar in appearance to MMC (multimedia cards), and many cameras that use modern SDHC also can use MMC or original SD—but not vice versa! You can’t use an SDHC card in an older camera that was built to use SD. Ditto for card readers, and that’s an important point because when you step up from a 2 GB SD card to an 8 GB SDHC, be sure to upgrade to a compatible Hi-Speed USB or SDHC card reader, too.

All SDHC cards that conform to the rules of the SD Card Association are marked according to their performance class. For example, Class 2 cards (the slowest) must deliver sustained read and write speeds of at least 2 MB per second (MB/s). Class 6 equates to 6 MB/s. The speed-rating system makes it easier for consumers to select the right card for a particular application, particularly video recording. There’s still some confusion, however, because some cards are capable of higher burst rates and are promoted as such. The key spec is sustained read and write speed—that’s the one that determines the speed class rating.

Camcorders that use SDHC—and there are many, many of them—have different requirements than typical digital cameras, and memory card manufacturers have fine-tuned their approach to that market segment. Because camcorders deliver a continuous stream of data to the memory card, the card must be capable of flawlessly sustaining read and write speeds of at least 4 MB/s. These are known as Class 4 SDHC as explained above.

Savvy card makers have designated certain cards as “video cards” and labeled them to match their videotape counterparts. Consequently, we find some Class 4 cards marked “4 hours” in addition to “16 GB.” We also encounter Class 2 cards labeled as “netbook cards” because small PCs typically don’t require cards capable of high-speed data transfer. Add this all together, and the result can be confusing; it’s possible to see two or even three cards from the same manufacturer—and of the same capacity—being sold for wildly different prices. In most cases, the difference is the speed class.


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