SanDisk has just announced its latest 32 GB Extreme series SD Card, due in August with 32 GB capacity -- and running at up to 30 MB/sec. read and write data rates.
Capacity is easy to get a handle on -- more bytes of storage gives more room for bigger photos, music, and high-definition video. But what's all this about speed and performance?
SD cards are marked with several types of information (besides the manufacturer), including the "SDHC" logo (SD card, High Capacity format), the storage capacity (i.e., 32 GB), and the speed "Class" -- the circle "C" with a number (i.e., 2, 4, 6, or 10).
The Class rating specifies the guaranteed data transfer rate, in megabytes per second. This is the baseline or minimum guaranteed performance of the card, so you can be sure it matches the recording rate requirements of your particular device. For example, AVCHD (H.264) format recording on an HD video camcorder requires a maximum of 24 Mbps (bits/sec), which corresponds to 3 M Bytes per second -- which means a Class 4 device (at 4 MB/sec.) provides all the performance required for the device, and a higher-performance class 6 card would provide no additional benefit, at least for recording.
So as SanDisk announces that it is stepping up its top-of-the-line Extreme cards from Class 6 to the new Class 10 (for a minimum of 10 MB/sec. recording rate), what's this about also promoting a 30 MB/sec. maximum speed?
What SanDisk, for one, would like you to understand that there's more to using memory cards than just recording photos or videos. When you upload all those HD videos to a computer, for example, or sync your media library of photos, music, and movies, a higher maximum speed gives headroom for a quicker transfer time.
And, interestingly, shooting photos can require higher data rates than HD video. Digital images now have higher resolution than HD video, and also are moving to less-compressed formats like RAW, so individual images are growing larger in size. And cameras aren't just for shooting one photo at a time, they support rapid burst modes to record continuous sequences of photos in fractions of a second.
In addition, the lines between different categories of devices are blurring -- today's digital cameras also shoot HD video, and today's HD camcorders also shoot high-quality high-res stills. So a camera's minimum Class rating may be set by its video recording mode, while a camcorder's read/write performance may be stressed by its photo burst recording mode.
So when shopping for a memory card, remember that while the Class defines the minimum baseline required data rate for a specific product, additional headroom in terms of the maximum speed rating can add visible performance benefits including longer burst recording.
See my full article: Flash Memory: Technology Summary for more on memory card formats and features
See previous posts: 32 GB SD Cards and Future Terabyte Memory Cards
See my Portable Storage Gallery for more on storage formats and devices.
SanDisk Ultra II and
Extreme III SDHC cards