SD Memory Card Performance And Standards
We’re dealing with full-sized SD flash memory cards in this piece. These measure 32 x 24 x 2.1 mm, and they currently store up to 128 GB of data. There are many vendors and various speed classifications, but they all operate within the SD card memory standards SD 1.1, SD 2.0, or SD 3.0.
You typically find speed ratings like 133x or 300x printed on most of the products or product boxes. These are comparisons to the single-speed data transfer speeds of the good old CD-ROM. A 1x transfer rate is equivalent to 1.2 Mb/s or 150 KB/s.
Effectively, a 400x SD memory card can theoretically reach 400 * 0.15 MB/s, or 60 MB/s.
Don’t be blinded by these numbers though, as they typically reflect peak performance capabilities and do not reflect everyday throughput. To make matters worse, most vendors refer to read performance numbers and hide away the fact that the large majority of SD cards deliver much lower write performance.
The initial SD cards compliant with revision 1.1 of the SD spec were limited to 2 GB, which is why SDHC (SD 2.0) was introduced to enable capacities of up to 32 GB. In addition to capacity, the SD 2.0 specification also provides performance classification to guarantee certain minimum throughput. There are four classes in the spec: Class 2, Class 4, Class 6, and Class 10. The number tells you the minimum transfer speeds in MB/s that can be sustained by the memory card.
We find that the Class ratings are more useful than the CD-ROM-based multiplier rating, as equivalent MB/s throughput is guaranteed and usually even exceeded by many SD cards. Speed ratings are important if you want to use your memory card on devices that require a certain minimum bandwidth, such as HD camcorders and digital SLR cameras. Once you want to store photos in RAW and JPEG mode, you need a fast memory card to support continuous shooting at several pictures per second.
The SD 3.0 (also known as SDXC) specification provides a ceiling of up to 2 TB. Although there are only a few SD memory cards with up 64 GB of capacity available, the SDXC standard also includes the modified UHS-I specification (Ultra HighSpeed I), supporting up to 104 MB/s. This is the successor to the High-Speed Bus I/F (Class 10) and Normal Bus I/F (Class 2, 4, 6), and it is the basis for products that can deliver more serious bandwidth. Be sure that any card reader you choose works with SDXC cards, as you cannot fully exploit SDXC cards with SDHC card readers.
In addition to conventional SD cards, you'll also find miniSD and microSD cards on the market. Those are well-suited to mobile devices like smartphones. They max out at 32 GB and they are electrically compatible with conventional SD cards, meaning that simple adapters allow you to access them. The miniSD form factor measures 20 x 21.5 x 1.4 mm. However, miniSD has never been very popular, and is on its way to disappear soon. The industry is either using SD or microSD.
microSD cards only measure 11 x 15 x 1.0 mm and are even smaller than a GSM cell phone SIM card. These are increasingly popular, as most smartphone designs utilize microSD cards as flexible storage. We currently find microSD cards at up to 32 GB, and it’s only a matter of time until they're also available at higher capacities. However, only few microSD cards deliver performance that is anywhere near the throughput we find on the devices tested in this article.