Speed matters. Especially when you’re capturing live audio and video using your digital camcorder, digital camera or digital audio recorder. As the demands of high definition video and audio increase, flash memory is stepping in as the de-facto solution for those who need high speed memory. High speed SD cards boast fast write times that ensure that no data is lost and that audio and video can be captured with minimal latency.

Choosing reliable, high speed SD cards is an obvious necessity. But the plurality of manufacturers offering SD cards makes it difficult to tell which is the fastest, or at least fast enough. The SD Association addresses this issue by maintaining a number of standards that help consumers choose SD cards based on speed.

Currently, the SD Association recognizes two Speed Classes: “Speed Class” and “UHS Speed Class.” Speed Class applies to SD, miniSD, microSD, SDHC, miniSDHC, microSDHC, SDXC and microSDXC cards. These are denoted by a class number within a C-shaped logo. The higher the class number, the faster the card. In order to bear the SD Speed Class logo, card manufacturers have to adhere to the minimum standards set out by the SD Association.

Class 2 is the slowest SD card speed. Class 2 has a write speed of about 2 Mb/s and is suitable for standard definition video recording. Class 4 and Class 6 have write speeds of about 4 Mb/s and 6 Mb/s, respectively, and can be used for HD video recording. These three classes of SD cards are designed to operate under the normal bus interface.
Class 10 SD cards have write speeds of 10 Mb/s and up and are designed to operate under the high speed bus interface.

Beyond Class 10 is the UHS Speed Class. UHS stands for ultra high speed and includes cards that have speeds up to 312 Mb/s. The UHS speed class logo includes a number contained within a U-shape. Currently, there is only one UHS speed class: UHS Speed Class 1, which is designed for the UHS bus interface. Cards in this class will usually be noted as SDHC UHS-I or SDXC UHS-I.

Note that in order to read/write to a SDHC UHS-I or SDXC UHS-I card, you’ll have to have a device that is capable of using both SDHC/SDXC and UHS-I cards. Note that you can use normal Speed Class cards in a device that supports UHS class cards.

One last thing to consider when choosing an SD card: capacity matters. That’s because for the fastest write speed, the host automatically writes to memory units that are contiguous. However, as you use continue to use your card, the availability of memory units that are next to each other decreases. As such, the host has to skip around to the free memory units. This is called non-linear or fragmented storage, and it can slow down your write speed. Larger capacity cards have less issues with fragmentation.

These tips and definitions should help you understand the world of high speed SD cards. But your best bet is to check the manual that came with your host device to learn which is the best SD media to use for the best performance.