Like HowStuffWorks on Facebook!

How Secure Digital Memory Cards Work

        Tech | Memory

Speed Class and UHC Speed Class

Another measurement you'll see on an SD card is its speed. Speed isn't very important if you're just storing files or taking still photographs. However, speed is very important when it comes to shooting and playing back video, particularly in high-definition (HD). When you have a higher speed SD card, you can capture better quality video with smoother playback.

The SD Association has two standards associated with speed: Speed Class and Ultra High Speed (UHS) Speed Class. These two standards are not compatible, so be sure to use the standard appropriate for your recording device. Let's take a closer look at each class and how to determine what speeds an SD card is capable of.

Speed Class applies to all SD cards. You'll see the Speed Class for an SD card next to its SD logo as a number inside a circular "C" shape. The following are the Speed Classes defined by the SD Association and the cards, as well as the video recording and playback qualities usually associated with those classes:

  • 2 - Standard-definition (SD), maximum speed of 12.5 MB per second
  • 6 or 4 - HD (720p, 1080i, 1080p), maximum speed 25 MB per second
  • 10 - Full HD (1080p), maximum speed of 25 MB per second

The Ultra High Speed (UHS) Speed Class is available in SD cards that feature the UHS-I bus-interface. That means the SD card has circuitry designed to read and write to memory at speeds up to 312 MB per second. A UHS SD card packaging might feature the Speed Class plus an additional class, written as a number inside a "U" shape. On the card itself, though, the UHS Speed Class is denoted by the Roman numeral "I" to the right of the SD logo.

Now that you're up to speed on SD card speeds and capacities, let's see what makes Secure Digital cards "secure."