Speed Class Rating

The Speed Class Rating is the official unit of speed measurement for SD Cards, defined by the SD Association. The Class number represents a multiple of 8 Mbit/s (1 MB/s), and it measures the minimum sustained write speeds for a card in a fragmented state [11].

The following are the ratings of some currently available cards: [12]

* Class 0 - These cards do not specify performance, which includes all legacy cards prior to class specifications.
* Class 2 - Minimum of 2 MB/s performance. Lowest speed for SDHC cards.
* Class 4 - Minimum of 4 MB/s performance.
* Class 6 - Minimum of 6 MB/s performance.
* Class 10 - Minimum of 10 MB/s performance.

Even though the class ratings are defined by a governing body, like × speed ratings, class speed ratings are quoted by the manufacturers but unverified by any independent evaluation process. In applications that require sustained write throughput, such as video recording, the device may not perform satisfactorily if the SD card's class rating falls below a particular speed. For example, a camcorder that is designed to record to class 6 media may suffer dropouts or corrupted video on slower media.[citation needed] On slower class cards, digital cameras may experience a lag of several seconds between photo-taking, whilst the camera writes the picture to the card.

Important differences between the Speed Class and the traditional CD-ROM drive speed measurement ("×" speed ratings) are that speed class:[13]

1. may be queried by the host device
2. defines the minimum transfer speed.

Since the class rating is readable by devices, they can issue a warning to the user if the inserted card's reported rating falls below the application's minimum requirement[14]. On 21 May 2009, Panasonic announced new class 10 SDHC cards, claiming that this new class is "part of SD Card Specification Ver.3.0".[15] Toshiba also announced cards based on the new 3.0 spec.[16]

On 1 June 2010, Pretec announced the new Class 16 HD video grade SDXC 64GB card at Computex Taipei 2010.[17]