hmm whats the difference in classes?
recording speed basically...
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 Mb/s (1 MB/s), and meets the least sustained write speeds for a card in a fragmented state.[16] These are the ratings of some currently available cards:[16]
* Class 0 cards do not specify performance, which includes all legacy cards prior to class specifications.
* Class 2, 2 MB/s, slowest for SDHC cards.
* Class 4, 4 MB/s.
* Class 6, 6 MB/s.
* Class 10, 10 MB/s.
Even though the class ratings are defined by a governing body, like × speed ratings, class speed ratings are quoted by the manufacturers and not verified 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. 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:[16]
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.[16]
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".[17] Toshiba also announced cards based on the new 3.0 spec.[18]
On 1 June 2010, Pretec announced the new Class-16 HD-video grade SDXC 64GB card at Computex Taipei 2010.[19]
[edit] × rating
The × rating is equal to 1.2 Mb/s. It is derived from the standard CD-ROM drive speed of 1.2 Mb/s (approximately 150 kB/s). Basic cards transfer data up to six times (6×) the data rate of the standard CD-ROM speed (7.2 Mb/s vs 1.2 Mb/s). The 2.0 specification defines speeds up to 200×, but unlike the class rating system, does not mandate that ×-ratings measure the card's least sustained write-speed. So, typically, manufacturers provide ×-ratings based on maximum read/write speeds. Furthermore, for most cards, the fastest read speed is typically swifter than its fastest write speed, leading some manufacturers to use read-speed as the ×-rating measurement. Other vendors, such as Kingston, use write-speed.[20]
This table lists common ratings, the minimum transfer rates, and the corresponding Speed Class.
Read more at
Secure Digital - Wikipedia, the free encyclopedia 
