What do SDHC memory card class designations mean?
April 3rd, 2008 by Scot Drew
You may have noticed that SDHC memory cards are available in different speed classes (2, 4 and 6). Each class designation directly reflects the minimum sustained Data Transfer Rate (DTR) of the card.
- Class 2: minimum DTR of 2 MB/second
- Class 4: minimum DTR of 4 MB/second
- Class 6: minimum DTR of 6 MB/second
These cards are the perfect solution for today’s SDHC-compatible devices that run MPEG-2, MPEG-4 and other high-quality video compression used by HD and DVD video.
May 6th, 2008 at 10:08 am
[…] the video. Now maybe there is some user error involved but my experience tells me the problem is in the speed of the card as my old Lexar platinum card worked fine for […]
June 26th, 2008 at 12:59 pm
Sandisk sells the Class 4 “featuring fast 15MB/sec Read/Write speeds”. So which is it, 4MB/sec or 15MB/sec? Shouldn’t this be a class 15? or at least a class 6?
SanDisk Ultra® II SDHC 32GB High Performance Card
SDSDRH-032G-A11
High speed card featuring fast 15MB/sec Read/Write speeds
http://www.sandisk.com/Products/ProductInfo.aspx?ID=2582
June 27th, 2008 at 3:08 pm
Based on information provided by Wikipedia the speed class rating is the minimum write speed. Sandisk is claiming up to 15MB write speed.
The SD Speed Class Ratings specify the following minimum write speeds based on “the best fragmented state where no memory unit is occupied”:
 Class 2: 2 MB/s
 Class 4: 4 MB/s
 Class 6: 6 MB/s
SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. One critical difference between the Speed Class and the maximum speed ratings is the ability of the host device to query the SD card for the speed class and determine the best location to store data that meets the performance required.
“Maximum speed” ratings are unofficial and have no formal evaluation process.
Source: Wikipedia http://en.wikipedia.org/wiki/Secure_Digital_card
Hope this helps.