Speeds
There are different speed grades available, measured the same as CD-ROMs, in multiples of 150 kB/s (1x = 150 kB/s). Basic cards transfer data up to six times (6x) the data rate of the standard CD-ROM speed (900 kB/s vs. 150 kB/s).
Note that maximum read speed and maximum write speed may be different. Maximum write speed typically is lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. This requires a certain sustained speed, or the video stops recording. For recording, a high maximum speed with a low sustained speed is no better than a low speed card. The 2.0 specification defines speeds up to 200x.
Some manufacturers use the read speed in their X-ratings, while others (Kingston, for example) use write speed.[7]
This table lists common ratings and minimum transfer rates.
Rating Write Speed (MByte/s) SD Class
6x 0.9 n/a
10x 1.5 n/a
13x 2.0 2
26x 4.0 4
32x 4.8 5
40x 6.0 6
66x 10.0 10
100x 15.0 15
133x 20.0 20
150x 22.5 22
200x 30.0 30
266x 40.0 40
300x 45.0 45
[edit] SD Speed Class Ratings
SD Cards and SDHC Cards have Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum write speeds based on "the best fragmented state where no memory unit is occupied":[8]
* Class 2: 2 MByte/s - 13x
* Class 4: 4 MByte/s - 26x
* Class 6: 6 MByte/s - 40x
SD and SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. Important differences between the Speed Class and the traditional "X" speed ratings are; 1) 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, 2) class speed defines the minimum transfer speed. Even though the class ratings are defined by a governing body, like "X" speed ratings, class speed ratings are quoted by the manufacturers but unverified by any independent evaluation process.
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".[9] Toshiba also announced cards based on the new 3.0 spec[10] As of December 2009[update], the SD Association's Web site does not include information on this new class or new specification.
I use QMemory 150x SD cards from flashmemorystore.com. They work good for me. I just bought two Transcend 8gb class 6 SDHC cards but haven't used them yet.
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