SD Class Break Down
SD cards come in all sorts of GB sizes and speeds.Â Today I thought it a good idea to take a look inside an SD card along with breaking out the speed differences.
To start, the SD media is broken down into “Classes”Â The Class depicts the speeds at which a device reads and writes.
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).
The 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.
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”:
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:
- Class 2: 2 MByte/s – 13x
- Class 4: 4 MByte/s – 26x
- Class 6: 6 MByte/s – 40x
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 so keep this in mind if the specification is different than real world results.
- 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
- Class speed defines the minimum transfer speed.
Next, lets take a quick break down of the SD card itself.Â In the image at the top of the page you can see the NAND flash memory as the larger chip – the place where information is stored – and the controller chip.Â The smaller chip is the controller chip and controls the flow of information between the flash memory itself and the host.Â What I mean by host is the computer, digital camera or video camera that is trying to access the SD card and save information to it.
Tags: class speed, sd, Secure Digital
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Kicking around in technology since 2002. I like to write about technology products and ideas, but at the consumer level understanding. Some tech, but not too techie.
Posting on Quora.com as well