What is Class Speed Rating?

You may have heard or seen speed ratings starting to appear on SD, Micro SD and Mini SD memory cards, these ratings are split into three categories as defined by the Secure Digital Association (SDA). These categories were introduced to make it easier for the consumer to identify the minimum speed capabilities of their SD cards. The three categories are defined in the below table:

		Transfer speed is a minimum of 2MB/sec.
		Transfer speed is a minimum of 4MB/sec.
		Transfer speed is a minimum of 6MB/sec.

Previously different manufacturers have indicated data transfer speeds in various ways and there has been no standard. The existing speed ratings in use state the memory cards maximum Data Transfer Rate (DTS) which may vary, the new SDA version 2.0 speed rating states a guarantee minimum sustained DTS.

The new SDA class ratings will be adopted by all memory card manufacturers which will make life a lot easier when comparing different brands. Speed classes logos will appear near the SD cards logo as shown on the below Sandisk SDHC 8GB memory card:

As technology moves forward devices such as Digital Cameras and Digital Video Recorders are requiring higher specification memory cards. Manufacturers of these host devices are now starting to stipulate the appropriate speed rating class to achieve the best performance. Having this information readily available and in an easy to understand format is definitely a great step forward and will save us all a lot of time researching what is the best memory card for my Camera!

Written by msowa at 14/08/2008 17:58:00

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Comments made about this article

Written By John on 27/08/2008 16:27:00

Very helpful notes. If you have the expertise, it would also be good to have the class ratings linked to likely needs. I have a 10MP camera that uses Sd cards and higher rated cards seem to make no difference at all. I am going to try a HD camcorder that records to SD cards, but apart from the manufacturer wanting me to buy their own brand, I don't know what spec of card is best. If you know the data transfer rate such a camcorder needs, that would be a help.

There is a second concern that may just be marketing spin. Sandisk do an SD card called 'Video', which is class 4. Is it any different from any other class 4, including Sandisk's own Ultra?

Thanks John Reed

Written By pderbyshire on 28/08/2008 14:10:00

Not all devices can take advantage of higher speed rated cards. High performance cards such as the Sandisk extreme IV and extreme III range of memory cards are designed for the professional or high end amateur DSLR camera. These digital cameras specifically benefit from high performance read/write speeds when shooting in RAW format or high res JPEG format. As these formats produce larger files a high speed card allows these files to be saved at a faster rate which in turn will increase your continuous shooting capabilities.

In regards to Digital Camcorders a high speed card will definitely be of advantage as data is continually being written to the memory card. Once again it would be advisable to contact the manufacturer as to what is the best suited speed class rating card for your device. As a general rule we would always recommend a class 4 speed rating as a minimum for digital Camcorders.

When transferring data from your memory card to your PC the benefits of a high speed memory card will be in the time it takes to transfer data. Obviously large amounts of data will take less time but, it is important to note that this will also depend on the Card Reader and the connection that you are using. For example to take advantage of the full 40MB/sec transfer speed of Sandisks CF IV cards you will need to use a Sandisk Extreme Firewire reader.