ShopinJA Classifieds

Speed Class Rating
32GB SDHC card

The SD Association defines standard speed classes indicating minimum performance to record video. These are defined in terms of suitability for different applications:[39]

    Class 2 for SD video recording
    Class 4 and 6 for HD ~ Full HD video recording,
    Class 10 for Full HD video recording and HD still consecutive recording
    UHS Speed Class 1 for real-time broadcasts and large-size HD videos

The specification defines these classes in terms of performance curves which translate into the following minimum performance levels (on an empty card):[40]

            Class 	Minimum performance
            SDHC Speed Class 2.svg Class 2 	2 MB/sec
            SDHC Speed Class 4.svg Class 4 	4 MB/sec
            SDHC Speed Class 6.svg Class 6 	6 MB/sec
            SDHC Speed Class 10.svg Class 10 	10 MB/sec

Speed Classes 2, 4, and 6 assert that the card supports the respective number of MB/sec as a minimum sustained write speed for a card in a fragmented state. Class 10 asserts that the card supports 10 MB/s as a minimum non-fragmented sequential write speed.[40] By comparison, the older "×" rating measured maximum speed under ideal conditions, and was vague as to whether this was read speed or write speed.

The host device can read a card's speed class, unlike the earlier "×" speed ratings. A device can warn the user if the card reports a speed class that falls below an application's minimum need.[40]
“×” rating

The “×” rating is a multiple of the standard CD-ROM drive speed of 150 KB/s (approximately 1.23 mbit/s). Basic cards transfer data up to six times (6×) the CD-ROM speed; that is, 900 KB/s or 7.37 Mbit/s. The 2.0 specification defines speeds up to 200×, but is not as specific as Speed Classes are on how to measure speed. Manufacturers may report best-case speeds and may report the card's fastest read speed, which is typically faster than the write speed. Vendors including Transcend and Kingston report their cards' write speed.[41]

This table shows the approximate relationship between certain “×” ratings and the newer Speed Class designations:

            Rating 	Approx.
            (MB/s) 	Comparable
            Speed Class
            16× 	2.34 	SDHC Speed Class 2.svg
            32× 	4.69 	SDHC Speed Class 4.svg
            48× 	7.03 	SDHC Speed Class 6.svg
            100× 	14.6 	SDHC Speed Class 10.svg