SD Card Speed Class Rating, Details and Specs
An SD card’s speed is measured by how quickly information can be read from, or written to, the card. In applications that require sustained write throughput, such as video recording, the device might not perform satisfactorily if the SD card’s class rating falls below a particular speed. For example, a camcorder built for a Class 6 card may suffer dropouts or corrupted video if a slower card is used. Digital cameras may experience a noticeable lag between shots, while the camera writes the picture to a slower card.
A card’s speed depends on many factors, such as the following:
- The likelihood of soft errors that the card’s controller must re-try
- The fact that, on most cards, writing data requires the controller to read and erase a larger region, then rewrite that entire region with the desired part changed
- The possibility of fragmentation: that a body of information the host views as a unit is, for historical reasons, written to non-contiguous regions of memory. (This possibility does not cause rotational or head-movement delays as with magnetic media, but it does vary the amount of computation the card’s controller must do.)
In early SD cards, the speed was measured with the “×” rating, which compared the average speed of reading data to that of the original CD-ROM drive. Currently, the official unit of measurement is the Speed Class Rating, which guarantees a minimum rate at which data can be written to the card.
The newer families of SD card improve card speed by increasing the bus rate (the frequency of the clock signal that strobes information into and out of the card). Whatever the bus rate, the card can signal to the host that it is “busy” until a read or a write operation is complete. Compliance with a higher speed rating is a guarantee that the card limits its use of the “busy” indication.
Speed Class Rating
The Speed Class Rating is the official unit of speed measurement for SD cards. The class number guarantees a minimum write speed as a multiple of 8 Mbit/s (1 MB/s). The SDA defines several speed class ratings, but manufacturers may claim conformance to those ratings without independent verification.
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.
These are the ratings of all currently available cards:
Class Speed Class 2 2 MB/s Class 4 4 MB/s Class 6 6 MB/s Class 10 10 MB/s
Speed Classes 2, 4, and 6 assert that the card supports the respective number of MB/s 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. 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 “×” rating is a multiple of the standard CD-ROM drive speed of 1.2 Mbit/s (approximately 150 kB/s). Basic cards transfer data up to six times (6×) the CD-ROM speed; that is, 7.2 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.
This table lists common ratings, the minimum transfer rates, and the corresponding Speed Class (though the comparison is not always exact).
Rating Read Speed
6× 0.9 10× 1.5 13× 2.0 2.0 2 26× 4.0 4.0 4 32× 4.8 5.0 5 40× 6.0 6.0 6 66× 10.0 10.0 10 100× 15.0 15.0 133× 20.0 20.0 150× 22.5 22.5 200× 30.0 30.0 266× 40.0 40.0 300× 45.0 45.0 400× 60.0 60.0 600× 90.0 90.0
UHS Speed Class
The Ultra-High Speed (UHS) interface is available on some SDHC and SDXC cards. The following ultra-high speeds are specified:
- UHS-I cards, specified in SD Version 3.01, support a clock frequency of 100 MHz (a quadrupling of the original Default Speed), which in four-bit transfer mode could transfer 50 MB/s. UHS-I cards declared as UHS104 also support a clock frequency of 208 MHz, which could transfer 104 MB/s. UHS-I is the only class for which products are currently available.
- Double data rate operation at 50 MHz (DDR50) is also specified in Version 3.01, and is mandatory for microSDHC and microSDXC cards labeled as UHS-I. In this mode, four bits are transferred when the clock signal rises and another four bits when it falls, transferring an entire byte on each full clock cycle.
- UHS-II cards, to be defined in Version 4.0, further raise the data transfer rate to a theoretical maximum of 312 MB/s.
UHS memory cards work best with UHS host devices. The combination lets the user record HD resolution videos to tapeless camcorders while performing other functions. It is also suitable for real-time broadcasts and capturing large HD videos.
Cards that comply with UHS show UHS-I or UHS-II on the label, and report this capability to the host device. Use of UHS requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation and select the 4-bit transfer mode.