The 3 form factors are: the original size, the "mini" size, and the "micro" size (see illustration). There are many combinations of form factors and device families.
Speed Class Rating
The SD Association defines standard speed classes indicating minimum performance to record video. Both read and write speeds must exceed the specified value. These are defined in terms of suitability for different applications:
- 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, for both read and write, on an empty card:
Class Minimum performance Class 2 2 MB/sec Class 4 4 MB/sec Class 6 6 MB/sec 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. 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.
Speed Class designates minimum writing performance to record video. The Speed Classes defined by the SD Association are Class 2, 4, 6 and 10.
UHS Speed Class is designed for UHS** products only.
* (Speed Class and the UHS Speed Class are not compatible.)
**UHS (Ultra High Speed), the fastest performance category available today, defines bus-interface speeds up to 312 Megabytes per second for greater device performance. It is available on SDXC and SDHC memory cards and devices.
SD Speed Compatibility
It’s important to remember that Speed Class and UHS Speed Class are two different speed indication symbols for different devices. However, a UHS-I memory card may also indicate a Speed Class.
The host device Speed Class requirements are minimum requirements for optimum performance. If your host device requires a Speed Class 4 SD memory card, you can use Speed Class 4, 6 or 10 SD memory cards. If your host device requires a Speed Class 6 SD memory card, you can use Speed Class 6 or 10 SD memory cards.
For the sheer variety of high-performing, feature-rich devices, the SD Association introduced higher speed bus interface (I/F) and its minimum writing speed specifications for SDHC memory cards and SDXC memory cards.
|Bus Interface||Card Type||Bus Mark||Bus Speed||Spec Version|
|Normal Speed||SD, SDHC and SDXC||---||12.5MB/s||1.01|
|High Speed||SD, SDHC and SDXC||---||25MB/s||2.00|
|UHS-I||SDHC and SDXC||50MB/s (SDR50, DDR50)|
|UHS-II||SDHC and SDXC||156MB/s|
*Maximum speed differs from the bus I/F speed. It varies depending upon the card performance. The average speed that a device writes to an SD memory card may vary depending upon the device and the operation it is performing. The speed may also depend on how other data is stored on the SD memory card.
Normal and high-speed cards can also be used with UHS-I host devices, but the high performance enabled by a UHS-I host device can only be achieved with a UHS-I memory card.
Difference between Speed Class, UHS Speed Class,
and Speed Ratings (performance) for SD/SDHC/SDXC cards
The speed rating measures maximum transfer speed for reading and writing images to and from a memory card, expressed as megabytes per second. However, video doesn't need as big a data pipe because the video format is a smaller "fixed stream" that uses only a portion of the data pipe.
SanDisk products transfer speed (from SunDisk Outlet)
SanDisk has THREE main lines for consumer products. Standard, Ultra, and Extreme
The standard line of SanDisk products are entry level storage devices. This product line includes most USB drives, Blue labeled SD/SDHC, Memory Stick Pro Duo (MSPD), CompactFlash cards and all microSD/microSDHC cards (excluding the Extreme and Ultra line). This product line does NOT have any defined speed specification and will generally be a slower transfer speed product.
This product line is sufficient for shuttling small files from one computer to another, storage, and low end entry level cameras or phones.
The Ultra line of products is a step up from entry level products. This product line includes SD/SDHC/SDXC cards, microSD/microSDHC/microSDXC cards, Memory Stick Pro Duo (MSPD) and CompactFlash cards. Ultra products have moderate transfer speeds and are suitable for shuttling many files from one computer to another and mid level cameras or phones. There several revisions of Ultra line products. The transfer speeds can range from 10MB/s to 30MB/s.
Extreme and Extreme Pro Line
The Extreme lines are professional grade products. The products include all SanDisk products labeled Extreme or Extreme Pro. Extreme and Extreme Pro cards are used by those who demand the highest performance from their products. Extreme products are suitable for shuttling large files such as High Definition videos from one computer to another and High end professional DLSR or HD Video cameras. There several revisions of Extreme/Extreme Pro line products. The transfer speeds can range from 20MB/s to 100MB/s.
Other factors that can affect performance
The transfer speed of a storage product can greatly be affected by the host device. If a host device such as a camera can only achieve for example a 20MB/s write speed, placing a card with a 90MB/s write speed in the camera will NOT improve the performance of the camera. The card WILL be limited to the write speed provided by the host. When shopping for a storage device, it is best to look at the performance specifications of the host and choose an equivalent storage device.
Host limitations are commonly seen with low end card readers such as internal or built in card readers on laptop and desktop computers. To achieve the best performance, a high speed external USB 2.0, USB 3.0 or ExpressCard reader is recommended.
PC Environment, Filesize and Type, and Transfer Type
The PC environment or host type can affect transfer speed. For best performance external high speed card readers are recommended.
Filesize and Type
Large files are transferred faster than many smaller files. This is due to the overhead created by transferring multiple small files.
Transfer Type: Sequential vs. Random
The defined transfer speed of a product is referring to the maximum Sequential transfer speed of the product and is calculated using benchmark programs. Sequential transfer is one continuous write or read. Random write and read speeds will be lower than sequential.
USB Flash Drives
Most of the SanDisk USB flash drives fall under the entry level category unless otherwise labeled as Ultra or Extreme. Entry level USB drives do NOT have any defined transfer speed. The Ultra and Extreme USB flash drives MAY vary from 10MB/s to 25MB/s depending on the specific product.
The standard (Blue labeled) line has a minimum transfer speed as defined by the Class Specification printed on the label.
For example, a Class 2 SD/SDHC card will have at least a read/write speed of 2MB/s. Class 4 SD/SDHC card will have a minimum transfer speed of 4 MB/s.
The transfer speed of the Ultra, Extreme and Extreme Pro SD/SDHC/SDXC cards are listed on the card and the packaging. Depending on when the product was manufactured, the Ultra cards speed can range from 10MB/s to 30MB/s. The Extreme and Extreme Pro cards speed can range from 20MB/s to 30MB/s. The defined transfer speed is calculated using benchmark applications and refers to maximum sequential transfer speed.
The standard line CompactFlash cards do NOT have any defined transfer speed. These are entry level cards and will generally be slower cards suitable for low end entry level cameras.
The speed of the Ultra, Extreme, and Extreme Pro CompactFlash cards are listed on the card and the packaging. Depending on when the product was manufactured, the Ultra cards speed can range from 10MB/s to 60MB/s. The Extreme and Extreme Pro cards speed can range from 20MB/s to 100MB/s. The defined transfer speed is calculated using benchmark applications and refers to maximum sequential transfer speed.
Unlike card write speeds that measure maximum performance, class ratings measure the minimum sustained speed required for recording an even rate of video onto the card. The class rating number corresponds to the transfer rate measured in megabytes per second. Class 2 cards are designed for a minimum sustained transfer rate of 2 megabytes per second (MB/s)1, while Class 10 cards are designed for a minimum sustained transfer rate of 10MB/s2.