Different Classes and Speeds of SD Cards

SD Card : SD Card Stands for Secure Digital (SD) Card. it is a non-volatile memory

Types of SD Card : 

1. Secure Digital Standard-Capacity (SDSC).
2. Secure Digital  High-Capacity (SDHC).
3. Secure Digital eXtended-Capacity (SDXC).
4. Secure Digital Input Output (SDIO)

Physical Sizes of SD Card : The SD Card specification defines three physical sizes.

• Standard Size
• Mini Size
• Micro Size 

Class of Memory Card : It is divided based on the size of the memory card

• SD Standard Card  : 2 GB and below capacity memory cards.cards are the slowest and oldest in the range with a typical speed of 12.5MB/s. 

• SD High Capacity : 4 GB above and 32 GB below capacity memory cards cards have a range from 4GB to 32GB. These cards will only have a maximum read/write speed of 45MB/s.

SDXC (Secure Digital Xtended Capacity) : cards are the highest memory capacity and are typically made in sizes larger than 32GB. I imagine the highest size in this type will be 2TB before we see the next series of cards. Sandisk SDXC cards come in two ranges, Sandisk Extreme Pro and Sandisk Extreme. Their write speeds currently have a maximum of 95MB/s.

If the SD card is 4GB or above, it is classed as a SDHC card. Currently, SDHC specifications allow for memory cards of a capacity between 4GB and 32GB. SDA (SD Card Association) has also established a Speed Class rating (Class 2, 4, 6 and 10) for the standardisation of data transfer speeds (compulsory for SDHC cards).

Different Classes of SDHC Memory Cards :

CLASS 2 -   a guaranteed minimum write speed of 2MB/s or higher*

CLASS 4 -   a guaranteed minimum write speed of 4MB/s or higher*

CLASS 6 -   a guaranteed minimum write speed of 6MB/s or higher*

CLASS 10 - a guaranteed minimum write speed of 10MB/s or higher*

Note : All Class 2, Class 4, Class 6 and Class 10 SDHC cards are compatible with SDHC devices. Please note that if the device manual suggests a SDHC card of speed Class 4 as a minimum, the user should install a Class 4, Class 6 or Class 10 (faster) SDHC, NOT a Class 2 card as this may reduce performance of the device. 
Standard SD Cards will work in SDHC compatible devices such as Memory Card Readers. In a SDHC camera a standard SD may be of limited use due to the slower read and write speed of this older technology.
 SDIO : secure digital cards (sometimes called flash cards) have become the industry standard for portable memory in cell phones, laptops, digital cameras and other devices. But SD cards can do more than store and recall data. 
Secure digitial input/output (SDIO) cards can actually expand the functionality of your device. SDIO cards haven’t been widely adapted yet, but as electronic devices become more portable and this standard takes hold, we may see SD cards transforming our computers and smartphones in a big way. 

NARESH CHINNAMMAGARI