SD is the abbreviation of Secure Digital and is one standard of memory card. It is widely used in the portable devices , such as digital cameras ,PDA, mobile phones and multimedia players. The technology of SD is based on the MMC format , but the SD card is thicker than MMC . There is higher speed for data transmission ,and also changing the standard frequently . The profiles of most SD cards set the anti-wirte control to avoid writting the data accidently , less of the SD cards supports the technology of DRM. The SD card provides different speeds through the method of 150KB/s of CD-ROM as one times speed to count . Basically , its transmission speed is more 6 times（900KB/s）than the normal CD-ROM , the high-speed SD card can transmit 66x（10 MB/s）and 133x or higher speed.