Secure Digital Memory Card (SD) is a portable flash memory card used in digital cameras, mobile phones, etc.
SD was developed in 2001 by SanDisk on the basis of MMC. Its size is 24x32x2.1 mm.
The card is supplied with its own controller and a special area able, in contrast to MMC, record data in a way that prevents unauthorized reading, in compliance with the “Secure Digital Music Initiative” requirements, which is indicated in its name – “Secure Digital”.
SD uses a special inscription protocol which is not accessible to ordinary users.
In most cases SD can be substituted by MMC. Back substitution is usually impossible as SD is thicker and may not fit into the slot for MMC.
The memory capacity may be 8 MB, 16 MB, 32 MB, 64 MB, 128 MB, 256 MB, 512 MB, 1 GB, 2GB, 4 GB and 8 GB. Cards with 16 GB memory have been developed. The transfer rate, as in case with the CD-ROM, is set by the multiplier number. The simplest cards have the speed 6 (900 KB/sec), the newest ones: 150X (22500 KB/sec).
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