Secure Digital Memory Card

[drew]

Introduction

Much smaller than the older Compactflash standard, Secure Digital cards can be integrated into more compact devices. Dispite this small size, these cards can provide similar performance to many larger formats. As such, Secure Digital has all but suplanted CF cards in the compact camera market and is begining to move into the professional DSLR market.
[edit]
Design

Fig.1 - The most common form of memory card used in compact cameras, Secure Digital is begining to make inroads into the Professional DSLR marketplace.


Secure digital cards can be up to 32mm x 24mm x 2.1mm in size and connect to the host device via a high-speed serial link. This allows them to take up much less space than larger cards, making it practical to use these cards in extremely compact devices.
SD cards typically have a write protect switch (see Fig.1) that allows users to protect the contents of the card when desired.
[edit]
Revisions

As with most memory card formats, Secure Digital has evolved over the years. The original format was limited to transfer rates of 10MB/s or less and capacities of 2GB or smaller. Over time, a number of minor revisions have boosted performance up to 20MB/s. Further, several major revisions added more functionality to the standard.
[edit]
SDIO

The SDIO extension to the standard allows Secure Digital cards with functionality beyond that of simple memory cards. Devices supporting this standard can use the SD interface for WiFi cards, GPS recievers and other compact peripherals. This is not a critical function for photographic purposes (as cameras don't support SDIO), however it can be useful for PDAs and other compact computers.
[edit]
SDHC

A relatively recent addition, SDHC allows Secure Digital cards with capacities of 4GB or larger. In addition to removing the 2GB limit on card sizes, this standard also calls for cards to be classified based on their ability to provide specific sustained transfer rates. Unfortunately it is important to note that SDHC cards are not backwardly compatible with older devices. To accept SDHC cards, the device must be designed for SDHC compliance.
[edit]
DRM

Secure Digital cards were originally designed for use in digital music players and include some basic DRM systems within the cards. As these features are generally not used by digital imaging devices, they are beyond the scope of this article.
[edit]
See Also

• Compactflash - A larger memory card format typically used in professional DSLRs.

[edit]
External Links

• Rob Galbraith's CF/SD Performance Database - Comprehensive database comparing the performance of many different memory cards in both cameras and computers.
• SD Card Association - The body that governs the Secure Digital format and all related standards.
• SD Card Specification - A simplified version of the formal SD Card specification used by vendors to produce compatible devices. The complete version is only available to SDA members.