Flash Memory Cards

What is SD/MMC/RS-MMC/MMC Micro/miniSD/MicroSD?

Secure Digital (SD) media is a very common format that has grown out of the MultiMedia Card (MMC) standard. MMC dates back to 1997 and describes a 24x32x1.5mm storage card of varying capacities. MMC cards have largely been superseded by SD equivalents, although some Nokia mobile phone devices still use MMC cards. SD cards measure 32x24x2.1mm, making them slightly thicker than MMC cards, but as the two devices are identically configured, any SD-compatible device can read and write to MMC cards. Owing to the increased thickness of SD cards, the reverse isn't true. SD and MMC cards are supported by a wide variety of digital camera makers, as well as manufacturers of portable media players and PDAs, including Kodak, Canon and Samsung.

SD cards are capable of storing no more than 2GB, a redesign of the SD card's internal workings is required to make it possible to store 4GB and higher of data, and thus SDHC (Secure Digital High Capacity) was created. SD and SDHC are identical in physical size and shape. Previously, SDHC-compatible products will be able to accept both the new SDHC and SD cards. Unfortunately, due to the way the host device reads the card, standard SD devices are unable to read SDHC cards.

There are SD cards on the market that are capable of storing 4GB of data, but beware, if the SDHC logo is not present, these cards are not SD 2.00 compliant. If these cards are used to record data in a SDHC device and then used in a SD device, problems such as data corruption and even loss of data are likely to occur. The safest option is to stick to SD 2.00 compliant cards.

For more information, Sandisk also provides a compatibility guide on http://www.sandisk.com/SDHC

The secure part of Secure Digital comes from the included but virtually never activated encryption hardware within an SD card. It's the equivalent of Sony's MagicGate technology, and about as welcome to the average consumer.

SD Card

Just as Sony has several variants of the Memory Stick, there are smaller mobile-phone centric variants of both the MMC and SD standard. On the MMC side, there is the Reduced-Size Multimedia Card (RS-MMC), which at 24x16x1.5mm is essentially a half-size MMC card. The interesting factor with RS-MMC is that it's the same width as a regular SD or MMC card, and can thus be read (with an adapter) in any SD or MMC slot. RS-MMC is manufactured by SanDisk, while Samsung has another reduced size MMC variant, known as MMC-micro. MMC Micro cards measure in at 12x14x1.1mm, making them smaller than RS-MMC, but without the cross-compatibility of that format.

Not to be outdone in confusing consumers, there are also two small SD card formats to ponder. The MiniSD card format calls for a card 20x21.5x1.4mm, and was introduced as a mobile media standard in 2003. One of these standards is seemingly never enough, and the SD Association announced an even smaller standard, MicroSD, in 2005. MicroSD holds the distinction of being the smallest portable media format, narrowly edging out Sony's Memory Stick Micro. MicroSD cards are tiny -- just 11x15x1mm, and while it's a format that has been accepted by the SD Association, it was originally solely developed by SanDisk under the TransFlash moniker.

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