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Gadget Friday

A very quick, 'relay' version of Gadget Friday today. Lots of people have been asking me about what type of Memory Card they should stick with. Its a hard choice to make, as it depends on your personal requirements. However, I thought I'd share with you some basics.

MultiMedia Card (MMC) vs. Secure Digital (SD)

The form factor or "size and shape" of the MMC Card and the SD Card are identical. As such they are generally interchangeable types of flash memory. If your camera or PDA will take SD Cards, then it is a good bet that it will take MMC Cards as well, and vice versa.

As a rule, SD cards are slightly more expensive than MMC cards. The question is, what do you get for the extra money when you purchase SD?

There are 5 main differences between the two: speed, durability, write-protection, copyright protection, and size.

Maximum transfer rates of SD are faster than that of MMC. SD has the capacity of being 4 times as fast as MMC. Whereas MMC can transfer data at 2.5MB per second, SD can transfer data up to 10MB per second. These are maximum speeds and will depend on the device utilizing the card.

The thicker casing and, apparently, changes in the design of the cards means that the SD card is more durable and resistant to electrostatic discharge. Having said this we have had no MMC card failures or complaints from users regarding reliability.

SD includes a small switch on the side of the card that has two settings: Locked and Unlocked. If you set the card to the locked position, devices will not be able to write to the card, erase data from the card, etc.

Copyright Protection
This is the most mysterious difference in the two media. According to the spec sheet provided by SanDisk (one of the consortium that developed the media), SD includes "cryptographic security for protection of copyrighted data." Presumably, this is to prevent people from copying information from a card (e.g., a card with games on it) and distributing it to other people.

It appears that this copyright protection may actually consume several 100kb of space on the card - which you cannot get back. The amount of space lost on a card is higher for SD than for MMC, or so it seems. Not necessarily a bonus for the consumer.

Another question that arises frequently is due to the discrepancy between the cards "claimed" capacity and what you see when you view it through your file manager or card reader connected to your computer. For example, if you buy a 64MB card and look at the free space shown, you will see that there is less space than this available on the card.

Where did the space go?
To understand why your 64MB card has only 60.9MB of space, it is necessary to define what we mean by a MB or megabyte.

The "dictionary" definition of MB, and that which is used by software such as Windows Explorer is that 1 Megabyte is equal to 1,048,576 bytes. This is the case because there are 1,024 bytes in a kilobyte and there are 1,024 kilobytes in a megabyte. Remember that computer capacities are measured in powers of 2. A kilobyte is 2 to the 10th power. A megabyte is 2 to the 20th power.

Manufacturers who produce storage media, however, use a different definition of megabyte: 1 MB = 1 million bytes. It's a simpler definition to understand, but it causes a large discrepancy when you start talking about large numbers of megabytes.

A "64MB card" will hold only 64,000,000 bytes (not 67,108,864 bytes) worth of programs, data, etc. If you divide that by 1024 (the number of bytes in kilobyte), you find that it will hold 62,500 kilobytes. If you divide that number by 1024 (the number of kilobytes in a megabyte), you find that the card will hold only 61MB. The two different definitions "cost" you approximately 4MB on a "64MB card."

It is a confusing concept if you're not used to thinking of bytes, kilobytes and megabytes being determined by powers of 2. What this means, basically, is that if you have a 62MB file (say, a video), it will not fit on a 64MB card.

Note that there is also a chunk of space that is taken by system files, which amounts to around 100k on an MMC card (which is why the 64MB card shows up as 60.9MB; you start with 61MB and subtract about 100k). These system files seem to be much larger (more than 900k) on SD cards.

Also, remember that the discrepancy gets bigger as file sizes get bigger. A 512MB card, will actually hold only about 488MB worth of data, for a loss of 34MB!

The main difference we have found that is of use to our clients is the write protect tab. If you are concerned about accidental erasure of crucial data, SD card could very well be the way to go.

A very good link that has sizes (form factors) of each can be found here.


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