MMC (Multi Media Card) VS SD (Secure Digital Card)

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I am a Memory card seller.  I've been in this business for around 1 years.  The common questions from my buyers are: What is the different between MMC and SD card?  Especially when their device are MP3s or PDAs which can accomodate both memory cards. So here it is the ANSWER of that question!

If you read the literature available on the web, you'll find that there are primarily 5 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, (the fastest one, the MMC High Speed. can transfer data almost 6MB per second.) but still, SD can transfer data up to 10MB per second. These are maximum speeds and will depend on the device utilizing the card. The Palm does not approach these speeds for tasks such as writing data to the card, but it is possible to see a performance difference.



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.  I have heard no complaints about the durability of MMC and mine has worked without any problems. This does not seem to be a compelling reason to purchase SD, unless perhaps you live in a climate prone to static discharge.



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, the Palm (and other devices) will not be able to write to the card, erase data from the card, etc.  I must admit that I do not see this as particularly useful, but if you are concerned about important data being overwritten, this might be an appealing feature to you.


Copywrite 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 form a card (e.g., a card with games on it) and distributing it to other people. Unless you are planning to sell cards with data on them, this is of no direct benefit to the purchaser.  It appears that this copyright protection may actually consume a 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, from what I can tell so far.  This is certainly not a bonus for the consumer, and may actually be a problem.


Another questions that arises frequently is due to the discrepancy between the cards "claimed" capacity and what you see when you view it through your Palm (or a card reader connected to your computer). For example, if you buy a 64MB card and look at the "Card Data" application on your Palm, you will see that there is only 60.9MB of space on the card.

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 (or the Palm OS) 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, which is why the screenshot of the 16MB card shows only 14MB capacity instead of 15MB.

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


I hope this guide can help you choosing the right memory card for your device.  Many thanks, Reny


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