Secure Digital vs. Multimedia Cards

n1ywb asks: "I recently cajoled myself into buying a Dell Axim. Since the compact flash slot is obviously taken up by my 802.11b card, that leaves me with the OTHER slot for adding additional storage. This other slot is billed as a 'Secure Digital Card' slot, although I understand it is backwards compatable with the 'Multimedia Card' standard. The name 'Secure Digital' is somewhat misleading it seems. It has some kind of digital rights management technology onboard, which nobody seems to want to elaborate on. It has hardware encryption, which sandisk.com touts as 'Cryptographic security for copyrighted data based on proven security concepts from DVD audio.' Hah! DeCSS anyone? Magic markers? There isn't a lot of REAL information about SD cards out there. I like cheap and fast storage, but I'm paranoid of DRM. _I_ am god here; my hardware is slave to me. I don't want my PDA telling me I can't play my Grateful Dead bootlegs because they aren't digitally signed. Should I buy MMC or SD? Where can I find more info? Any real world experiences? What do you think is the bottom line?"

"Here are a couple of other choice quotes from the Sandisk page:

'...included are unique and proprietary security features implemented within the card's controller which facilitate the secure exchange of content between host devices and the card.'

'The SD Card is designed to comply with all three levels of SDMI security requirements. Both non-protected (category 1) and copy protected (categories 2 & 3) material can be stored on the card. The copy protected material can be secured either by a unique card bound identification (category 2) or by an active cryptography algorithm (category 3), that involves challenge/response protocols against a private key. The SD Card security features also have the capability to revoke non-compliant SDMI components using a Media Key Block if security is breached. Under this security scheme, a content provider's data (music, books, software applications, maps, schedules, etc.) can be Checked-In (moved to the card), Checked-Out (moved from the card) or copied to other SDMI compliant cards with necessary copy restriction. The Check-In/Check-Out feature gives the content owners the confidence that their content is protected.'

I read somewhere (trying to find the link) that the DRM technology in the card consists of a chip similar to what would be found in smart cards. It can limit, for example, the number of times a file can be copied off the card.

SD does have some real technical advantages over MMC. It is much faster, up to 10 megabytes per second. The cards have a physical write protect switch, and they have a very high ESD tolerance of +/- 10K volts for contact discharge and +/- 15K volts for air discharge. They also seem to be slightly cheaper now."

Comments

There's not that much difference...

[Gordonjcp]

SD is used by certain hardware mp3 players as part of the DRM stuff. My girlfriend's Panasonic player uses SD and a specially modified copy of Realjukebox to send files to the card. If you're not using it for "secure" stuff, it just acts like an ordinary MMC card. I've mounted it in a card reader under Linux to download digital camera photos, and used it as a handy way to transfer files. There's no problem with it at all.

You can use SD for both, but it tends to be a little more expensive (presumably because of the crypto deelie). At least, that's in my experience, using Windows 98 and Linux. Your mileage may vary, especially if you're using a newer version of Windows.

Speed and Brand

[Ben Jackson]

SD is faster (4x?) than MMC, in theory. But make sure the slot in your device can take advantage of that -- the Zarus SL-5x00 cannot.

Some devices only support memory (not SD accessories like bluetooth).

Stranger still, some support only MMC. The Franklin e-Bookman (a palm-class device recently on sale for $50 at costco!) manual claims to support only MMC. (and an aside, the MMC storage in the e-Bookman is not general purpose -- only certain types of data can be put on the card rather than RAM)

For whatever reason, some devices support a limited range of flash sizes. The e-Bookman is limited to 64M. The Zaurus requires a software upgrade to use 256M cards.

And finally, make sure you can easily return/exchange your card. The Zaurus, for example, can mount and read any brand of card. But a problem (driver bug? hardware bug? card bug?) causes some to become inaccessible after a suspend/resume event. You can find tables of user-tested cards online, but I wouldn't necessarily risk postage or a restocking fee on their accuracy.