Memory cards is something every one of us uses because digital cameras, PDAs, smart-phones are all equipped with one or even several such cards. You can see them in cell phones, players and car stereos, too. Memory cards are also used with ordinary PCs even though the small size of this storage doesn’t seem to be an important factor for them: the low power consumption and shock tolerance make such cards a good replacement to hard disks in industrial PCs, compact servers, notebooks. You may also want to use a memory card with your desktop PC because Windows Vista can utilize flash memory to accelerate disk access (thanks to the relatively low access time of such memory at reading) by means of ReadyBoost technology.
Memory cards have been improving their key characteristics, capacity and data-transfer rate, year by year. But besides the theoretical speed, there is the real one that depends not only on the card’s capabilities but also on the card-reader’s. The reader is built into the device’s electronics in a camera or PDA, but if you want to use the card with your desktop PC, you can choose the reader yourself. It would be wise to do so because cheap readers, based on slow chips, cannot reveal the potential of a modern card fully.
Before proceeding to the devices I am going to test, I will tell you about the SDHC and MMCplus formats that have come to replace SD and MMC, respectively. I’ll mention these formats below in connection with some models of readers, but some users have a rather vague notion of the difference of these formats from their predecessors.
The story of these form-factors (they are thicker than the full-size versions but it is the only difference) began in 1997 with the introduction of the MultiMediaCard by Siemens and SanDisk. As opposed to the older Compact Flash format, which mostly employed NOR flash memory chips, the new standard was based on more compact NAND chips. The resulting card was very small at 24x32x1.4mm. Later on, the Secure Digital standard was developed on that basis. An SD card was 0.3mm thicker but offered data security features (not very demanded by the users, though) and a somewhat higher speed due to the higher interface frequency, 25 or 50MHz as opposed to the MMC’s 20MHz. The further progress was logical enough: the cards were getting larger in terms of capacity, yet retained their compact size. They eventually won a share of the market from the larger Compact Flash format. Their speed has always been somewhat lower in comparison with Compact Flash, but the difference is not too big in practical applications. Reduced-size versions of these cards appeared under the pressure from the manufacturers of compact devices (cell phones in the first place): RS-MMC, SecureMMC, mini-SD, and microSD. RS-MMC is just a twice shorter MMC while the others have differently positioned connectors and can work in SD/MMC-supporting readers by means of an appropriate adapter.
After a while, both formats approached a point when the steadily accumulating problems called for an introduction of serious changes into the design of the cards. For MMC the main problem was the data-transfer speed across the interface, which was too low. For SD, there was a fundamental capacity limit of 2GB resulting from the addressing mechanism originally implemented in the standard. That’s why the attempts to introduce 4GB SD cards were not a success as most devices just wouldn’t work with them. Anyway, both standards solved their problems one way or another. There appeared MMC 4.1, also known as MMCplus, in which the bus frequency was increased to 52MHz while the bus itself was enlarged with two additional contacts. This helped maintain backward compatibility, even though at low speeds (if you insert an MMCplus card into an old card-reader, the latter will work with it as with an ordinary MMC), and achieve excellent speeds, up to 290x, in new card-readers (to remind you, 1x equals 150KB/s as in CD drives). The SD standard developed to version 2.0, better known as Secure Digital High Capacity, in which byte-based addressing was replaced with sector-based addressing. The maximum capacity of a card thus increased to 2TB. Unfortunately, the introduction of the new addressing type doesn’t allow SDHC cards to be read by old, SD-supporting readers.
Summing it up, MMCplus cards can work in any MMC-compatible device but can only show the full speed in new card-readers specially designed for MMCplus. SDHC cards do not work at all in old SD-compatible readers. You should make sure your card-reader (PDA, camera, etc) supports SDHC before purchasing such a card.
Finally, the SD 2.0 specification introduced speed class ratings. Each SDHC card can be class 2, 4 or 6. The number means the minimum write speed the empty card provides. These speeds are 2, 4 and 6MB/s, but the manufacturers can install faster chips for higher write speeds. The speed just cannot be lower than the declared class.
You can go to the MMC and SD websites for more information about these flash card standards. Now that we’re done with the theory, it’s time to get to practice. I mean, to the card-readers to be tested.