Specifications

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The SD card has become the most common flash memory card format for electronic products. As
a flash card, however, it still has the limitation typical of all flash cards—the need for its own
particular card slot in order to be read or be written. USB flash devices owe their enormous
popularity to the “universality” of the USB port that is standardized throughout the computer and
electronics industry. In order to take advantage of the widespread use of USB ports, the Memorex
SD/USB 2-in1 Interface Card has a patented design that allows users to use either SD slots or
USB ports. The card works as a standard SD card in SD card slots and card readers. It can also
turn into a fast USB 2.0 memory device when the USB interface is extended from the card (Figure
13). The Dual Interface design allows this particular card to function as both a “travel drive” and as
a standard SD card for the most practicality possible.
Although 2GB seems to be reasonable capacity (capacity equal to almost 1400 double-sided
floppy disks), the developers of SD cards realize more capacity is always welcome. They have
introduced SDHC format (Secure Digital High Capacity) according to a new SD 2.0 standard. SD
and microSD cards greater than 2GB in capacity will fall into the SDHC class with a capacity range
of 4GB to 32GB. Unfortunately, these SDHC and microSDHC cards are not compatible with
standard SD and microSD cards and devices. However, SDHC readers and devices are
compatible with SD cards as well as the SDHC cards. These new cards have speed classes
associated with them. Unlike standard SD cards that had speeds specified as a maximum limit,
SDHC cards get their classification from a minimum sustained write speed limit.
SD specification 1.01 maximum read/write speed of 10 MB/second 66X*
1.1 20 MB/second 133X*
SDHC Class 2 minimum read/write speed of 2 MB/second 13X*
Class 4 4 MB/second 27X*
Class 6 6 MB/second 40X*
*The speed rating is based on a 1X speed of 150 kB/second, the transfer rate of an audio CD and using a FAT32 file
format. The multiplication factors use decimal calculations rather than binary calculations. That means that 1,000 kB =
1MB rather than 0.976 MB.
The speed of SD, SDHC, and other flash cards has not been a significant consideration until their
capacities became large enough for them to be used for storing video. Digital cameras that allow
continuous shooting still slow things down a bit with their internal buffers and digital processing, but
their ability to shoot good quality video at 30 frames per second puts some significant demands on
the speed of the flash card to keep up. One digital camcorder is capable of 30 fps high definition
recording at a speed of 4.375 MB/second, so that is some indication of the maximum speed
required for recording good quality standard video onto a flash card in a digital camera or
camcorder.
Figure 13