SDHC is SD with High Capacity

SDHC® is the new designation for Secure Digital cards exceeding 2GB in size. The HC series adheres to the SD Specification Version 2 which allows the cards to reach 32GB in size.

Aside from the larger storage capacity, SDHC cards offer optimized recording performance with support for FAT32 file formats. The SD Association has defined 3 speed classes in order to identify speed and performance capabilities of the SD/SDHC cards and the host products. The SD speed Class Ratings specify the minimum sustained write speed for SDHC cards, as follows:

Class 2 : 2 MB/s
Class 4 : 4 MB/s
Class 6 : 6 MB/s*

SDHC-compliant cards have both the SDHC and Speed Class logo in their packaging.



Though SDHC cards are identical in size with the standard SD card, only SDHC-compatible products will be able to accept the newer SDHC cards. SDHC host products can use both SD and SDHC memory cards, while SD host products can use only SD memory cards of 2GB or lesser size.

*1 megabyte (MB) = 1 million bytes
Note: SDHC, the SDHC logo and SD are trademarks

SD Ver 1.1 : The faster SD

Whereas the SD 1.01 standard specifies a maximum clock rate of 25 MHz, the newer SD 1.1 version doubles the clock rate to 50 MHz. Using 4-bit mode data transfer, this increases the theoretical data transfer rate of the newer SDs to 25MBps or 166X.

There are sites which state that the SD 1.1 standard allows for an optional 8-bit data bus width. If this is so, this can only be possible by adding more pin connectors like what was done with the 13-pin MMCplus (original MMC had only 7). The current 9-pin SD bus currently uses 4 pins for data, 3 for power/ground and a line each for clock and command/response signals.

As of this writing, the fastest SD card available is 133X~150X and comes in a 9-pin package.

Flash Card Speed Rating - The X-factor

You may have noticed that some flash cards, notably Compact Flash and Secure Digital, have X-ratings emblazoned on their packaging. Just like in the optical drive industry, the X-ratings give a measure of how fast the device can handle data.

So how fast really is a 133x SD card? A 1x speed rating corresponds to a transfer rate of 150 kilobytes (KB) per second, so a flash card rated at 133x can attain speeds of around 20 megabytes (MB) per second.

Some vendors use different names for their high-speed flash cards. Kingston labels their higher speed SD as Elite Pro SD (45x-50x) and SD Ultimate (120x-133x). On the other hand, Sandisk has their Ultra II SD (10MBps read/9MBps write) and Extreme III series (20MBps).

Now, tell me, what is the equivalent X-rating of a Sandisk Ultra II SD?