The SD Card Association has created and defined a SDHC Class Rating to help you identify speed and performance capabilities / minimum requirements of SDHC, mini SDHC and micro SDHC cards and the host products.

The SD Speed Class Ratings specify a MINIMUM sustained write speed for SDHC/mini SDHC/micro SDHC cards (Class 2: 2 MB/s; Class 4: 4 MB/s; Class 6: 6 MB/s; Class 10: 10 MB/s; Class 16: 16 MB/s).

Please remember these are MINIMUM write speeds, many cards will write data a lot faster, such as the SanDisk Extreme III and Extreme Pro SDHC Memory cards.

It is with video use that the class ratings come more into play and are a more important factor when purchasing a memory card. Many camcorder manufacturers will designate a recommended "class rating" to ensure optimum video recording.

A new SD format has been announced! The new format will be called SDXC (an abbreviation of Secure Digital eXtended Capacity) which will allow up to 2TB of storage on one card. The New SDXC Card will execute read and write speeds to 104MB/s this year, with the possibility to achieve up to 300MB/s in the future.

The SD association are introducing the new format cards in order to make HD content a lot easier to handle and make large volumes of professional-quality HD video and high resolution pictures easier to capture smoothly.

Also read and write speeds are set to increase on the currently available SD formats (SDHC and Embedded SD) SDXC Cards will turn mobile phones into HD multi-media centres allowing years of photo collections and weeks of HD video to be stored.

The new extended capacity cards will use Microsoft’s exFAT file system as opposed to the currently used FAT16 and FAT32 file systems.

Shigeto Kanda, general manager at Canon said “SDXC is a large-capacity card that can store more than 4,000 RAW images, which is the uncompressed mode professionals use, and 17,000 of the fine-mode most consumers use. That capacity, combined with the exFAT file system, increases movie recording time and reduces starting time to improve photo-capturing opportunities,”

With the introduction of this new format it may not be long before we see 64GB iPhones and flash based MP3 players being released. The 64GB SDXC card is set for release in the first quarter of this year, surpassing the current largest SDHC card which is at 32GB.

SDHC - Secure Digital High Capacity memory cards are flash memory cards with a minimum capacity of 4GB (gigabytes). SDHC cards are a removable storage solution for compatible digital devices including digital cameras, camcorders, MP3 players, PDAs and more.

SDHC memory cards are the same size as an oridnary SD Card - about the size of a postage stamp. 

Secure Digital Cards have been around for nearly 10 years, SDHC Cards are a newer version developed for use with digital cameras and other such devices that not only a require a higher capacity card, but also one that comes with a performance to match. SDHC memory cards are designed for devices that are compatible with the SD 2.00 specification. Products designed exclusively to support previous SD specifications 1.0 and 1.1 will not be able to utilise SDHC cards. To check if your device is compatible with SDHC look in your device manual or on the packaging for the SDHC symbol below:

Remember, if your device accepts SDHC cards, it is backwards compatible with standard Secure Digital (SD) cards.

The growing demand for high-capacity flash memory springs partially from the increasing use of high-definition video and high-resolution digital photography. SDHC memory cards meet the challenge of these demanding products not only by providing ample storage but also by introducing a new feature - Classification of Data Transfer Speed (DTS).

Consumers can get the best performance value out of their digital products by using flash memory cards that support the device’s highest standards for data transfer speed. The SD specification 2.00 calls for cards to be classified according to the minimal sustained DTS as follows:

* Class 2: minimum sustained DTS of 2MB/sec (13x Speed)
* Class 4: minimum sustained DTS of 4MB/sec (26x Speed)
* Class 6: minimum sustained DTS of 6MB/sec (40x Speed)
* Class 10: minimum sustained DTS of 10MB/sec (68x Speed)
* Class 16: minimum sustained DTS of 16MB/sec (109x Speed)

The issue with regular SD memory cards and other cards for that matter, is that if the card has a write speed of say 6MB/Sec or 40x Speed, then it’s maximum speed is that. So the device (eg. camera) will start writing the image or data to the card at 0MB/sec and then work up to the top operating speed of 6MB/sec and then slow down again to 0MB/sec as it finishes wrting the data, thus meaning the average write speed may be around 3MB/Sec.

Therefore the SDHC format, which has a guaranteed minmum write speed of say 6MB/Sec (40x Speed) will start at this speed and may in fact have a higher top speed - like the SanDisk Extreme III Secure Digital Card - SDHC 4gb Memory Card, this is a Class 6 card -therefore having a minimum sustained write speed of 6MB/Sec - 40x Speed, but has a top speed of 20MB/Sec - 133x Speed.

In simple terms this means SDHC cards start writing data a specified speed and sustain that speed, whereas regular SD Cards start writing data at 0MB/Sec and build up to a top speed; SDHC memory cards start writing the data at a higher minimum speed.

SDHC memory cards are classified to guarantee a specific sustained DTS. This potentially saves consumers money, as flash cards are priced not only according to capacity, but also to speed. For example, if a product’s maximum DTS is 2MB/sec, spending the extra money for Class 4 or Class 6 SDHC cards would be a waste of money. Conversely, devices that can utilise the 4MB/sec or 6MB/sec DTR (Dat Transfer Rate) will perform significantly better with Class 4 or Class 6 SDHC cards, respectively. 

Secure Digital was forced to create a new specification for SDHC cards when the previous specification topped out at a capacity of 2GB. This occurred previously when SD cards hit the 512MB wall. The new 2.00 specification should last a bit longer, as it allows SDHC cards to reach a maximum capacity of 32GB. Secure Digital is so-named because of its ability to protect copyright content through Digital Rights Management or DRM. Because of this, it is a favoured flash memory format in the audiovisual industry.

Before purchasing and SD or SDHC memory card, ensure your device is compatible. Picstop have a use Product Finder, where you can check memory card compatibility. There are several card formats available on the market, and devices are proprietary. If a device manual does not list SDHC cards, or state it is “SD specification 2.0 compatible,” the device cannot utilise these cards.