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 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 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 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)

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 ther 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 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.

Source: http://www.picstop.co.uk/blog/2008/07/what-is-sdhc.html/