Secure Digital High Capacity (SDHC) Explained

 

Following on from the news of the launch of the Panasonic SDR-S200 (in Japan, at least), I thought this might be a good time to explain what the new SDHC format is and means. The name is short for Secure Digital High Capacity, and the basic idea is that it is a higher capacity version of the conventional SD Card, which tops out at 8GB capacity; SDHC cards can have a maximum theoretical size of 32GB (although it will probably be some time before we hit that). But there’s another aspect to this: there will also be versions of the cards which are guaranteed to have a minimum sustained data transfer rate: Class 2 cards will run at 2.2MB/s, Class 4 at 4MB/s and Class 6 at 6MB/s. This is important because camcorders (particularly high definition ones) require storage that can deal with a constant flow of data, while still cameras only need it to work in sudden bursts as the images are saved.

 

The cards themselves are the same size as current SD Cards, and all of the electrical connections are the same. This should mean that an SD card will work in an SDHC card reader, but you won’t be able to use an SDHC card in an SD Card device or reader.

 

The SD card format is defined by an industry association (the appropriately named SD Card Association) which includes companies like Matsushita (parent company of Panasonic), Sandisk, Toshiba and many others. Sony is noticeably absent from the list. They haven’t released the final specification yet, but it can’t be far off, as Panasonic has announced plans to make, a 4GB SDHC card to go with the SDR-S200 (read their press release here). Other manufacturers will no doubt have similar announcements soon.

 

UPDATE: I did more checking on this, and it seems I wasn't completely accurate. Version 1.1 of the SD Card spec increases the maximum capacity of an SD Card to 32GB, but the difference with SD and SDHC is that the SDHC cards have a guaranteed transfer rate: even the new, 133X SD cards don't have that.