SD stands for Secure Digital - a non-volatile memory card format. SD cards have become one of the most popular forms of memory cards to be used in digital devices including cameras, PDAs, smartphones and MP3 players.
They have all but replaced the older Compact Flash (CF) cards - except for some high-speed professional photography and legacy applications.
As technology improves, so do the cards and their capacity. There are four families of SD card, available in 3 different form-factors.
SD card families
- Standard Capacity (SDSC)
- High Capacity (SDHC)
- eXtended Capacity (SDXC)
All of the above families can be produced as original SD, mini SD or micro SD form factors. The HC and XC families were designed to expand the theoretical limit of how much SD cards could store.
Most new devices can use HC and XC cards, although there are exceptions. If unsure, check your device to see if it is compatible with HC or XC cards. If you have older equipment, it is definitely woth checking before buying a new card.
SD High Capacity (SDHC) was based on the SD 2.0 specification, introduced in January 2006. This specification introduced a higher speed and enabled SD cards to reach higher capacities of up to 32GB.
SDHC was semi backwards compaitble - HC devices were required to be able to read standard cards, however standard readers could not read HC cards.
To ensure compatibility, look for the SDHC logo on cards and host devices.
SD eXtended Capacity (SDXC) was based on the SD 3.01 specification, introduced in January 2009. This specification upped the theoretical limit of cards up to a massive 2TB (2,000 gigabytes / GB!)
The same issue of backwards compatibility was raised with XC cards. XC devices are required to read HC and standard cards but standard/HC readers cannot accept XC cards.
exFAT file system
SDXC also uses Microsoft's proprietary exFAT file system as a mandatory feature. exFAT is a licensed technology, which means only operating systems which have properly licensed the ability to read and use exFAT cards should be able to use them.
This means that XC cards may not read on all modern devices the way you would expect them too. For instance many Linux based systems require a third party plugin to read exFAT correctly. XC cards can be reformatted to use a different file system such as FAT32 (although this comes with limitations, such as a maximum 4GB file size).
So as to to comply with the official SDXC card specification, most XC capable devices are programmed to expect the exFAT file system on cards larger than 32 GB. Due to this, they may not accept SDXC cards reformatted as FAT32, even if the device supports FAT32 on smaller cards (for SDHC compatibility).
Therefore, even if a file system is supported in general, it is not always possible to use alternative file systems on SDXC cards, depending on how strictly the specification has been implemented. This bears a risk of accidental data loss, as the device may treat a card with an unrecognized file system as blank or damaged then attemot reformat the card - although you should be prompted first.
This stands for Secure Digital Input Output and these types of cards are not specifically for storage. They are often used to provide a 'bridge' that allows for extra features tpo be added to a device.
Laptops, computers and mobile products may have an IO ready SD card slot. SDIO equipemtn could be a GPS receiver, modem, barcode scanner, FM / DAB radio tuner, TV tuner, WiFi antenna, Ethernet port etc.
Their use has declined dramatically due to the miniaturisation of technology and the convergence of equipment, culimating in the SoC (System on a Chip) processing units now powering many mobile devices.
Speed Class Rating
The SD Association has created and defined several speed classes. This allows you to identify the speed and performance capabilities of a card before you purchase.
Generally speaking, a higher class number results in faster read / write speeds for data.
All speed classes should be fine for recording still images, although those looking for continuous shooting / burst modes may want Class 10 to take advantage of the added hgh speed bus. Applications for higher speed classes are generally reserved for video recording:
- Class 2 - 2 MB/s - Standard definition video recording
- Class 4 - 4 MB/s - High definition video recording (720p - 1080i)
- Class 6 - 6 MB/s - High definition video recording (720p - 1080i)
- Class 10 - 10 MB/s - High definition video recording (1080p)
Class 10 asserts that the card supports 10 MB/s as a minimum non-fragmented sequential write speed and uses a high-speed bus mode. This is why it is preferred for continuous shooting of high definition stills.
Since 2009, UHS has been defined. This uses a high speed data bus that is not supported in non-UHS capable devices.
This new data line allows UHS cards to be especially useful for recording and broadcasting real-time high definition content, whilst simultaneously working on other projects.
If you use a UHS card in a non-UHS device, it will revert to the Speed Class above that should also be marked on the card. This is usually Class 10 but may be lower.
- UHS 1 (10 - 104 MB/s)
- UHS 3 (30 - 312 MB/s)
You can look for the Speed Class Logo to identify the performance class of your memory cards and host devices.
*1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes.