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A Secure Digital (SD) card is a memory chip in portable format, about the size of a postage stamp and thickness of a credit card. It is non-volatile memory, meaning the chip does not require power to maintain data. SD cards are popular storage devices for digital cameras, camcorders, cell phones, portable computers, personal digital assistants (PDAs), gaming consoles and other mobile electronic devices. A 4G SD card refers to a card with four Gigabytes (GB) of capacity.
Not every device that supports SD cards will support a 4G SD card. The Secure Digital Association (SDA) responsible for issuing specifications and standards for the cards revised version 1.x of the standard, allowing for capacities up to 4GB. This was accomplished by extending the block length on the card up to 2048 bits. The problem arises when an older product does not take the block length data into account when reading the identity string of the card, assuming a smaller length of 512 bits, thereby misidentifying the card as having a lower capacity.
A 4G SD card might also refer to the newer 2.0 SD High Capacity (SDHC) standard. SDHC cards have capacities between 4 and 32 GB. The 4GB boundary overlaps with the previous 1.x version. SDHC cards are imprinted with "HC" under the SD logo. A standard 4G SD card will be missing this designation.
The main difference between a 4G SD card and its SDHC cousin is that the newer 2.0 specification guarantees a minimal data transfer rate (DTR). Standard cards (1.x versions) begin writing data from nil speed, working up to the maximum speed supported, then as the write process nears completion, speed drops, finally reaching zero. This process results in an average write speed that is many times less than the maximum speed the card is rated at.
SDHC cards are assigned a classification (Class 2, Class 4, Class 6, etc.), indicating a minimum, guaranteed, sustained write speed. The most common classifications follow in megabits per second (mbps) and Megabytes per second (MB/ps):
Typically, faster cards are more costly than slower cards of the same capacity, but in some cases, speed is not a luxury, but a requirement. For example, a digital camcorder that relies on a flash card (versus a hard disk) will need to be able to write to the card extremely quickly to record movie frames. If a slow card is used, the result will be lost data or dropped frames. Conversely, if a product can only support slower write speeds, purchasing a faster card will be a waste of money.
Products are rated for their capacity to write at a particular maximum speed, and this specification can generally be found in the manual. A product with a rating of 13x correlates to a Class 2 SDHC card; while a rating of 26x would be commiserate with a Class 4 SDHC card, and Class 6 cards support products rated at 40x. Using a Class 3 card in a device that only supports Class 1 speeds will not make the device work faster.
While any product that supports the newer SDHC cards is backward compatible and can read a standard 4G SD card, products made only for 1.x SD will not be able to read 2.0 SDHC cards. SD eXpanded Capacity (SDXC) is an even newer specification, picking up where SDHC left off at the 32GB boundary. SDXC has a maximum capacity of 2 Terabytes.