Secure Digital - Speeds

Speeds

An SD card's speed is measured by how quickly information can be read from, or written to, the card. In applications that require sustained write throughput, such as video recording, the device might not perform satisfactorily if the SD card's class rating falls below a particular speed. For example, a camcorder built for a Class 6 card may suffer dropouts or corrupted video if a slower card is used. Digital cameras may experience a noticeable lag between shots, while the camera writes the picture to a slower card.

A card's speed depends on many factors, such as the following:

• The likelihood of soft errors that the card's controller must re-try
• The fact that, on most cards, writing data requires the controller to read and erase a larger region, then rewrite that entire region with the desired part changed
• The possibility of fragmentation: that a body of information the host views as a unit is, for historical reasons, written to non-contiguous regions of memory. (This possibility does not cause rotational or head-movement delays as with magnetic media, but it does vary the amount of computation the card's controller must do.)

In early SD cards, the speed was measured with the "×" rating, which compared the average speed of reading data to that of the original CD-ROM drive. Currently, the official unit of measurement is the Speed Class Rating, which guarantees a minimum rate at which data can be written to the card.

The newer families of SD card improve card speed by increasing the bus rate (the frequency of the clock signal that strobes information into and out of the card). Whatever the bus rate, the card can signal to the host that it is "busy" until a read or a write operation is complete. Compliance with a higher speed rating is a guarantee that the card limits its use of the "busy" indication.