SD cards are memory cards most often used in digital camera and video devices. While all cards do essentially the same thing (hold data), the performance of a card is greatly dependent on the speed of it. A slower card may be a pain in some situations.
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Official Rating System
The speed of an SD format is officially rated by classes. For example, an SD card may be rated a class 5 card, while another may be a class 7. The numeric rating of each class is the minimum writing speed of the card, which basically means how fast files can be created on it. A class 2 card provides a writing speed of at least 2Mbps, while a class 10 card provides 10Mbps. As of 2010, a class 4 card is a standard speed.
The "X" Scale
Some cards will describe themselves compared to a CD-ROM drive, which writes at 150Kbps. If a card is rated as 20x, which stands for 1 times the speed of a CD-ROM, it writes at 3,000Kbps or 3Mbps. This scale is proportional, so a 100x card is five times as fast as a 20x card. The reading speed of the card is almost always higher than the writing speed. Be careful: Some manufacturers use the reading speed of the card instead of the write speed. Be sure to inspect the actual speed of each before buying. As of 2010, common speeds are between 13x and 66x.
Compare the reading and writing speeds of cards directly to each other. A card's specifications are located on its packaging or available from the manufacturer. The write speed of a card is how quickly files can be created on it, while the read speed is how quickly files can be copied from it. In general, the read speed is slightly faster than the write speed. As of 2010, common read and write speeds are between 4 and 8Mbps.
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