vendors, including Transcend and Kingston, report their cards' write speed. When a card lists both a speed class and an "×" rating, the latter may be assumed a read speed only. Real-world performance 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 high-definition camcorder may require a card of not less than Class 6, suffering dropouts or corrupted video if a slower card is used. Digital cameras with slow cards may take a noticeable time after taking a photograph before being ready for the next, while the camera writes the first picture. The speed class rating does not totally characterize card performance. Different cards of the same class may vary considerably while meeting class specifications. A card's speed depends on many factors, including: The frequency of soft errors that the card's controller must re-try Write amplification: The flash controller may need to overwrite more data than requested. This has to do with performing read-modify-write operations on write blocks, freeing up (the much larger) erase blocks, while moving data around to achieve wear leveling. File fragmentation: where there is not sufficient space for a file to be recorded in a contiguous region, it is split into non-contiguous fragments. This does not cause rotational or head-movement delays as with electromechanical hard drives, but may decrease speed⁠ ⁠ ⁠— for instance, by requiring additional reads and computation to determine where on the card the file's next fragment is stored. In addition, speed may vary markedly between writing a large amount of data to a single file (sequential access, as when a digital camera records large photographs or videos) and writing a large number of small files (a random-access use common in smartphones). A study in 2012 found that, in this random-access use, some Class 2 cards achieved a write speed of 1.38 MB/s, while all cards tested of Class 6 or greater (and some of lower Classes; lower Class does not necessarily mean better small-file performance), including those from major manufacturers, were over 100 times slower. In 2014, a blogger measured a 300-fold performance difference on small writes; this time, the best card in this category was a class 4 card. Features Card security Cards can protect their contents from erasure or modification, prevent access by non-authorized users, and protect copyrighted