Secure Digital (SD) Card

A non-volatile memory card format that is widely used in portable / hand-held devices, including digital cameras, tablet computers, smart phones, etc. SD cards are specified with speeds described in multiples of the standard CD data speed – 150 kB/s, and capacity.

The original SDSC (Standard Capacity) offer up to 2 GB storage and a bus speed of 25 MB/s. Next up SDHC (High Capacity) provides from 2 GB to 32 GB storage and support for FAT32 file systems (as is used on many PCs). SDXC introduced in 2009 supports capacities beyond 32 GB to 2TB and a maximum data speed of 300 MB/s. For yet more performance there is now Ultra Speed Bus. UHS-I can have a clock speed of 100 MHz, so handling 50 MB/s in 4-bit mode. UHS104 has a 208 MHz clock and can transfer 104 MB/s. UHS-II raises the transfer rates to a possible 156 MB/s (312 MB/s half duplex).

Currently the highest capacity on the market is a SDXC from SanDisk, offering 512 GB. This was introduced at the IBC 2014 trade show where many of the visitors and exhibitors are constantly seeking higher capacities and speeds to handle ever larger TV formats and higher frame rates.

Markings on the cards indicate their performance. A ‘C’ refers to a speed Class. The number in the big C is the minimum sustained transfer speed in MB/s; typically 2, 4, 6, 8, and 10 are used. Above that you will see a ‘U’ on the card. A ‘I’ in the U is for 10 MB/s UHS-I, and III for 30 MB/s.

SD cards have transformed video storage in both amateur and professional video cameras. Panasonic was an early adopter with its professional camcorders using P2 cards. Each card includes SD chips, a micro computer and RAID controller. There are no moving parts. There is no noise. It takes only tiny power and it is compact and robust – and a long way from tape and disk-based recording. P2 cards can read at 8x real-time for quick transfers to editing equipment. They can be used as edit stores in themselves.

See also: Flash memory, ING, P2, RAM