House of cards

Media isn’t what it used to be. In the days of film, your choice of stock was an important and individual thing. Not only was it a question of making the right choice for technical reasons, personal preference was also a factor when choosing from available black and white, colour negative and transparency emulsions.

Though your choice of digital media won’t affect the quality of your images in the same way, it still has important consequences and you need to think about more than just capacity when buying. Reliability, durability, read and write speeds and compatibility are all issues you need to consider – and, of course, price.

Other than first-time digital camera buyers, many of us are influenced in our choice of media by the format used in our existing camera. Being a Canon DSLR user, I’ve tended to favour Compact Flash card cameras when I’ve been on the lookout for a compact camera. These days, though, with the cost of media becoming less expensive, it isn’t so much of an issue.

Sony’s Memory Stick aside, two formats – CF (Compact Flash) and SD (Secure Digital) – account for the lion’s share of the digital media market. If there were any advantages in one format over the other, it was that SD cards used to be slower and more costly. These days there’s less to choose between them in either cost or performance.

The technology
Both CF and SD cards use solid-state, non-volatile Flash memory to store data. In addition to the memory chips, the cards contain a controller that handles communication between the card and the device that’s writing to or reading from it.

Generally speaking, cards use two kinds of Flash memory cells. The fastest cards use Single Level Cell (SLC) memory. Multi Level Cell (MLC) memory is cheaper, but slower and less durable. Eventually, after many thousands of erase/write cycles, Flash memory cells lose their capacity to store data and this happens sooner in MLC memory than in SLC.

Memory cell failure doesn’t render a card unusable because the controller flags the expired cells and avoids them. Controllers ‘average out’ write/erase operations across all the available cells on a card in order to reduce wear and tear on individual cells.

There’s a third type of card that uses a miniature disk drive, or microdrive, in a Compact Flash type II format (see later under ‘Compatibility’). Though less robust than Flash memory cards, microdrives previously enabled larger capacity cards to be manufactured. Now Flash memory card capacities have increased, microdrives are fast becoming obsolete.