Buyer's Guide Until the day when broadband is much faster, all devices have wireless internet access and sync'n'store services such as Dropbox or Google Drive are ubiquitous, memory cards will remain the most convenient general-purpose, pocketable storage.

They help everyone to move documents, photos, music and videos around safely and simply. But which to choose?

Memory cards come in half a dozen different shapes and many more capacities and speeds. They are the de facto portable storage standard, but selecting the best match to the gear that uses them is not as simple as it might seem.

It is not just a question of buying a Memory Stick for a Memory Stick socket, or a Secure Digital (SD) card for an SD slot. There are ranges of different capacities and different speeds. You don’t need the same speed for saving audio in a media player as you do for saving video in a camera.

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Card types

Some memory card types have fared much better than others over the years. SmartMedia has completely died away and xD-Picture Card (xD), only used by Fujifilm and Olympus, is supported by very few new cameras.

CompactFlash mainly survives because high-end digital SLR cameras from the likes of Canon and Nikon still use them.

MultiMediaCards, which have the same physical dimensions as SD, have been subsumed and offer no real advantage over SD.

SD and Sony's Memory Stick between them, in their various guises, are by far the most popular types of card. SD is particularly so since it is an open standard backed by dozens of companies, including Samsung and a host of others.

More recently, the high-capacity SDHC and extended capacity SDXC cards have enabled storage up to 32GB and 2TB, respectively, although the 1TB and 2TB cards are not yet readily available.

The SD card has spawned Mini SD – now a dying breed – and Micro SD variants, mainly used in smaller devices such as smartphones and media players. Memory Stick is available in Duo, Pro Duo and Pro-HG Duo, as well as its basic form.

All cards are available in different capacities and some also in different speed grades; the choice is, of course, dictated by the device you are using. Few bits of kit can take more than one type of card, with the exception of multi-card readers in PCs and printers.

If your computer lacks a certain kind of memory card slot, USB-connectable adapters are commonplace.

Capacities

As memory technology has advanced, card capacities have grown from kilobytes (KB) through megabytes (MB) to gigabytes (GB). Specifications are already in place for terabyte cards.

Typical capacities today range from 1GB up to 32GB, with 64GB, 128GB and 256GB available for specialist uses.

The best advice is generally to go for the highest capacity you can afford, but there’s a proviso. If you have the cash for, say, a card that can store four hours of video or for two that can store two hours each, the two-hour ones might give you more flexibility.

You might find it useful to categorise your videos and keep one card for each category. It could help with the data housekeeping, and also prevent you losing all your data if a camera goes missing.

Buying options

Check that the card type your device uses can cope with the capacity of the card you are considering.

The high-capacity SDHC cards, for example, use a different Card-Specific Data register from SD, which means that many earlier SD devices can’t read them. SDHC cards also come pre-formatted with the FAT32 file system, and SD cards don't.

The Memory Stick Pro is a higher-capacity version of Memory Stick and can have a maximum capacity of 32GB, the same as for SDHC.

Duo and Micro are smaller versions of Memory Stick, similar to Mini SD and Micro SD cards in application.

CompactFlash cards range from 1GB to 64GB, though the specification supports capacities up to 128GB.

Speed grades

You can pay over the odds if you buy a fast card for a slow device, but more awkward is buying a slow card that can’t store data fast enough. This may well lead to dropped frames in recorded video, for example.

Memory Stick Pro cards with capacities higher than 1GB support a high-speed mode, which is normally enough to record even HD video.

The Memory Stick Pro-HG Duo uses an 8-bit parallel interface and an increased clock rate, making this class of card up to three times faster than Memory Stick Pro Duo. This is a better bet for 1080p – aka full HD – recording.

Classes of SD card are more varied. There are currently four different classes: 2, 4, 6 and 10, which correspond to minimum write speeds of 2MB/s, 4MBps, 6MBps and 10MB/s.

You may also see SD cards rated in the older ‘x’ system and the speed classes correspond to 13x, 26x, 40x and 66x respectively.

Typical applications for the different SD classes are SD video for class 2, HD recording at 720p and 1080p for classes 4 and 6, and full HD video with consecutive stills for class 10.

CompactFlash cards also use an ‘x’ rating system, though confusingly, not the same one as SD cards did. CompactFlash cards are rated using the same scheme as for CD-ROMs, where the maximum read rate equals the ‘x’ rating times 150Kb/s. So, for instance, a 133x card has a notional read speed of 20Mb/s.

And finally…

Just in case it crosses your mind, both SD and Memory Stick cards incorporate Digital Rights Management, using a built-in processor and a protected area of memory, so you can’t use them to circumvent copy protection on commercial videos. ®