What’s the difference between… SD classes?

QI want to upgrade my digital camera’s 1GB SD card. I found no shortage of choice online but I quickly became confused by all the different ‘class’ ratings. Many cards are described as ‘Class 10’, for example. The card I took out says ‘Class 4’. I also saw cards labelled as ‘UHS-1’. Can you explain the difference between these? Ian Lord

AThe explanation isn’t as simple as it could be because the tech industry seems to have gone out of its way to make matters as complicated as possible for consumers (not for the first time!).

SD memory cards have a speed rating that signifies how fast they will reliably record data. This is known as the ‘absolute minimum sustained write speed’, and the class numbers directly indicate the rate in megabytes per second (MB/S). So, a Class 4 carcardard will record data at an absolute minimum sustained write speedeedee of 4MB/S, while a Class 10 guarantees 10MB/S. Note the ‘absolute minimum’ aspect here: a card won’t drop below the minimum specifiedd by its class, but it could manageageag more — so you might sometimesmes see the relevant MB/S potential printed alongside the class.

Where it gets complicated is in the marketing. The SD Association ( www. sdcard.org) has additionally come up with Ultra High Speed (UHS) Classes 1 and 3. UHS Class 1 guarantees a write speed of 10MB/S, so is equivalent to a standard Class 10, while UHS Class 3 ups this to 30MB/S — which is the kind of speed needed for recording video at 4K resolution. The SD Association has also coined Video Speed Class, with classes up to V90 (90MB/S). V30 is 30MB/S – the same speed as UHS-3. All three standards are still actively promoted in the market.

If your digital camera works satisfactorily with a standard Class 4 card, then a higher-capacity version of the same will do the job just as well. However, use of a higher class might result in faster operation, if the camera itself is capable of exploiting the extra storage speed. Want to know the difference between technical terms? Email us at noproblem@ computeractive.co.uk.