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Leo Wattenberg

SD cards: It's not just the capacity that matters

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Posted (edited)

Moin.

It sometimes happens that you go into a store and see one pile of ridiculously cheap SD cards, even though all other SD cards next to it are about as expensive as you'd expect. Why is that?

Well, the reason for that is simple: Speed. SD cards have an awful lot of different speed specifications as can be seen in this Wikipedia Table:

Bildschirmfoto von 2018-08-23 14-44-09.png

And the speed matters: If you're filming in 1080p, you're probably filming somewhere between 20 MBit/s (2.5 MB/s) and 50 MBit/s (6.25 MB/s). If your card has a lower write speed than your video bitrate, you won't be able to film. Further, note that video typically gets recorded with a variable bitrate (VBR), meaning that on average, your bitrate will be around a certain number and that errors may happen when attempting to write, reducing the effective write speed. 

 

So, long story short: If you at all have the means to do so, always get an SD card that says any one of the following on it:

  • If recording 1080p30: C10/U1/V10/A1
  • If recording 1080p60 or 4K: U3/V30

 

 

 

But wait, there's more!

So far, we've been discussing write speeds. While write speed definitely is the more important metric, read speed makes using the cards more pleasant. You don't want to wait for 4 hours to copy the contents of your 16GB SD card to your computer after all, do you? 

The read speed of SD cards typically is better advertised than the write speed, simply because it's always higher. But even here there are some differences that are specified and found on the card itself:

  • UHS-I (represented on the card simply as roman numeral I) has a bus that can transfer between up to 50 MB/s and up to 104 MB/s
  • UHS-II (represented as II) can transfer between up to 156 and 312 MB/s
  • UHS-III (represented as III) can transfer between up to 312 and 624 MB/s
  • PCIe 3.0/NVMe (represented as EXPRESS) can transfer up to 985 MB/s.

To achieve these speeds, you will an SD card with a high read speed and an appropriate bus, an SD-card reader that can handle the UHS bus as well as the higher bus clock speed of UHS-II and III, a hard drive that can write at those speeds, and, if you're using an USB SD-card reader, a USB cable and port that can handle those speeds. In other words, when reading the contents of an SD card, pretty much every part of your PC can be the bottleneck with higher-performing SD cards. 

Lastly, to talk a bit about the capacity anyways: SD cards come in the version SD (up to 2GB), SDHC (up to 32 GB), SDXC (up to 2TB) and now SDUC (up to 128TB). While these cards all fit into the same cameras and readers, note that this doesn't mean that all cameras and readers can handle them, especially the newer formats XC and UC.

 

So, before buying a newer SD card, check whether your equipment is compatible with the newer SD versions

Edited by Leo Wattenberg
simplified
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