SD v SDHC v SDXC ?

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The choice explained

A little background as to the difference between SD (Secure Digital) SDHC (Secure Digital High Capacity) and SDXC (Secure Digital eXtended Capacity) may be useful for you when it comes to selecting the best card for you.

Firstly, there is no quality or security difference between the SD, SDHC and SDXC formats.  Cards labelled SDHC usually have a capacity of 4GB – 32GB, whereas SDXC is typically larger.

SD CARD CLASS RATINGS V RATED SPEED[1]

class ratings (2, 4, 6, 10) refers to the maximum transfer speed for reading and writing images to and from a memory card, expressed as megabytes per second MB/s.  Unlike card write speeds that measure maximum performance, class ratings measure the minimum sustained speed for recording an even rate of video onto the card.  Class 2 cards are designed for a minimum sustained transfer rate of 2 megabytes per second, while Class 10 cards are designed for a minimum sustained transfer rate of 10 megabytes per second.

Speed class is a minimum speed based on a worse case scenario test. The speed class is important for video mode or camcorders, where the device is actually saving a steady stream of data.  The resolution and format of the video determines the amount of steady stream data. This translates to a minimum speed you need to guarantee that the video captured on the cards is recorded at an even, sustained rate with no dropped frames.

Rated speed, e.g. 15 MB/s, 30 MB/s, etc is the maximum speed of the card and also what you would expect to approximately see in typical usage of writing or reading files on the card.  This measurement is pertinent to Still photography, especially for taking pictures with high resolution, and / or saving in RAW format where the files created are very large.  The faster the card, the faster it can save the file and be ready to take another picture.  You can really notice speed differences with high megapixel DSLR cameras when using multi-shot burst mode.

Still images shot on high megapixel cameras should utilise fast data throughput (a large data pipe), higher speed cards for improved performance.   Higher speed cards can also improve how fast you can transfer the files between the card and your computer. 

Compared to high megapixel photography, video doesn’t need as big a data pipe because the video format is a smaller “fixed stream” that uses only a portion of the data pipe.  But, you do need a minimum guaranteed speed for the SDHC card that satisfies the requirement of the data stream.  Your cameras specifications should state the minimum SDHC class rating required.

Using a card without the proper class rating on a more advanced camera, such as a high definition (HD) Camcorder or Digital Single Lens Reflex (DSLR) camera with HD video record settings is likely to result in an error message indicating that video can only be recorded at a lower definition setting.

[1] Taken from SanDisk article and research

Current classifications

The current SDHC specification defines the classes as follows:

  • Class 2: Minimum speed of 2MB/s
  • Class 4: Minimum speed of 4MB/s
  • Class 6: Minimum speed of 6MB/s
  • Class 8: Minimum speed of 8MB/s
  • Class 10: Minimum speed of 10MB/s

UHS Speed Class was introduced in 2009 by the SD Association and is designed for SDHC and SDXC memory cards.  UHS utilises a new data bus that will not work in non-UHS host devices.  If you use a UHS memory card in a non-UHS host, it will default to the standard data bus and use the “speed class rating” instead of the “UHS Speed class rating”. UHS memory cards have a full higher potential of recording real time broadcasts, capturing large size HD videos, and extremely high quality professional HD.

  • UHS Class 1: Minimum speed of 10MB/s
  • UHS Class 3: Minimum speed of 30 MB/s

Video Speed Class or “V Class” was created by the SD Association to identify cards that can handle higher video resolutions and recording features.  This speed class guarantees minimum sustained performance for recording video.

 The other speed classes are either not optimised or are unable to accommodate the recording of multiple video streams, 360° capture, virtual reality content or 8k and higher resolution video.

  • V Class 6: Minimum sustained speed 6MB/s
  • V Class 10: Minimum sustained speed 10MB/s
  • V Class 30: Minimum sustained speed 30MB/s
  • V Class 60: Minimum sustained speed 60MB/s
  • V Class 90: Minimum sustained speed 90MB/s

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