SD Cards Explained

SDHC, SDHX, MicroSD - Card Sizes Explained 

In today’s market you will come across SSD (Standard size SD cards), they are mostly used in larger devices like DSLR cameras and larger video cameras, which also happen to use Compact flash cards, CF, which is another type of even larger memory card.

The smaller Micro SD Cards appear in smaller devices, such as mobile Smartphone’s, Tablets, Action Cameras, Portable Music Players, GPS Map readers and GPS Finders, Car Dashboard Cams and other smaller hand-size electronics.

The physical dimensions of the 3 cards are as follows:

(Full Size) SD – 32mm x 24mm
Mini SD – 21.5mm x 20mm
Micro SD – 15mm x 11mm

They include optional content security features, which enable SD memory cards to be used as storage for content-protected audio, video and books due to the official SD Association memory classification. The Association further defines SD standards into an input/output (I/O) classification. SDIO extends device for a range of capabilities including Internet service, GPS and camera capabilities.

SDHC & SDXC Capacity Explained

SDHC stands for Secure Digital High Capacity and SDXC stands for Secure Digital eXtended Capacity, it’s that simple.

Card Capacities - Each type of card has a memory capacity range, and these are:

  • SD standard - Up to 2GB SD memory card using FAT 12 and 16 file systems
  • SDHC standard - Over 2GB-32GB SDHC memory card using FAT32 file system
  • SDXC standard - Over 32GB-2TB SDXC memory card using exFAT file system

Each SD or Micro SD card has a speed rating, called a Class. Larger Class numbers correspond to a faster level of writing/recording (minimum performance) allowing files to be written to the card or recorded at a higher speed or definition (HD/4K).

The Class system rating corresponds to the minimum read speeds in MB/s as below:

  • Class 2 – 2MB/s (minimum recording speed),
  • Class 4 – 4MB/s
  • Class 6 – 6MB/s
  • Class 10 – 10MB/s

Class 2 is sufficient for SD video recording, whilst Class 4 and Class 6 support HD recording. The Class rating is displayed on the card, by a number which is displayed inside a circle.

After the Class 10 classification, it becomes a little more complicated. An additional standard called UHS was then introduced, which allows cards to reach higher speeds. Here we see a similar system:

UHS Class 1 (or just U1 or U-I), which has a minimum performance rating of 10MB/s writing speed.
UHS Class 3 (or U3 / UHS-III) has a rating of 30MB/s.
UHS Class 3/III supports 4K video recording.

(Note that the UHS speed classes are shown on the card with a number inside a U).

Beware, because the UHS Class 1 and Class 10 refer to the same 10MB/s speed, so sometimes you’ll see cards that are labeled both as Class 10 and UHS-1.

For example, if you have an Action Camera, such as GoPro or EvoDX, that records HD at either 1080p or 4K you really need to be thinking about purchasing a UHS-3/III card as you’ll get a better performance and writing speed.

Some less expensive action cameras will buffer the recording, but this is not practical in the slower card speed classes and may cause juddering or stuttering of the recorded footage, as well as other issues.

Similarly with the latest Digital SLR Cameras (especially in modes such as sport, multi-frame or burst-mode) where multiple images are taken, you should consider having a faster card which will allow you to review the images much sooner, it will mean less waiting for the images to write to the card.

SD Cards Compatibility Explained

Most devices such as cameras, mobile phones and card readers will generally work with older SD Cards, but newer SD Cards with higher Class and Speed classifications may not work in the older devices – check your owners’ manual or search online for cards that will work with your device before making a purchase.

Newer cards come with some limitations compared to the ones supplied with older devices, meaning your device might not be able to read the full size of the card, or read the file system of the card, or it have an older/obsolete hardware interface.

Watch out for Full-size SD cards and Micro SD cards, as they may not work in devices that are specifically designed for one or the other, even though smaller cards can fit into larger card slots using card adapters.

Even if the newer Card (U-1 or U-3) can be read by your older device, your device may not support the newer UHS-1 standard, which will result in slower-than-expected writing speeds when using these U1 or U3 cards (ie: it will write at the maximum of a Class 2/4 or 6 card) .

For these reasons, it’s important to search for manufacturer specifications for your device before buying an SD or Micro SD card.

Understanding SSD Card Adapters

Mini and Micro SD cards can be placed inside Full-Size SD Card ‘adaptors’, allowing them to be used in devices designed for Full Size SD Cards. This can be useful when using a single card in multiple devices (like a phone & / or digital camera)

Using Full Size SD Card Adaptors with Micro SD Cards allows the Micro SD Card to be read by external and also built-in SD card readers (like those found on laptops and desktop PC’s).

SD Card ‘Bus’ Speeds Explained

SD bus speed of 12.5MB/s Default Mode was defined by SD1.0 and 25MB/s High Speed Mode was defined by SD1.1 to support digital camera. Its high performance function for continuous shooting required higher write speed memory card where SD Association introduced higher speed bus interfaces for SDHC and SDXC memory cards named UHS-I, UHS-II and UHS-III.

UHS-I can provide ultimate bus speed using 1st Row pins. UHS-II and UHS-III have ability to provide much higher bus speed than UHS-I using 1st Row and 2nd Row pins. 2nd Row interface is supported with Low Voltage Differential Signaling (LVDS) technology.

UHS-II and UHS-IIIhas two lanes. Full Duplex (default mode) assigns a lane to downstream direction (host to card) and the other lane to upstream direction (card to host) and then packets can be transferred both directions at the same time. Half Duplex switches lanes to the same direction during data transfer to make data transfer rate twice (156MB/s in Full Duplex can be switched to 312MB/s in Half Duplex). However, UHS-III Card does not support Half Duplex for 312MB/s Full Duplex and 624MB/s Full Duplex to make PHY design simple.

Maximum speed differs from the bus I/F speed. It varies depending upon the card performance and the average speed that a device writes to an SD memory card may vary depending upon the device and the operation it is performing. The speed may also depend on how other data is stored on the SD memory card.

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