SD Cards

1. Speed Classes (Video & Bus Speed)

These are the most important labels for filmmakers and photographers, as they guarantee a minimum sustained sequential write speed.

Parameter	Name	Minimum Sustained Write Speed	Best For
V-Classes (Video Speed Class)	V6, V10, V30, V60, V90	$6 \text{MB/s}$ to $90 \text{MB/s}$	4K & 8K Video: The most reliable rating for sustained video recording. V90 is the top-tier standard.
U-Classes (UHS Speed Class)	U1, U3	$10 \text{MB/s}$ and $30 \text{MB/s}$	HD and lower-bitrate 4K video. U3 is often used interchangeably with V30.
C-Classes (Original Speed Class)	C2, C4, C6, C10	$2 \text{MB/s}$ to $10 \text{MB/s}$	Older standard; C10 is the most common and means $10 \text{MB/s}$ .

The Hierarchy: V-Class ratings are the most stringent and reliable for modern video. For $4\text{K}$ video, you generally want V30 or higher. For professional $4\text{K}$ or any $8\text{K}$ recording, you will need a V60 or V90 card.

2. Capacity Formats (SDHC, SDXC)

These labels relate to the maximum storage capacity of the card and how the data is formatted. You must ensure your camera supports the format you buy.

Parameter	Name	Maximum Capacity	File System
SDSC	Standard Capacity	Up to $2 \text{GB}$	FAT16
SDHC	High Capacity	$4 \text{GB}$ to $32 \text{GB}$	FAT32
SDXC	eXtended Capacity	$64 \text{GB}$ to $2 \text{TB}$	exFAT

Crucial Note: Most modern cameras use SDXC cards because they offer the necessary high capacity (over $32 \text{GB}$ ) and use the exFAT file system, which allows for single files larger than $4 \text{GB}$ (essential for long video clips).

3. Bus Interfaces (UHS-I, UHS-II)

The bus interface dictates the maximum potential speed of the card by defining how the data is transferred between the card and the device. This is often the biggest speed difference.

Parameter	Name	Max Potential Speed	Physical Difference
UHS-I	Ultra High Speed - Phase I	Up to $104 \text{MB/s}$	Single row of pins (standard)
UHS-II	Ultra High Speed - Phase II	Up to $312 \text{MB/s}$	Second row of pins on the back of the card.
UHS-III	Ultra High Speed - Phase III	Up to $624 \text{MB/s}$	Newer, less common standard.

Identifying UHS-II: Look at the back of the card. A UHS-II card has a second, smaller row of gold contacts.

Benefit: A UHS-II card can reach significantly higher speeds than UHS-I cards, even if they share the same V-Class rating, making it a favorite for clearing the camera's buffer after a burst of photos.
Compatibility: A UHS-II card will work in a UHS-I slot, but it will only run at the slower UHS-I speed.

4. Application Performance Classes (A1, A2)

These ratings are specifically for devices that run applications from the card (like smartphones, tablets, or even some drones/action cams). They focus on Random Read/Write Speed (IOPS), which is key for launching and running apps.

Parameter	Name	Minimum IOPS (Input/Output Operations per Second)	Use Case
A1	Application Performance Class 1	$1500 \text{ Read IOPS} / 500 \text{ Write IOPS}$	Running mobile apps smoothly from the card.
A2	Application Performance Class 2	$4000 \text{ Read IOPS} / 2000 \text{ Write IOPS}$	Faster app loading and better performance in mobile devices.

General Rule: If you are only using the card for photography and video recording in a dedicated camera, you can mostly ignore the A1/A2 ratings, as they don't directly correspond to the sequential video speeds (V-Class).

SD Express and microSD Express

While UHS-II and CFexpress have been the dominant players, the memory card industry is already pushing the boundaries with a next-generation standard: SD Express and its smaller sibling, microSD Express.

This new format essentially marries the universality of the SD card form factor with the lightning speed of CFexpress and SSD technology.

What is SD Express?

SD Express is the newest memory card specification that brings the incredible performance of PCIe (Peripheral Component Interconnect Express) and the NVMe protocol directly into the traditional SD card form factor.

In simple terms: It puts the technology of a powerful internal computer SSD into an SD card.

📈 The Speed Leap

Card Type	Interface	Max Potential Speed
UHS-I (Standard SD)	UHS-I Bus	$\sim 104 \text{MB/s}$
UHS-II (Fast SD)	UHS-II Bus	$\sim 312 \text{MB/s}$
CFexpress Type B	PCIe 3.0 x 2 Lanes	$\sim 2000 \text{MB/s}$
SD Express	PCIe 3.0 x 1 Lane	$\sim 985 \text{MB/s}$

SD Express cards are nearly 10 times faster than the current top-tier UHS-II SD cards and offer speeds that begin to compete with CFexpress, all while maintaining the same physical card size.

How Does it Work? (The Pins)

SD Express achieves this speed boost using the same clever trick as UHS-II: a second row of pins.

For SD Express, the second row of contacts (the ones used by UHS-II) is repurposed to carry the high-speed PCIe lanes.
This means an SD Express card has the standard UHS-I pins, plus the PCIe pins, allowing it to work in three modes:
1. SD Express Mode (Fastest): Using the PCIe lanes for up to $985 \text{MB/s}$ .
2. UHS-II Mode: If the camera only supports UHS-II, it uses the contacts for $\sim 312 \text{MB/s}$ .
3. UHS-I Mode (Standard): If the camera only supports basic SD, it defaults to the slowest speed.

📱 microSD Express

microSD Express is the same concept shrunk down. It's designed to bring near-Gigabyte-per-second performance to devices like smartphones, action cameras, drones, and handheld gaming consoles, overcoming the speed limitations of current microSD cards.

The Current Status & Future Outlook

SD Express is still in the early adoption phase. While the specifications exist and some cards are starting to appear on the market, most current cameras (even high-end models) do not yet have SD Express slots. They rely on the existing UHS-II or the dedicated CFexpress slots.

Why SD Express is Important:

CFexpress Alternative: It provides a near-CFexpress speed in the universally accepted, cheaper, and smaller SD card form factor.
Wider Adoption: As more manufacturers adopt the technology, it will bring high-speed recording to a wider range of mid-range cameras, drones, and consumer electronics.
The Future Standard: SD Express is positioned to be the replacement for UHS-II, offering a massive performance jump while retaining backward compatibility with millions of existing devices.

In summary: SD Express is the future of high-speed memory cards, promising the best of both worlds—the speed of an SSD and the convenience of the SD format. However, it will take time for camera manufacturers to fully integrate the new technology into their hardware.