In an era where digital content is not just consumed but created at unprecedented volumes, the role of storage technology has never been more critical. To meet the escalating demands for speed and capacity, the SD Association (SDA) continues to innovate and expand its range of speed classifications. Specifically, the recent introduction of the SD Express Speed Classes signifies a significant leap in terms of data performance and efficiency, catering to a new standard of digital content creation and consumption.

The SDA regularly updates its speed class criteria to keep pace with the evolving technology landscape, where the need for faster and reliable data storage is constant. Speed classes are designed to offer a guarantee for the minimum sequential performance capabilities of memory cards, ensuring users can match the card’s sequential read/write capabilities with the demands of their devices and applications. The latest addition is a direct response to the introduction of the PCIe® interface and NVMe™ interface in the SD Express cards serving performance needs in ultra-high definition content creation and use.

For those host device manufacturers which minimum assured sequential WR/RD performance is critical for their proper operation, understanding and adapting to these speed classes is pivotal. The flash memory device requires a specific method of access to the data for reading and writing. Usage of the optimized access method may best utilize the memory device’s usage.

The speed class specification definition defines the optimized access method to the cards. In fact, the reason that SDA defined several different speed class specifications was due to the evolution of NAND Flash memory technology.

Speed Class, in general, provides a clear means:

1. For product vendors to ask for memory cards with minimum speed classes to ensure proper operation
2. For memory card vendors to declare an assured minimum performance levels for their cards
3. For consumers to know which memory cards to purchase to ensure the proper operation of their devices.

Speed Classes vs. Speed Ratings

The speed rating represents the maximum speed of the card. The speed rating is not standardized by SD specifications and is used sometimes by card vendors to express their maximum performance capability of WR/RD bursts of data and usually written in megabytes per second, for example 60MB/s, 90MB/s.

While speed ratings measure maximum performance, the various speed class types, on the other hand, assure minimum sequential card access performance (including multiple stream access in SD Express Speed Classes) under defined conditions specified in the standard. In other words, the minimum speed needed to ensure the video, for example, is recorded on the memory card at an even, sustained rate with no dropped frames, which could result data loss or rough and uneven playback.

The Marks

The official marks for the various classes have symbols or pictographs to easily identify capabilities. These pictographs can be found on both memory cards and device’s user manuals.

Host devices, such as DSLRs, camcorders, laptops, etc. should display the appropriate speed class pictograph, guiding consumers to select compatible memory cards and make informed decisions based on the performance they need for their needs.

Video Speed Classes

Early speed classes, including Speed Class and UHS Speed Class were optimized for older technologies. Video Speed Class was introduced in 2016 to identify memory cards that can handle higher video resolutions while guaranteeing minimum sequential performance for video recording using the latest evolved NAND flash memory technologies.

Understanding the Layers of SD Express Speed Classes

With the introduction of SD Express in 2018, the SDA expanded its horizons to accommodate advancements in the market. The latest SD Express Speed Class specification, in fact, represents the adoption of the same access method as defined by the existing Video Speed class specification over the NVMe protocol used through the PCIe interface of SD Express cards.

SD Express Speed Classes	SD Express Speed Class Pictographs	Minimum Read/Write Performance
SD Express Speed Class 150		150 Mbytes/Sec
SD Express Speed Class 300		300 Mbytes/Sec
SD Express Speed Class 450		450 Mbytes/Sec
SD Express Speed Class 600		600 Mbytes/Sec

The SD Express 9.1 specification introduced a new range of Speed Classes, each denoting a minimum assured sequential access performance under the specified condition:

These classifications enable users to understand the card’s performance capabilities more clearly, particularly when handling tasks like high-bitrate video recording, multi-stream recording, 3D content capture, and high-performance computing applications while using SD Express cards.

In terms of multi-stream performance, SD Express is a game-changer. It ensures that a memory card can handle up to eight multiple data streams simultaneously, a crucial feature for applications that require several streams of recorded data. In such cases the total accumulated performance of the individual streams is assured by the given speed class. For example – a system that is expected to handle 4 stream recording – two of high-resolution cameras requiring 50MB/s each and 2 ultra-high resolution cameras requiring 100MB/s each… a card that supports SD Express Speed Class 300 may be sufficient (50×2 + 100×2 = 300). Examples for multi-stream applications can be found in car-dash cameras using multi-stream surveillance systems capturing several high-resolution video feeds onto one memory card.

Maximizing the Potential of SD Express Speed Classes

To assure speed classes for faster recording speeds of hundreds of MB/s, Power and Thermal Management features were introduced.

Power Management – An SD Express card indicates several Maximum Power (MP) values supported by the card. The card consumes power up to one of the MP values set by the host device. Sometimes, a card’s best performance is not always required for every action; therefore, it is unnecessary to always specify the largest MP value. The SD 9.1 specification also introduces a new power management feature. The card suggests necessary and sufficient MP values according to the targeted SD Express Speed Class and the selected PCIe bus mode. Using this feature, the host device can select an appropriate MP value depending on the card’s capabilities so that the card does not consume excessive power for the targeted recording.

Thermal Management – Power saving and card temperature management are essential to maintain targeted recording speed. There are two internal card temperature thresholds related to the thermal throttling set by the host device. The first threshold is for activating a light throttling, and the second is for a heavy one. The second threshold is critical for the speed class recording because the card cannot continue to record due to heavy throttling. In addition, the first threshold is effective to reduce temperatures through light throttling, not to exceed the second threshold. These thresholds are key parameters for controlling the card temperature to maintain the target recording speed.

The SD 9.1 specification introduced Thermal Management where the card indicates a group of its specific thermal thresholds. The host device may then set appropriate Thermal Management parameters for the card according to the target class and the selected PCIe bus mode, much like an MP value for power management. With this feature, the host can adequately control the card’s internal temperature to maintain the target recording speed for each connected card.

In Summary

By understanding and applying these standards, manufacturers can ensure their products are at the forefront of the digital revolution, offering users a seamless and powerful data storage experience while assuring a smooth and fluent operation of their performance-sensitive applications and maintain a clear means of communication between host vendors, card vendors and consumers.

By: Yosi Pinto, Chairman of the Board, SD Association