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SD Speed Class standards were created to support video recording. SD memory card manufacturers use different types of flash memory to create an SD memory card, so actual transfer speed can vary. Varying speeds make it difficult to determine which card will provide reliable recording of streaming content because recording video requires a constant minimum write speed to ensure a smooth playback. The SD Association defines Speed Class standards indicated by speed symbols to help consumers decide what card will provide the required performance. There are two kinds of speed indications regarding SD bus generation:

SD速度分类标准是被制定于支持视频录制。SD存储卡商使用不同的闪存来制造SD存储卡，所以实际的传输速度会不同。因为录制视频要求一个恒定最低写入速度以确保流畅的回放，但是（各种卡的实际的传输）速度的不同，造成很难判定哪种卡在录制流媒体上提供更高的可靠性。SD协会用速度符号定义速度分类标准以帮助消费者根据具体的要求来挑选SD卡。共有两种不同的速度指标，基于SD卡的总线代。

Speed Class and UHS Speed Class* symbols indicate minimum video recording performance to ensure smooth video playback. This is important for camcorders, video recorders and other devices with video recording capabilities. The Speed Classes defined by the SD Association are Class 2, 4, 6 and 10.

Speed Class 和 UHS Speed Class 符号标明了最低的视频录制速度以确保流畅的视频回放。SD协会定义的Speed Classes分类包括2、4、6和10。（这就是原来大家常见的SD卡分类标准。）

UHS Speed Class is designed for UHS** devices only and designates minimum writing performance to record video on UHS products. The UHS Speed Classes defined by the SD Association are UHS Speed Class 1 (U1) and UHS Speed Class 3 (U3).

UHS速度分类是用于UHS（超高速）设备的，并标明了超高速产品上录制视频的最低写入速度。SD协会定义的UHS速度分类有UHS速度分类 1 (U1)and UHS速度分类3(U3)。

* (Speed Class and the UHS Speed Class are two different speed classes.)
Speed Class and the UHS Speed Class 是两种不同的速度分类/种类。

**UHS (Ultra High Speed), the fastest performance category available today, defines bus-interface speeds up to 312 Megabytes per second for greater device performance. It is available on SDXC and SDHC memory cards and devices.

以上内容之原文来自：
https://www_sdcard_org/consumers/speed/speed_class/

下图来自：
http://panasonic_net/avc/sdcard/features/speed_class.html

UHS-II Pin Layout

A new pin layout was introduced to support the UHS-II bus interface. The new second-row pins on the full-size SD memory card and microSD memory card are used for the UHS-II signals. The first-row pins are used for the normal speed, high speed and UHS-I bus interface signals to provide interoperability with existing non-UHS-II host devices.Normal Speed/High Speed/UHS-I CardUHS-II Card