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I am considering purchasing one of these:

https://static.cloner-alliance.com/manual/ca-1080fl_user_manual_v1.1.pdf

It claims to be able to capture "uncompressed video", but in the list of output formats in the document above (page 67), it lists "MJPEG,YUY2". I know MJPEG is a compressed format, but I'm less certain about YUY2.

The source I'm considering is a Surface Pro 7 which outputs HDMI via a USB-C port. I am under the impression that the Surface is likely to output sRGB by default. My reading of the Wikipedia article on YUV leads me to believe that if the capture device receives an HDMI signal using the sRGB color space, and outputs a stream using the YUY2 color space, information must be lost (some discussion here seems to support this theory).

Is there any sense in which this device can claim to support "uncompressed" video or is that misleading?

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It's a bit of a semantics game. The YUY2 video mode is not "compressed," but it won't result in a bit-exact copy of RGB input. The luminance is full resolution in that mode, but the chrominance is sampled at a lower resolution. This is technically decimation rather than compression, but some information is indeed lost.

That said, if you are going to wind up posting your video online or something, it'll be even more decimated, and also subject to lossy compression. So in most use cases, the loss of information from going into that YUY2 colorspace wouldn't be anything to really worry about one way or another. You probably need to use some lossy compression to be able to reliably record it in real time unless you have very fast storage or are having the device output a low resolution. If you really care about truly perfectly exact output for some sort of scientific image analysis process, you'd need more specialized hardware.

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