Technically speaking, this is sub-sampling rather than compression. As you pointed out, the image sensor's Bayer Filter outputs 12-bit RGBG data from a 12-16 Megapixel array. When this RAW data is converted into Y:C:C format in a 4:2:2 ratio, the chroma components are sampled at half the horizontal and vertical resolution as the luma. Both luma and chroma are sampled at 8 bits. This YCC data stream is fed directly into the HDMI cable and is not compressed by any codec. Since there are no standard formats for streaming RAW Bayer Filter output, it would not be possible to do so through HDMI or any other type of external connector.In my mind, chroma sub-sampling (4:2:2) is a form of compression. Additionally, the "sensor signal", what you get in a RAW file is 12-bit, so will [likely] get down-sampled to 8-bit.