Default colorspace conversion matrix for argb input in nvidia video codec

What is the default color conversion matrix when the input is in argb format(H.264, d3d11), and where do I specify or get it from?
I feed a d3d11 argb texture into nvenc directly and get H.264 bitstream. There’s no color conversion matrix info in the bitstream vui info, and I haven’t found any documentation on the default colorspace conversion matrix in this case yet.

Is it rgb or ycbcr?

When nvenc’s input is an argb texture, I think nvenc converts it internally to NV12 before encoding it in H264. I would like to know what conversion matrix is used for this intermediate conversion?