Does NVENC device internally converts all user input format to YUV420?


I noticed that NVENC API enables user to provide different data format such as ARGB or YUV444, but does NVEVC internally converts all those format data to YUV 420?

But conversion from RGB to YUV420 is a lossy process and we will never get a high quality image, is there anyone know how to get a high quality image without losing color information?