vga is analog and hdmi is digital. That is the main difference as to the picture quality.
http://www.cnet.com/news/hdmi-vs-displayport-vs-dvi-vs-vga-which-connection-to-choose/
How come HDMI have way better picture quality than VGA when I connect from PC to a 1080p 55' HDTV? My PC resolution is 1920x1080p.

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