Are HDmi cables needed to watch HD tv?

Both HDMI and DVI carry up to 1080p and are the two common domestic interfaces. If the signal source and the TV are both capable of handling 1080p, they will almost certainly have an HDMI interface. However, the fact that an HDMI interface is fitted to a device does not mean that it will handle 1080p nor does it mean that it will handle HD at all - there are many SD DVD players that have HDMI but still only deliver SD material.