Connecting STB to plasma T/V

Hi,

I have successfully connected my STB to my NEC TV, and have excellent digital signals and pictures/sound, however I am not getting anypictures/sound for the HD Digital channels, even though signal shows at 100% (I only see the magnavox symbol).
For connection I have gone from magnavox component video, and downmix audio to TV component input and associated audio socket
The plasma does have an HDMI interface, do I have to plug into this to get HD pictures/sounds?