How do i connect my Digital TV box Via my dvd home cinema then to tv?

Hi there I have a LG HDTV 32” (RZ32LZ55) and a LG home cinema system (LHT550TB) the DVD player is connected via a component cable to the TV, all of that works fine sound, picture fine, but when I come to connect my digital box (set-top box, homechoice) via an output scart to the AV in jacks on the back of the DVD player I get sound but no picture when I go to aux av1, I want the digital TV to go in through the DVD player and into the TV via the component cables for superior picture quality. Now I thought as there are the AV in jacks L sound, R sound and video in the back of the DVD player, the video in must be there for a reason so I’m wondering why it isn’t working??