Catalyzes the introduction of C-3,4 double bonds into 1-hydroxyneurosporene (1-HO-Neu) to yield demethylspheroidene (DMS). A + rhodopin = (3E)-3,4-didehydrorhodopin + AH2 Carotenoid biosynthesis; spheroidene biosynthesis. Belongs to the carotenoid/retinoid oxidoreductase family.