I recently bought ONKYO 7.1 receiver and a set of speakers (7+1sub woofer). The DVD player I have has only Digital Coax Output and no Optical output. The retailer where I purchased the system says that using Digital Coax cable, I can't get the full 7.1 channel effect. They say it only supports 5.1 Channel. Is this true ? Does optical cable interface is the only thing that supports 7.1 ? (besides HDMI)? Appreciate your opinions.