New to this forum...thank for any help.

My family just purchased a new 42" plasma and it is connected to our Onkyo receiver for surround sound. I don't have anywhere to put my center speaker so I changed the tv speakers to be the center speaker. However, the sound from the tv does not sound like it is in unison with the rest of my speakers, it almost sounds like my surround speakers are a split second behind the tv speakers and it almost echoes. I am using HDMI connections from cable box to tv and an optical connection to my receiver. Is there any way around this? Let me know if you need more info.

Thanks.