You don't necessarily need to have everything go through the receiver. In fact, you will probably do better otherwise given the receiver doesn't have HDMI. The main problem is I don't know which comcast box you have (please post that info if you can). You will need two sets of component video cables (R/G/B), one digital optical cable, and one digital coaxial cable. Monoprice.com is a decent and inexpensive place to obtain these cables. The big box stores normally overprice all of them. Radio Shack is starting to do it too unfortunately.
Give the following connections a try:
Sony DVD player (Component video R/G/B out) -> (Component video in) Toshiba HDTV
Sony DVD (digital coxial out) -> (digital coaxial in) Yamaha receiver
Comcast cable box (Component video R/G/B out) -> (Component video in) Toshiba HDTV
Comcast cable box (digital optical out) -> (digital optical in) Yamaha
You would have to set the input on the Toshiba HDTV for one of the component video connections to be paired with the DVD player. If that procedure isn't clear in the Toshiba manual I can post the PDF version. No worries 
If you wanted to use the receiver for upconverting SD video from the DVD player to your HDTV you could also try that with another set of component cables. In other words, instead of going from the DVD player directly to the HDTV, the receiver would be in the middle to process the video.
-Pedro
I recently purchased a cheap DVD player to hook up to my Toshiba REGZA 32RV530U through my Yahmaha HTR 5850 receiver. I have my comcast cable and surround sound/subwoofer hooked up as well. The DVD player came with the red, yellow and white cables but I am at a lost as to what else I might need to hook up to my receiver?? I assume everything goes throught the receiver and not directly to the TV? Do I need a coaxial (digital out)
In addition what do I need to set my TV to in order to than play the DVD's.
I will apologize in advance as to being electronics savvy.
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