I have an SA 8300 HD DVR from Charter Cable. The problem is that I cannot get captions to display on TV when using HDMI input. Captions will display OK with TV input set at "TV". When I change input to HDMI, I lose the captions. The TV is a Vizio SV470XVT though I understand that captions are decoded by 8300 and not the TV.
Enable captions on the 8300HD itself, not on the TV. There is no established way to send caption data over HDMI, so you have to do the caption decoding in the receiving device. I'm guessing the reason you are seeing them when you put the TV in "TV" mode is because you have a cable from the 8300HD's RF out to your TV's RF in? You won't be getting HD that way, so it's a pretty terrible way of handling the issue.
I'm not sure, I believe it depends on specifically which software your cable operator is running (SARA, Navigator, iGuide, Passport, ...) on their boxes. You might have to call up support, if they don't provide any online documentation covering the basics of using it.