I'm working on a mythtv box but one thing I was wondering was, will the mythtv box eliminate the need for my cable box. Will I be able to view the higher channels (digital calble) without the cable box provided for me by my cable provider (Charter).

no and no.

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Have a question search the forum and have a look at the KnoppMythWiki.

Xsecrets

Then how will the channel be changed on my cable box and on the mythtv box at the same time? I know on my tivo I have to use the ir remote cable that connects to the back of the tivo.

Bingo. You need to set up an IR blaster.

any recommendations for the ir hardware I'd need to do the channel changing?

I don't use one myself, but a search for "IR blaster" should turn up plenty of info...