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Can the Linux projects for PVR media centers be used to create a personal content firewall?
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Most of the newer TVs and cable boxes have parental controls built-in. You can set the parental controls to lock-out individual channels, lock-out anything that is rated TV-14 or TV-MA. You can also combine these with timed lock-out so that the channels are unlocked after a certain time or so nearly all channels are locked-out before Family viewing time starts (time for kids to do homework). I can attest to this system working, BUT beware, it can make it a PITA for you the adult to watch TV. You constantly have to enter the pin code to watch the channel, even if you are just skipping around to find something to watch.
One of the easiest things you can do is remove the bad channels from the line-up. Cancel a movie channel subscription of remove it from the remote. That way if the kids are scrolling thru the channels they won't roll thru the baddies, but you can manually enter the channel number.
Until MythTV and Freevo get something like Dan's Guardian for TV channels, there are your answers. Or you could just unplug it.