Modern versions of MythTV can do this for DVB-T/S/S2, at least for unencrypted channels. The only restriction is that you need as many tuners in the server as there are different multiplexes (not channels) being watched. Even that thing I lashed up can do it for DVB-T radio, though I never bothered trying to get video to work. It's all in UPnP AV v2.

[Edit on rereading your post] It doesn't "decode" them as such, of course; it distributes the raw, encoded MPEG2 stream around the house. But that's what you actually want.

Peter