@skriefal: Is your old RCA receiver capable of decoding a Dolby Digital (AC-3) encoded digital signal? Yes it recognizes Dolby 3 and it is not really that old, maybe 5 years.
the receiver may need to be told to "map" the digital input to one of the named inputs (DVD, TV, etc). If so, and you haven't done that, then it's probably ignoring the digital input and is instead using the analog input I have no idea to what you are referring, it is Greek to me. As far as using the analog input the cable tv signal is entirely HD digital. There is no more analog tv broadcast in Canada for many months now.
As an aside, what the heck is LG's Triple XD Engine? Can't find a simple description of what is is, only advertizing gobbledy gook.