Your Cable company does not broadcast digital PCM on all channels. Only on your Digital ones. Also your Cable Box does not have A/D converters in it. D/A yes.
Motorola came out with the DTC2400 to correct this issue. By installing a A/D converter in the set top box. For non PCM channels. their A/D converter sounds like crap. Keep it analog for now.
A lot of Satellite boxes on the market have the same thing. They might output Digital 2.0 and only on certian movies will they output Digital 5.1 It also has to do with the broadcast.

Hope this will help