It's not the codec that is the problem - as I understand it, it is the recovery of the digital stream from the AM or FM signals that is the problem. A 400MHz Blackfin is not sufficient for that. The first HD decoders used an array of Sharc DSPs. Later ones (TI chips) use single C64xx class DSPs. More than that I cannot say without breaching non-disclosure agreements.
Roku M1000, M1001, M2000, R1000, Roberts WM-201, Stream 83i