The Oppo BDP-105 does not currently play ripped HDCD files “decoded” so therefore, I have to make do with a workaround. Thankfully I own few HDCD titles, so this is not very time consuming at all to convert them into a higher resolution FLAC file which any digital player can render properly.
HDCD, in simplest terms, is essentially closest to a 20-bit sample encoded into a 16-bit sample. The data for this encoding is hidden in the least significant bit of the signal.
Some digital players, primarily computer based, can decode these files as though they are 20-bit and play them back as such. But for network media players, including those built into BD/SACD/CD players like the Oppo, support is variable. The BDP-105 is one of those which does not play this file back decoded.
The trick? HDCD can be decoded into a 24-bit file; the extra four bits of empty padding can be at the top or bottom of the digital file. If this padding is placed at the bottom (in the least significant bits), the signal played back will be six decibels louder; if placed at the top in the most significant bits, playback will be softer. Sound quality will remain the same either way, however.
The decoding to a 24-bit file can be done during ripping, or after the fact from existing FLAC files. I use dBpoweramp as my primary ripping and conversion suite for files. dBpoweramp has a DSP plugin for HDCD. You can run it on any CD rip or FLAC file. It will save a 24-bit file if it finds the HDCD encoding, but if it doesn’t, it will save it as the original 44.1kHz/16-bit file. (If you are converting FLAC to FLAC, it will leave an unaltered 44.1/16 file, so you need not worry about it having been altered by the program if HDCD encoding was not found.)
Thankfully I know which titles of mine are HDCD encoded, and it took only a matter of minutes to convert them.