Using an oscilloscope on the DB60XG, the AK4510 digital inputs (CMOS/TTL) are pretty simple:

Pin 17 SDTI = Serial stereo input A/D data, MSB first
Pin 13 STD0 = Serial stereo output D/A data, same format
Pin 12 SCLK = Serial Data Clock = 48xFs = 2.1168 MHz
Pin 11 L/R frame clock - Fs = 44100 Hz sampling rate, High=Left, Low=Right
Pin 9 CMODE = H -> MCLK = 384 Fs

For a clean (pure digital) signal, the digital inputs to the AK4510 could be buffered and sent to a SPDIF transmitter IC for digital SPDIF (coax/fiber) input to a sound mixer.