Optical output / bit transparency / external clocking.

The Macbook Pro hardware is capable of bit-perfect output. Of course, your software must also be capable of bit-perfect playback. iTunes is capable of bit-perfect playback if properly configured.

Bit-perfect output can be verified by playing a DTS encoded audio file.

You can download a DTS encoded WAV file here:
http://sverigesradio.se/sida/default.aspx?programid=2446
See Multichannelcheck at bottom of page. The unzipped file name is SURROUNDTEST_011212.wav

DTS audio cannot be decoded if it has been modified by sample rate conversion, lossy compression or Digital Signal Processing. If using iTunes for playback, all Digital Signal Processing must disabled as described in Apple article "Playing DTS audio files with iTunes":
http://support.apple.com/kb/TA47850?viewlocale=en_US

DTS audio playback requires a digital connection to a DTS decoder. Connect the Macbook Pro's optical output, or an Airport Express Base Station's optical output, to an audio receiver or processor that has a DTS decoder. Set up the receiver or processor to automatically detect the format of the digital audio signal.

If the bitstream is unmodified, the receiver or processor will automatically engage its DTS decoder and correctly reproduce the 5.1 channel audio signal. If the bitstream is corrupted, the receiver will indicate the source is digital PCM and will produce noise that sounds like static.