[mythtv-users] Audio - PCM, Master

[R]

In MthTV we have:
MythTV->Main->Utilities->Setup->General->Audio->Mixer Controls-> PCM or Master

In Fedora Core 4 we have:
KMenu->Sound & Video-KMix->Current Mixer->Input->PCM

Pardon my ignorance.

What is PCM?
What is the difference between PCM and Master?

Given that my MythTV box:
1.  Runs both frontend and backend.
2.  Has 1 TV Tuner card.
3.  Has 1 sound card.

When should I use PCM?
When should I use Master?