From the ALSA wiki
|The FAQs have been reorganized. The new location of this question is FAQ# How can I enable "digital out" on my soundcard?|
How can I enable digital-out on my soundcard?
Many of the details are explained in DigitalOut. If that doesn't help you, check the documentation for your driver module (kernel source or http://alsa-project.org/alsa-doc/ or driver homepage) and also the corresponding wiki page in the ALSA modules and Sound cards sections for more info.