I have an external USB DAC and to get it to work as the default I've created a $HOME/.asoundrc file containing:

defaults.ctl.card 1
defaults.pcm.card 1
defaults.timer.card 1

OK - this works, and (purely out of interest) I've Googled pretty extensively, have found out the purpose of 'ctl' and 'pcm' but so far I still haven't managed to find out what 'defaults.timer.card 1' actually does.

Can anyone enlighten me?