[LAU] Jack, HDA and mic input

Hello all, I got a new computer recently, with onboard audio (ATI Technologies Inc SBx00 Azalia (Intel HDA) according to ALSA). I discovered that if there's no microphone connected, jack (jackdmp 1.9.5) doesn't run and outputs a continuous stream of "alsa_driver_xrun_recovery JackAudioDriver::ProcessAsync: read error, skip cycle". As soon as I connect a mic, it works normally. When starting it with the -P option (playback only), mic presence doesn't affect jack operation, but of course there are no capture ports available. It doesn't make much of a difference for me right now, but just for curiosity's sake, I'm wondering if it's possible to start jack without a mic and have capture ports appear automatically when one is connected. Thanks, Alexandros