qjackctl launch an alsa driv with only 16 bits of capture for an audigy 2 zs (24 bits)

I have an audigy 2 zs sound card and i use qjackctl (configured in RT with lunux-rt). My linux is hardy 8.04 with KDE. Unfortunately Jack launch an alsa driver with only 16 bits for capture whereas the sound card supports 24 bits. I don't find any parameter in ALSA to set or force the use of 24 bits. By searching at some jackd log on google I realize also that some user had a jack log trace with 32 bits (how do they do ?!)

messages from qjackctl

creating alsa driver ... hw:0|hw:0|1024|2|44100|0|0|nomon|swmeter|-|32bit
control device hw:0
configuring for 44100Hz, period = 1024 frames (23.2 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 16bit little-endian
ALSA: use 2 periods for capture
ALSA: final selected sample format for playback: 16bit little-endian