[LAU] mystic 1ms border in sound cards or ALSA?

Hi all, in several audio devices I observe a minimal block size setting of 1 ms: - In the Echo Layla 3G, it is possible to configure smaller block sizes, but the driver or device seems to use 3 fragments instead of 2. - With the RME HDSP9652, I hear artifacts when I configure a fragment size which is smaller than 1 ms. - With the RME HDSP9632 I am not able to configure a smaller fragment size at all. Does anyone know about the source of this mystic border? If it is in the ALSA abstraction or driver layer, what is the reason for this? Is it simply the assumption that nobody needs shorter fragment sizes, and shorter fragment sizes are likely to produce xruns? Or is this just caused by the hardware? Thanks for any hints! Giso