>Thanks. but how to define a device ID? the help file doesn't say.

Assuming that the help files are your only resource is ill-advised. Even if you weren't enterprising enough to search the website for discussions of this topic, you have a nice tutorial on working with live input as well as the online reference manual page for the jit.qt.grab and jit.dx.grab objects.

This advice to beginners is given with some regularity: DON'T assume that the help file contains everything you need to know about the messages or attributes of a given object. DO assume that it's on the Reference manual page.

That advice will serve you well. Good luck.