Clipping a Frame
You can specify that the driver clip the frames. (If the other capture filter sections are omitted, this may be the only function of your capture filter). If the nFrameBytesToCopy field is not zero (0), its value specifies how many bytes of each frame received to retain. If the field is zero, then the whole frame is retained.
You may clip any frame that passes the other portions of your capture filter by setting nFrameBytesToCopy.