streaming video from a web cam isn't "simple" to begin with. your best bet is a PC with a video card that has analog S-video or composite video outputs and the appropriate software. a "standalone" converter would likely consist of a USB controller, a video DSP chip, about 1G of RAM and a high speed DAC with a 140+Mhz clock rate. the DSP code you would have to write from scratch. with larger cameras that have analog video outputs, the DSP hardware is already a part of the system.