I have some security cameras who's only interface is a BNC connector and I want to use them to do video processing. I want to find a simple way to interface a BNC camera with my computer so that I can get the data. I figured that the only way is to create a simple circuit that can forward data from the pin through USB, then write drivers for it so that I can register it as a directshow video camera (on windows). Is there a better way to do it?
It's not anywhere as simple as you think.
It is likely that your "BNC" camera is a standard composite video camera.
As such, you need a fast ADC, and a bunch of logic to properly decode the video signal. You cannot in any way just wire it up to usb.
Your best bet is probably to buy an inexpensive video-capture card (google "video capture", there's lots of options). They run as low as ~$30.
If you really want to do it yourself, there are a few people who have done so out there, and posted projects online.
Here is one.
It's worth noting that if you want to do this in real-time, you will have to use a FPGA or very fast microcontroller of some sort. The above project is just a frame-grabber (i.e. it can take one frame every few seconds). Commercial products for video-USB generally use an ASIC of some sort.