Device for capturing analogue signal at composite video frequencies
Stop and think about the bandwidth requirements.  You didn't say what kind of video, so I'll use NTSC as example.  The color carrier is 3.6 MHz, so the theoretical minimum sample rate is 7.2 MHz.  In practice, you'll probably need 15 MHz samples to do useful things.

There are certainly A/Ds that can convert at 15 MHz rates to 8 bits or more.  The problem is then moving and dealing with 15 Mbyte/s, or 120 Mbit/s, without counting any protocol overhead.  You'll need at least "high speed" USB for that.  "Full speed" uses 12 Mbaud, and the data transfer rate is even less due to protocol overhead.

120 Mbit/s sustained can be done, but you have to think about things carefully.  Then how are you going to process the 15 Mbytes/s data stream?  This would be doable in the right kind of FPGA, but you're not going to find a programmable processor to handle that, at least not in real time.

If you just want to store a frame or two and decode later not in real time, then things get easier.