Hi Paul,

As Erik said, the modulator/demodulator has already been invented. And, also as he said, it wasn't originally intended as an internet device. It was just a means of converting digital data into, and back from, an analog signal (Frequency Shift Keying) that could be sent over the phone lines (just as you're thinking).

Now for something of interest to you, at least as I understand your question. Back in the early to mid 1980s I worked in security (alarms, monitoring, access control, etc.). At the time the state of the art in sending video required SSTV (Slow Scan TeleVision). Basically, it meant sending a single frame of video every 5 or 10 seconds. Of course, at the time 1200 baud was pretty much the fastest modem you could find. Now that modems can get up to 56k, I have no idea how much video you could send over the phone lines. Calculate how many bits comprise each frame and see how many frames you can send each second. The bottom lines are that I suspect you can do it if you can accept jumpy video, and there is no need to invent new hardware to get it done.