I tried hooking my RF modulator to an upstairs cable jack and used a mini camera for the video source. I watched for my video on my TV downstairs. I tried four different unused channels, and got nothing but slight changes in the white noise.
One thing I noticed is that most of my unused channels have a mostly dark picture, unlike the even mix of black and white I get with the cable disconnected. I think I may be competing with signals that are not analog TV but still used for something. According to my cable modem status page, it is using a downstream frequency right in an unused cable TV channel frequency. I want to hook up some form of spectrum analyzer so I can make sure these channels are really not in use. I may try just hooking up a ham radio HT and checking the signals strengths.
I also might try disconnecting my cable TV line at the splitter and trying again, just to see.
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I have been looking for more info on this, and apparently, unused cable channels are used in this way quite a bit, usually for video surveillance systems. Typically a filter is used to prevent digital video or Internet users on the frequency from interfering with the signal and vice versa. If you go without that filter on a truly unused channel, you could broadcast to neighbors and get yelled at by the cable company.
The best post I found on this is:
http://forums.snapstream.com/vb/off-topic/43277-channel-modulator.html#post276172I could definitely see someone running a pirate station like this in a college dormitory. (Although I'm not suggesting anyone do it.)