I'm trying to figure out whether this is a problem with our cable installation or something else. I have hooked up one of our (Comcast/Xfinity) cable boxes to an agile modulator to broadcast cable throughout the house. Recently, I noticed that many of the lower channels (under 150 or so) are showing what looks interference with the vertical signal:
The interference bars fade in and out over a period of several seconds, and they seem to come down from the top of the screen. The interference only appears on vintage TVs (both B/W and color); modern TVs are immune.
I have seen similar interference when playing Macrovision-protected DVDs on old TVs, as in the following photo:
I can eliminate the Macrovision interference by running the signal through a video stabilizer, but I tried the stabilizer on this, just for kicks, and it had no effect.
The problem has nothing to do with my agile modulator (it appears when I connect the cable box directly to a TV with an RF modulator). I also swapped cable boxes and tried different locations in the house, to eliminate boxes and locations from the equation.
I don't have a photographic memory, so I can't swear that this interference never happened before, but it seems to be a recent development. Again, this is only present on some of the cable channels below about channel 150.
Any ideas about what's going on? At one point I wondered if it might be closed-captioning data, which is more likely to be present on our lower cable channels (largely local and network broadcasting) than on higher channels (HBO and the like). But I have no easy way to tell which broadcasts have closed captions.
I put my oscilloscope at a couple of spots in the vertical sweep circuit of my CTC-11, which is currently on the workbench, and didn't see any obvious signal distortions when the interference fades in and out.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html