Okay. So, the quest is this: record the entire spectrum of TV. Cable, broadcast, whatever. Let's say, for the sake of discussion right now, that I want to record all the ambient broadcast TV.
A few facts that I've managed to piece together:
A typical TV signal as described above requires 4 MHz of bandwidth. By the time you add in sound, something called a vestigial sideband and a little buffer space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC allocated three bands of frequencies in the radio spectrum, chopped into 6-MHz slices, to accommodate TV channels:* 54 to 88 MHz for channels 2 to 6
* 174 to 216 MHz for channels 7 through 13
* 470 to 890 MHz for UHF channels 14 through 83The composite TV signal described in the previous sections can be broadcast to your house on any available channel. The composite video signal is amplitude-modulated into the appropriate frequency, and then the sound is frequency-modulated (+/- 25 KHz) as a separate signal.
VHS tapes have about 3 MHz of video bandwidth. So that's why the quality is lower, I suppose, on VHS tapes. The signal is compressed somewhat.
To tape 100 channels at "full" quality would cost about 600MHz of bandwidth. At VHS quality, it seems like that would be 300MHz.
It looks like you'd need a special oscilloscope or other device to record the whole thing. I'll keep working. Hmm hmm.
I *love* projects like this.
A few useful sites:
http://electronics.howstuffworks.com/tv9.htm