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The virtual channel concept was also important during the transition from analog to digital broadcasting. I don't think many people appreciate the job the FCC did in overlaying a new TV broadcast service over the old one that minimized interference between the two while managing to essentially, not completely, replicate coverage. A little history:
Digital broadcast television was the first case where a new technology applied to a broadcast service was not backward-compatible with current recievers. Color TV and FM and AM stereo had to work on black-and-white TVs and mono radios. For digital TV, it was no longer a matter of adding extra information in a clever way to the existing signal. A whole new transmitting scheme was required.
Therefore, to implement DTV, broadcasters had to fire up a parallel broadcast on a separate channel. At the time, there was much wailing and gnashing of teeth about "giving away" spectrum to broadcasters. Those folks never had to equip a studio for digital, nor had to pay for erecting antennas and feed line up tall towers.
One question that came up was, how to tell people where the new channels are and make it simple to use. Since DTV is all-digital, adding additional information to describe what is in the stream, like a programming guide, channel identification, etc. is trivial. So, the idea of the virtual channel was born. This allowed labelling the digital channel with another number that corresponded to the analog channel number. You could then, with the digital tuners of the day, go from analog to digital with a single click on the remote (e.g., "4" for analog 4, then a click up to "4-1" which is RF 48 in my area.)
When analog went away, stations had three choices: continue with the existing DTV channel, move the DTV to the analog channel, or start completely over on a third channel. No matter where they ended up, the virtual channel allowed them to keep the same "identity," and you did not have to learn a third channel number for a station. (Note also that when analog went away, the TV spectrum went from 2 - 69 to 2 - 51.)
When we're working with antennas, we are dealing with, of course, the physical frequencies, so you have to figure out what those frequencies are. Enter tools like this one.
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