Problem: Stations have spent a lot of effort branding themselves as "Channel 4", but "Channel 4" was just an index in a table of frequencies, and with the DTV change, the TV stations don't get to keep those same frequencies.
The PSIP (Program and System Information Protocol) is a way of supplying metadata in the MPEG stream of a DTV broadcast. Part of this metadata is a new virtual channel, normally set to the old analog channel number, a separator (dot or dash) and a new number.
It allows TV stations to keep their Channel number, even if it no longer has anything to do with their frequency.
So, Channel 4 on the old analog system becomes Channel 4.1 on the new system.
The first sub-channel number (.1 or -1) in the virtual channel is normally where broadcasters will put their new main feed, with alternate feeds being sent on higher number sub-channels.
The Channel Scanner now needs to not only determine if there is a signal on a given channel, but what virtual channel number that signal represents. When scanning for channels, the channel you see being scanned likely has nothing to do with the virtual channel number you may now be familiar with.
Explain how to connect channels scanned via a channel scanner are connected to a channel in your listings source.