Old TV / New TV

With digital television there is an additional reason for the required scan.  It allows the tv to decode and remap the physical frequencies and sub-channel data streams to the logical channels you actually see (3.1, 13.5, etc).  In other words, with the old analog system channel 2 broadcast on a specific range of frequencies - always.  Channel 3 was just above it, etc.  Every tv was hardwired to use those frequencies all the time.  Now with the digital system the channel numbers don't have a direct relationship to the frequencies used to broadcast them.  Channel 2.1 might actually be broadcast using a frequency which on the analog system was used for channel 67.  To confuse it more, channels have sub-channels which are just compressed data combined with the data for the main channel.  Then to confuse it more, a sub-channel coming from one broadcaster may actually be remapped to a different main channel.  So the local CBS station might own the rights to or lease out to the local CW affiliate, one of its subchannels.  The CBS station broadcasts the CW programs on channel 13.5 but that gets remapped to channel 20 so it appears that CW is broadcasting its own stuff when it isn't.  The scan allows the tv to figure all of this out. 

Clear as mud.