I've gotten a couple of lineup tickets where member were confused about Broadcast vs Logical channel numbers. I thought I would make a post to try and resolve some of the confusion.
Under the old NTSC ( analog broadcast) life was simple. Your local station marketed as "Channel 5" broadcast on a frequency that was tied to channel 5 on your TV tuner. Even your cable company tended to retransmit "Channel 5" on their number 5.
ATSC ( digital broadcast ) is more complicated. Everyone still transmits on a frequency with a FCC channel number, but that number is usually hidden from consumers. The station transmits a "Logical Channel Number" and subchannel that is what most users see. The logical channel number often has no relation to the physical channel number. In addition, while only one station could transmit on the same physical channel, it's possible for two stations to transmit the same logical channel number (although it shouldn't happen).
There's also the concept of subchannels. A subchannel isn't a different frequency, but an additional audio/video stream mixed in with the "primary" signal. All subchannels coming out of a transmitter usually have the same logical major channel, but that's not always the case. Your "channel 27" and "channel 33" may actually be coming from the same transmitter now!
I've modified the lineup report on the SD web site to provide "logical" and "broadcast" channel numbers. Hopefully this will help you troubleshoot your systems as the physical (broadcast) channel number is often hidden.
Here are some examples:
In Dallas, TX the station that's marketed as "Channel 5" used to transmit analog on Channel 5. Digital transmission is on Channel 41 with a logical channel of "5-1". The analog transmitter is now shut down, but the station still calls itself "Channel 5" even though it has nothing to do with the frequency known as channel 5.
In Dallas, TX the station that's marketed as "Channel 11" used to transmit analog on Channel 11. Digital transmission *was* on another channel (44?) but when the analog station shut down, the digitial transmission was moved to Channel 11. The logical channel was always "11-1".
I don't have an example in front of me at the moment (I'll update this post when I find it), but I've seen a case where "11-1" is on one transmitter and "11-2" is on another!
Simple huh!
Robert