Logical vs Broadcast channel numbers

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Logical vs Broadcast channel numbers

Postby rmeden » Mon Sep 28, 2009 9:30 am

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
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Re: Logical vs Broadcast channel numbers

Postby the Goat » Wed Oct 21, 2009 1:20 pm

FYI:

When WCFE's (logical 57, broadcast 38) broadcast antenna collapsed in 2007, a different station, WCAX (logical 3, broadcast 53 [at the time]), broadcast WCFE as a subchannel. So broadcast channel 53 was providing logical channels: 3-1, 3-2, & 57-1. WCFE and WCAX are located in different states (New York and Vermont) and their regular transmitting locations are about 100 miles apart.

more info here:
http://en.wikipedia.org/wiki/WCFE-TV#20 ... r_collapse
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Re: Logical vs Broadcast channel numbers

Postby agideon » Thu Apr 28, 2011 8:51 am

While this thread is informative, it appears to be missing some basic matters of practicality. I've come into this with zero knowledge and there are some things I still don't grasp after reading this thread. I'm not even sure I know enough to fomulate a coherent question.

I've digital cable from Comcast, listed as "Comcast Union/Verona - Digital". That feed on SD perfectly lists what I get from the STB provided by Comcast.

But I just purchased a WinTV-HVR 950Q. This has the ability to receive the digital signals directly. What I don't grasp is how to map what the new device sees to any listing information. But this issue applies even to my digital-capable TV when connected directly to the feed from Comcast. Both devices appear to show me what I believe are the "logical channel number" and "subchannel". How does that get mapped to listing data from SD?

If it matters, the software that is grabbing the listing data is MythTV. It too shows me the "logical channel number" and "subchannel". For some, MyTV is showing me listings. But I'm not sure that this is coming from Schedules DIrect; it could be using information received via EIT. Or maybe not.

For example. 63_4 calls itself on the MythTV listing page "SinoVSN". This doesn't seem to correspond to anything I see on https://www.schedulesdirect.org/lineups ... J29444%3AX so I'm guessing that Schedules Direct isn't providing the listing data that MythTV is displaying. But maybe I'm just missing how 63_4/SinoVSN does somehow map to something in the SD feed.

Any help in mapping between the "logical channel number" and "subchannel" and the listing data would be very much appreciated.

Thanks...Andrew
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Re: Logical vs Broadcast channel numbers

Postby rmeden » Thu Jun 02, 2011 7:39 am

Digital broadcasts use the ATSC protocol.
Digital Cable uses the QAM protocol.

What you're seeing on cable is what's called "Clear QAM" (unencrypted QAM). Unfortunately, supposedly the cable companies do not want Tribune Media Services to distribute QAM tuning info.

This may be because cable companies have the ability to move the QAM stations around quite often (and historically have) transparent to uses of their cable boxes.

It also can be because they're jerks and want to make things difficult for those not using their hardware. :)

The best solution for clear QAM right now is manual mapping of tuning info and guide data. The HDHomeRun folks have some tools that can help with this on their web page.

Robert
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