Saturday, February 14, 2009

Understanding Subchannels and the DTV channel system

I just realized that information about the physical digital channels might be confusing to the average person, so let me try and explain how the channel system works.

We know for a fact that two things cannot occupy the same space at the same time. Right? Same is true with radio signals. You can't broadcast analog and digital on the same physical channel at the same time. Thus, every full power station is currently broadcasting a digital channel on a different physical than their analog. After stations cut their analog signal, some will move their digital channel to their old analog channel (ie, KTTC moving from 36 back to 10), some will stay on their current digital channel (ie, KXLT FOX 47 staying on 46), and some will move to a completely new channel (not in this market, though).

Obviously, this would create a mess if viewers had to completely re-memorize their channel lineup and stations had to change all their branding to match their digital channel. For this reason, the digital TV standard allows stations to broadcasting a little thing called remap data. This tells the tuner which channel to display it as. For example, KIMT-DT tells your digital tuner (converter box, TV, whatever) "display me as 3-1 and 3-2). Without this piece of information, it would be 42-1 and 42-2. In fact, it has been reported that KIMT's equipment malfunctioned once and the channel came up as 42-1 and 42-2 for a few hours until the engineering staff fixed it.

So why do I need to care about physical channels again? For antenna selection. If you have a VHF only antenna (ch. 2-13), you will no longer get ch. 3 because it is actually on ch. 42. Also, when troubleshooting it's really handy to go into manual tuning and look at signal levels of stations which are too weak to actually decode.

Subchannels. Digital TV isn't so much TV standard so much as a standard to transmit a fixed amount data, be it television, audio or data. TV stations can allocate their bandwidth (19.4 Mb/s) for different uses. For example, KAAL-DT allocates all of their bandwidth to channel 6.1 (ABC) which allows for high quality high definition video and 5.1 surround sound. KTTC-DT on the other hand still broadcasts in HD on 10.1, but uses a portion of their bandwidth for standard definition CW on 10.2. Meanwhile, KSMQ splits their bandwidth into four Standard def channels (15.1,.2,.3,.4).

Wisconsin Public Television used to have an arrangement where they had five subchannels, four of which were SD and one HD. During the daytime, the four SD subchannels would broadcast programming while the 5th sub would display a message notifying viewers when the channel would start broadcasting. At said time, three of the SD channels would cut programming and display a message notifying viewers when the channel would be active again, while the HD channel and one of the SD channels had programming on them. This is a case where the station's channel bandwidth allocations were automatically reconfigured.

[It should also be noted that every station uses a tiny amount of that bandwidth for channel re-map information, guide data, ratings, their callsign, etc.]

Stations can also broadcast audio only channels, encrypted subchannels and datacsts. While audio only channels have not seen much use, there was a company (USDTV) that bought bandwidth from local TV stations where they broadcast a few cable channels to viewers with USDTV boxes and subscriptions. The company went bankrupt a few years ago, and so far as I know nobody uses this feature anymore.

Digital stations can have up to 99 subchannels and the standard allows for remap channels 1-99. There has been talk of national translator networks, such as TBN or 3ABN using one remap channel across the entire country for consistency. In the Twin Cities, Twin Cities Public Television (tpt) owns two channels (KTCA and KTCI). KTCA will be on channels 2.1 and 2.2, while KTCI will broadcast on ch. 2.3 and 2.4.

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