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Re: How does HD Programs work?
Local stations, say the NBC affiliate on channel 10, will broadcast on their shows on the VHF band for channel 10, and it's an analog broadcast that your old TV could handle (plus it was in SD, and according to NTSC standards so that your old TV will display the broadcast properly). That same NBC affiliate will also broadcast a digital signal (according to ATSC specs, and in HD, 16x9 aspect ratio) in the UHF band (for this example, let's say it'll be on channel 25, sub-channel 1), a HDTV tuner that can pull in and tune to channel 25-1 will take the ATSC digital signal being broadcasted, and convert it so that a HDTV set will display it on the screen.
Even though it's the same programming, 2 different broadcasting streams are used, one in SD with NTSC specs, the other in HD with ATSC specs. The SD/NTSC broadcast is formated so that the content will show in a 4x3 aspect ratio display space on the TV, while the HD/ATSC broadcast is formated for 16x9 content, and sometimes you'll see pillarboxes on the sides of commercials with 4x3 aspect ratio to make it look normal inside the 16x9 display area. ESPN will pillarbox footage that is in 4x3 aspect ratio as well for their HD broadcast.
A station can also split their HD channel band (UHF) into multiple sub-channels if they want to simu-cast other content, so sometimes you'll find that your ATSC tuner can find 25-1, 25-2, 25-3. With March Madness NCAA basketball on the horizon, CBS affiliates can show many different games going on at the same time on their one UHF channel band within their sub-channels, but simu-casting and splitting that band into many sub-channels will degrade picture quality for each game shown on its own sub-channel.
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