View Full Version : How do I select a subchannel from a transport stream?
07-24-2003, 05:54 PM
I have a file I recorded with my MyHD card. It's the local PBS station, with a 1080i demo loop and their normal broadcast at 480P, each as a separate subchannel. It imported into the Library just fine after I added the extension to the sage.properties file. When I select it, SageTV shows the 480P signal, and I can't find any way to get it to show the demo subchannel.
07-24-2003, 06:14 PM
Is there anyway with the MyHD card to select which subchannel gets recorded to the file?
Isn't the bitrate extremely high for that file since it contains 2 video streams?
07-24-2003, 06:41 PM
Time for your HDTV edjumacation.
An HDTV "channel" is really a broadcast stream. That stream can contain several subchannels. Two is a very common number. Sometimes the content is the same, but the resolution or aspect ratio (or both) is different. Other times the content is different, as is the case with my local PBS station.
These subchannels will be identified by different PID's (I think that stands for Program ID) in the stream. The MyHD, and most other HDTV cards, record the entire stream unaltered to disk. They also allow you to select which subchannel gets decoded and displayed, whether you're watching live or from a stored file.
Regarding the bitrate, it never exceeds the 19.2Mbit/sec allowed, but it's possible to pack one 1080i and one 480p signal in there, believe it or not.
07-25-2003, 06:24 PM
I actually know about them (but not everything apparently), I've written an MPEG2 transport stream demultiplexor for Sage. :)
I hadn't recorded any PBS yet using an HDTV card (that's the only one in my area that does subchannels), so I didn't realize it saved them both to the same stream. I thought there were frequency sub-bands for each channel that could optionally be used and that the subchannels were carried each on a separate band. But now I see (thanks) that the subchannels are extracted in the demultiplexing operation, NOT in the tuning operation.
It's funny because I do have PID mapping in the demux already. It just uses whatever the PAT (Program Association Table) lists as the active streams.
So I guess you could get it to tune subchannels if you altered the PATs of the transport stream to indicate that another stream was the main one. Not that I know of a tool that'll do that for you. :)
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