[mythtv-users] Is the pchdtv card looking good?

Brandon Beattie bbeattie-maillist at linkexplorer.com
Thu Sep 18 00:56:57 EDT 2003


On Wed, Sep 17, 2003 at 09:56:21PM -0700, Jarod C. Wilson wrote:
> On Tuesday, Sep 16, 2003, at 21:03 US/Pacific, Brandon Beattie wrote:
> 
> >>> I
> >>> believe that saving a single stream is possible and could be very
> >>> beneficial from within software, It would be nice to see a stream
> >>> selection when HDTV really gets going so we can drop the lower 
> >>> quality
> >>> stream in transit(and vice versa) and just save the one we want, thus
> >>> saving LOTS of hd space.
> >>
> >> My understanding is that you actually can do this with the pcHDTV. Or
> >> at least, you can choose not to capture the unwanted audio streams 
> >> that
> >> are usually muxed in (i.e. SAP). I'm not certain about multiple
> >> resolutions of HDTV streams. Is it your understanding that there
> >> multiple HDTV resolutions broadcast for each HD channel? I thought it
> >> was just one, and any down-sampling was done at the client end... Or
> >> did I misunderstand what you were saying?
> >
> > An ATSC signal is up to 40Mb/s of data.  It can be used for almost
> > anything (Audio or video mpeg2).  It's an mpeg2-ts stream, meaning it's
> > a transport stream of an mpeg2 file that has built in redundancy for
> > packet loss.  In the 40Mb/s ATSC signal, you can have X number of video
> > streams and X number of audio streams _in the same channel space_.  In
> > my area, 4 stations broadcast only 1 HD channel and 1 audio channel.
> 
> I guess I just thought one HD, one audio was the norm. Goes to show how 
> very little I know about HDTV. I bow to your HDTV knowledge!

I bow to others, it's a great circle. :)

> > Both PBS stations have a SD (standard def 720x480p) that is the same as
> > their NTSC broadcast and they also have a second audio and video
> > (subchannel is what they are called) for HD broadcast that are almost
> > always a different program schedule than that on the first subchannel.
> 
> Odd. PBS here (on Comcast cable, that is) has one NTSC channel, then 
> three distinct HD channels. I'm thinking the cable companies probably 
> want to limit how much bandwidth gets sucked up, and thus break things 
> out to different channels, and only one resolution each. Just 
> theorizing though...

They could be doing that... never heard of it being done that way
though.
 
> If multiple streams are on the same channel (HD and NTSC), how does one 
> pick out which to use? I'm assuming the receiving hardware just 
> auto-picks what it thinks is best... I really ought to spend some time 
> on the AVSForum site...

No, tuners all you to select subchannels.  If I try to flip from 7.1
(PBS NTSC station) to the "next channel up" a HD tuner would go to 7.2
(PBS ATSC station).  It's the tuners job to change stations and then
display whatever subchannel.  And in the case of this "channel change
up" there is no change on frequency listening, it's just that it drops
subchannel 1, and shows subchannel 2 (as before it was showing channel 1
and dropping subchannel 2)

> > NBC and CBS both broadcast SD and HD versions of the same program
> > schedule at once, and ABC shows their HD broadcast an hour before teir
> > SD broadcasts/program schedules.  (And causes me to often record the
> > wrong show.. (No HD broadcast listings availible yet for Myth)
> 
> D'oh!
> 
> > This should clear things up a bit.
> 
> Yes and no! I'm still kind of muddled on a few things...
> 
> >>> Transcoding is yet another thing and is rife
> >>> with all it's own problems, but the newer transcode engine seems to
> >>> handle
> >>> everything now (last spring it didn't)...
> >>
> >> I'm curious to see how long it'll take to transcode a one-hour HD
> >> show...
> >
> > To transcode from mpeg2-ts to mpeg2-ps (Ie, used for dvd's, and it just
> > has the erducdency taken off, and you can also strip off the unwanted
> > subchannels if there are any) it takes a few minutes.  All you are 
> > doing
> > is dropping off data from the current file mostly.
> 
> So it isn't actually the pcHDTV card that drops the extra audio and 
> video channels, it is transcode that does this work?

The pcHDTV card tunes to a frequency (ie, channel 7) you get all
subchannels (7.1 and 7.2) at the same time.  Then in xine or mythtv
(which are being used for the "tuner") it displays subchannel .1, or .2
and ignores the other.  But if you use the dtvdump program on channel 7,
you get everything, 7.1, 7.2, all written to a single file.  Then it's
up to a player (Xine, mplayer, mythtv) to have support to pick which
subchannel to play in that stream/file.

--Brandon
 
> --Jarod
> 
> -- 
> Jarod C. Wilson, RHCE
> 
> Got a question? Read this first...
> http://catb.org/~esr/faqs/smart-questions.html
> 
> MythTV, Red Hat Linux 9 & ATrpms documentation:
> http://pvrhw.goldfish.org/tiki-page.php?pageName=rh9pvr250
> 

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