[mythtv-users] Suggestion: Splicing channels.

[Joe Votour]

I don't know what format captures from the software
cards come in, but I'm assuming it's mostly a raw
frame format (i.e. something like ARGB8888, where
there's full information).  This information is later
compressed by MythTV into something that doesn't take
as much space on the hard drive.  You might be able to
take advantage of this and strip away what you don't
need.

MPEG-2 is a different story though.  If you can tell
the capture card to capture only a certain screen
region (as opposed to a screen size), then you might
be okay.

If you can't specify the screen region to capture,
then you must parse every MPEG-2 packet, since any of
them could contain a change to anywhere on the screen
(unless the capture card is using I-frame striping,
which somewhat alleviates this issue, as you'd be
guaranteed that the changes are isolated to a strip of
the screen, not the entire screen).

I'm still learning about this stuff myself (it's my
job), but I have the advantage of having hardware
MPEG-2 encoding and decoding, with no processing
needed.  :)

-- Joe

--- Nicholas McCoy <mccoyn at gmail.com> wrote:

> Once you define the block you need, there really
> shouldn't be any need
> to decode any more of the video.  I'm not sure how
> much this could
> save, but it seems a waste to have the computer to
> struggle when I
> only want 10% of the video.  In fact, for software
> encoders it
> shouldn't be encoded at all since there is little
> use for a non-live
> (or close to live) crawl.  I've thought about this a
> bit, because I
> rely on a PVR 350 for my output.  Perhaps it would
> be possible to play
> the crawl as an overlay like the on screen display. 
> This would of
> course all have to be done in software.
> 
> On 4/23/05, Joe Votour <joevph at yahoo.com> wrote:
> > Part of it is who would care, but also, from a
> > technical standpoint, it's very difficult to do.
> > 
> > First of all, you need multiple inputs.  So, you'd
> > need two capture devices (one per channel).  If
> you're
> > using software capture, that requires a lot of CPU
> > (about 1GHz per tuner, according to the official
> > MythTV documentation).  If you're using hardware
> > MPEG-2 capture, that helps on the capture, but you
> > still get screwed over on the playback.
> > 
> > In order to playback that data though, you have to
> > make a composite screen of the two sources, and
> that's
> > where it gets tricky.  I don't know the internal
> > workings of the MythTV MPEG-2 playback engine, but
> I
> > can say that it will require a fair amount of CPU
> to
> > combine the streams (because processing has to be
> done
> > on each stream to determine what parts to cut out
> and
> > leave in).  (Even if you have a PVR-350 for
> output,
> > this part is required, because you have to feed a
> > contiguous signal to the decoder, which means
> > pre-processing in software).
> > 
> > So, to do this, I'd say that you're looking at a
> > minimum of a 2GHz machine (for all hardware
> encoding),
> > and you'd be unable to use two tuners for anything
> > else.  Probably not worth it for the average
> person
> > (heck, I'm a baseball junkie, and I get enough of
> it
> > by just watching a game, and monitoring the ticker
> on
> > the screen - I have yet to use PiP for that
> purpose).
> > 
> > -- Joe
> > 
> > --- Kyle Rose <krose+mythtv at krose.org> wrote:
> > > Nicholas McCoy wrote:
> > > > Well, the NFL draft is today and the folks on
> ESPN
> > > are yakking on.
> > > > Now, the most important information on that
> > > channel is the bottom
> > > > couple inches.  I'd like to watch something
> else
> > > on most of the TV.
> > > > It would be nice if I could take the bottom
> couple
> > > inches off one
> > > > channel and watch on another.  Of course, this
> > > feature would work for
> > > > sports, stocks and news as well.  Does anyone
> else
> > > think this feature
> > > > would be worth implementing.  Also any ideas
> of
> > > how to implement it.
> > >
> > > Not to muddy the discussion, but what the
> hell...
> > >
> > > In general, it'd be great to be able to combine
> or
> > > select from among
> > > multiple inputs in some way more interesting
> than
> > > picture-in-picture.
> > > For example, the way I watch baseball is
> generally
> > > to turn on the TV,
> > > mute it, and then put on the radio broadcast,
> since
> > > the radio announcers
> > > are typically much better than the TV
> announcers.
> > >
> > > Unfortunately, this doesn't work out so well
> when
> > > one wants to
> > > synchronize the sources, as the TV will always
> be
> > > 3-4 seconds behind the
> > >  radio broadcast.
> > >
> > > I can do something like this if I use MLB's
> Gameday
> > > Audio to listen,
> > > because I can insert almost any delay I want
> into
> > > that; but it's kinda
> > > ugly to have to do this when I could accomplish
> much
> > > the same thing with
> > > input selection and a radio tuner module.
> > >
> > > Anyway, I digress.  Being able to grab the crawl
> > > (or, really, any fixed
> > > block) out of one recording and superimpose it
> over
> > > another recording
> > > would be neat, but I don't know how many people
> > > would care.
> > >
> > > Cheers,
> > > Kyle
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