[mythtv-users] commercially-produced mythboxes -- an idea

[Brad Templeton]

On Sat, Jan 15, 2005 at 10:56:22PM +1100, Hamish Moffatt wrote:
> On Thu, Jan 13, 2005 at 12:53:48PM -0800, Brad Templeton wrote:
> > in the clear, on disk or on plain digital outputs.  (IIRC, you can
> > make it available as QAM or ATSC again, with the broadcast flag intact.
> > For a variety of reasons, a cheap ATSC modulator card would be a neat
> > thing for PVRs, providing high quality output to HDTVs over RF.  I
> > find this highly ironic.  In the old days, using RF from your VCR to
> > your TV was the deprecrated approach.  Today, it would be great, a
> > simple cheap cable with high quality signal bundling multiple
> > channels of audio and video together at perfect digital quality!)
> 
> I noticed you've mentioned this a few times. Do you think it would be
> better than DVI + S/PDIF, both of which are commonly available on PCs
> today?

DVI is uncompressed, hard to make work, and can only be sent over
a cable of limited length.  (For example you might not be able to run
it to a ceiling projector.)   For reasons unknown to me, many TVs treat
the DVI signal as just a digitized analog one -- it still has sync, it
does not contain the aspect ratio information or any data streams etc.

ATSC sends the original signal you recorded in the first place.  It
keeps the signal in its original compressed digital form -- possibly as
created in the studio that shot the show -- until the last moment, which
is the right thing to do.

Sending mp2 (by atsc or firewire) means the PVR needs minimal CPU to
watch.  (It still may need cpu, but not real time, for transcode and
other such events.)
> 
> It also means that your output signal must be MPEG2, even for the Myth
> menus, your X desktop etc. Encoding MPEG2 is CPU-intensive.

Indeed.  My understanding is that the mpeg2 standard used includes
a bitmap overlay channel which is not difficult to encode, for use
in doing on-screen display etc.   One could encode the backgrounds in
mp2 and the menus and text using this channel.

However, I don't know if you would want to use it as a full time X
display.

With firewire (now called dtvlink) there is another alternative called
DV.  DV is for camcorders and is very lightly compressed.  In theory
it should not be too CPU intensive to make a full X-server for that
when doing non-video display.


It is likely that the ideal setup would consist of both an mpeg2 streaming
method (with OSD overlay ability) over via ATSC or Firewire, and an analog
display ability (over vga or DVI).   This is possible with the firewire
because it includes a control channel so you can tell the TV, "OK, now
display the video I am sending you over the firewire.  Now switch back
to the analog input" and so on.

If you had this, you could do an HDTV PVR with the top quality, and
minimal hardware in the box.  The signal would be as good as it could be,
no skips or jitters, even if your computer is busy transcoding etc.