[mythtv-users] Re: HDTV, Myth, HD-2000, HD-3000, questions galore (and hopefully answers from the smart people... )

[Brad Templeton]

On Mon, Jan 24, 2005 at 03:04:24PM -0600, Paul Leppert wrote:
> Blammo,
> - How are HDTV channels set up? Using zap2it and an HDTV lineup
> resulted in a mix of NTSC and ATSC channels with incorrect freqid for
> the digital channels (e.g., in Chicago, Channel 7 (ABC) is actually
> broadcasting on 52).  How is this set up via mythtv?

The easiest way is mythweb.  Select your digital TV lineup as a subset
of the broadcast lineup on zap2it.  Mythsetup allows slower setup of
the frequencies.

If you want to get fancy, write a script that takes the output of dtvscan
(which came with your pchdtv) and builds the database.   Subchannels are
entered with a dash into myth though many sources write them with a dot.
> 
> - How are HDTV subchannels set up?  Again, based on above, ABC is
> multicasting on 52.  In mythtv, are the channels specified as freqid
> 52-1, 52-2?  or some other format?  How should these multi-channels be
> set up for names (mythtv channel input won't allow '.' (7.1, 7.2,
> etc.) or '-' (7-1, 7-2).  Do these channels need to be renumbered
> (e.g., 900, 901)?  For me, I cannot seem to get any other subchannel
> other than the first regardless of what I put in the freqid field in

Myth 0.17 will offer the ability to specify subchannels.  Today, you must
allocate everything to an integer channel.   Thus I just use spare
iteger channels

In mythweb/settings_channels.php, I have, for example, Channel 4-2
(which broadcasts as 57-2), show up as  "4"  (channum in mythweb.)
This is the "real" channel 4, that you get if you tune them analog.

I have channel 4-1, which broadcasts on 57-1 appear on 6, which is
a spare channel in the area.

> - Based on http://thegeorges.us/mythtv instructions, the mythconverg
> database needs to be tweaked to set channel format to ATSC (assuming a
> default of NTSC).  Is this necessary?  What are the correct commands
> for setting this up?
Don't recall having to do anything but set atsc in mythtvsetup.

> - If your mythtv setup receives OTA NTSC and ATSC, is there any reason
> to keep the NTSC channels within mythtv?  If so, how should these be
> set up (similar issues as above since you now have two (or more)
> channel 7's)?  Is there a priority scheme within mythtv to make sure
> it records a show off the ATSC stream before choosing the same show on
> the NTSC stream?

You can put priorities on channels but generally since all analog
channels are now broadcsting in digital -- or is that not true where
you live? -- why would you want to tune the analog.  Anyway, I think
that myth only recently got patches to support the analog tuning on
the pchdtv anyway.  I don't use it though.

> - Does transcoding need to be set up in mythtv for HD channels (my
> understanding of the HD stream is that it contains all subchannels. 
> Does mythtv filter out the "extra" subchannels or do you need to use
> the transcoding mechanism to reduce the file sizes?)  I have no idea
> how any of this works in mythtv.

Yes, myth extracts just the subchannel when you record.  Transcoding
could be used to make it smaller.  Mythtv 0.16 does not handle
anything but one mp2->mp4 transcode profile, so it is not really able
to deal with the fact that some shows are 1080 lines, some 720, some
480, it would transcode them all to the same size.

Manual transcode with mencoder is your best bet.

> 
> And questions for performance:
> 
> - What components of the mythtv / computer setup impact performance of
> HDTV (e.g., at a given hardware level, what is the CPU usage for
> recording?  what is it for playback, what is it for both)?

No cpu usage for recording that you would notice.

TONS and TONS of cpu usage for playback, you want a 3ghz processor or
even better.

Also lots for transcode, commercial eliminate etc.

Playback with xvmc takes less CPU but mythtv has issues with seeking
in such recordings.  You can playback in mplayer with xvmc, but
you would go out of myth to do that.

It's not out of the question to build an HDTV mythbox, for example,
using a lower powered machine with an xvmc video card (nvidia for now)
and be unable to play your videos in myth, but instead go to the shell
and use the mythname.pl program to give the files nice names, and play
them in mplayer.


> - Related to above, what is the impact of multiple HDTV captures
> (e.g., two HD-3000 cards) on the processing requirements of the PC? 
> Anybody here yet?  Anybody have an idea of what it takes
> processing-wise for HDTV PIP with two cards?

No impact for recording.   Why would you want PIP on a PVR, I have
always wondered?  It makes no sense to me.  PIP is for live programming,
to watch two things at once at least one of them live.  You don't watch
live on a PVR after using it for a while.

I have not tried it but I would imagine dual-HDTV playback would take
a lot of CPU.

> - What can be done to improve performance (how is resolution reduced? 
> Is there any impact (or even capability) of recording at lower
> resolution than 720p or 1080i/p? Is there an easy way to playback at
> different resolutions (via mythtv rather than different modelines).

You can't record at a different resolution, you are recording an
already encoded and compressed data stream.    It's just a file copy
and that's why it takes no CPU.

To change the resolution, you would have to transcode.  If you did
that, you could generate a file that could play on a lower speed
machine.  It would take a long time to do it on your lower speed
machine.

> - If a modeline is set up for a particular monitor at a particular
> resolution, does that automatically reduce the processing of an HDTV
> stream (in other words, if I'm playing a 720p ATSC stream (1280x720)
> back on a monitor at 480p (704x480), have I somehow reduced the
> processing impact, or is the mythtv box still handling the full
> processing impact and all that is happening is a reduction in quality
> on the output)?

Correct, no reduction in impact.  You send the full 1280x720 window
to the xvideo processor in your video card.  It shrinks it for you in
hardware.  Anything else would take even more CPU, though I can
imagine perhaps certain scalings that could be displayed faster.


> - For playback, what is the processing impact for different
> resolutions (playing back a 720p stream at 1280 x 720, 1024 x 576, 800
> x 450)?

No difference, the playback scaling is done in hardware.

> - For those trying to get the best performance (at the tradeoff of
> quality), what is the best way to play back HDTV?  Reducing playback
> resolution?  Transcoding to reduced resolution? etc. etc.

xvmc would be by far your best approach.  I know the mythtv team is
hoping somebody will step to the plate to work out the bugs that
existing in seeking on xvmc playback.

video cards with xvmc are cheap -- $30 or so.

Otherwise, it's transcode (which on a slow machine would take place
in more than real time -- ie.  multiple hours to transcode an hour of
video.)   Then you could play it back.

If I were stuck in this situation I would instead just quit myth and
play with mplayer/xvmc.

not that xmvc is all that easy to get working on the video cards!

> - What about audio?  Are there general rules about digital audio and
> performance or is this generally "not an issue" based on using a 5.1
> sound card (e.g., SB Audigy)?

Yeah, hardly anybody posts to this lists having problems with their
audio.  :-) :-)

(It is in fact one of the most common issue areas.)

> - What is XvMC and how does / will that help with the processing
> requirements of HDTV playback?  Are there particular video cards this
> will / will not work with?  Anyone have real world examples of
> processing improvements (e.g., CPU usage) with XvMC and without?

See above.  So far what I have seen is
    a) You can get xvmc to work in mplayer with nvidia cards
       But I have found it sometimes not to work under 2.6.9 kernels.
       Nvidia driver problems are legion.
    b) People report seeking and other activities will crash myth, at
       least on some platforms with xvmc.
    c) With other chipsets I have tried -- Intel Extreme, Unichrome,
       Radeon 9200 -- I have not even been able to get xvideo (a
       prequisite to xvmc) to work at 1920 wide.   It works at 1280
       wide on these platforms however.
> 
> And a general question:
> 
> - Is there a way to tell within mythtv what format the ATSC is being
> broadcast (720p, 1081i, etc.)?
> 
Not that I know of (other than looking at the stream on disk, there
are some tools that will tell you the file format of an mpeg file on
disk, mplayer is one of them.)

However, right now, all stations use the same format 24/7.  So if
you are watching ABC or Fox, it's 720p.  If you are watching CBS or
NBC or UPN or WB it's 1080i.