[mythtv-users] HD content on SD equipment

[Rod Smith]

On Sunday 02 November 2008 02:02:09 pm Jon Hoyt wrote:
> Hello,
>
> I have a BE/FE machine that I've been using happily for several years.  It
> has a Sempron 2600+ processor, and it gets OTA broadcasts with a PVR-150
> and outputs them through the S-Video output of a Nvidia MX4400 video card
> to an 18 year old CRT.  It has been working great for a long time, with
> just the occasional update using Knoppmyth.
>
> Now, however, everything has been screwed up by the upcoming Digital
> Conversion.  I bought a DTV converter (Tivaz brand) and it works fine, but
> I haven't got a channel change script going for it yet so I have to change
> the channels by hand, which simply doesn't work very well.  I bought an
> air2pc card thinking it would tune in the digital channels and work as
> transparently as the pvr-150, but after doing some reading it seems that
> with the HD content that's being broadcast, I'll end up with huge files

Three points:

First, the digital conversion is immediately relevant only if you're receiving 
over-the-air (OTA) broadcasts. If you're tuning in via satellite (DirecTV or 
Dish Network), then you're already receiving digital broadcasts; your set-top 
box converts to analog. If you've got cable, contact your cable company. My 
impression is that most cable companies intend to keep delivering analog 
signals for at least another couple of years.

Second, depending on your market and what channels are involved, you may be 
able to tune digital SD versions of some channels. For instance, I can tune 
one channel (channel 12) from my cable TV lineup in SD or in HD using my 
digital tuner. I'm not sure offhand how many OTA stations throughout the US 
are providing digital SD versions of their broadcasts in addition to their HD 
versions.

Third, it's possible to transcode programs from HD to SD resolution, or even 
lower if desired. This takes time, though -- probably about twice the running 
time of the program, given your CPU. Still, this may be a good option for 
saving disk space; if disk space used is a consideration, chances are you 
won't be watching the program for a while, so the time spent transcoding the 
recording won't be an issue. One caveat: The version of MythTV I've got (a 
0.21 fixes branch, I don't recall the exact fix version), or perhaps a 
support tool such as ffmpeg, has a bug that ruins the audio on transcodes if 
the audio is in 5.1 format. I don't know if this bug has been fixed or what 
versions of MythTV are affected. In a worst-case scenario, you could use your 
own transcode script to do the job.

> that I might not be able to play back smoothly. Is this true for my 
> Sempron 2600?  Does it matter that I'm only outputting S-Video?  Would a
> better video card help (I'd rather not use XvMC unless I have to, but I'l
> also rather not have to replace the motherboard, processor, and ram)?

I've got a system with an Intel Celeron 3.06GHz, running a 32-bit version of 
Ubuntu and a locally-compiled version of MythTV. I can play back HD content 
at SD resolution on my nVidia GeForce FX 5200 card running at 800x600 
resolution. When I do so, top shows that mythfrontend consumes about 50% of 
my CPU time and X consumes about 6-8% of my CPU time, so I've got CPU time to 
spare, at least in theory. This does require proper X configuration, though; 
in particular, the following line needs to be in the Device section for the 
video card:

    Option         "UseEvents" "True"

If this option isn't present, X CPU use shoots up to the 50% range and video 
playback pauses every couple of seconds, which is maddening. The same thing 
happens even with correct video configuration if a background process runs 
during HD video playback. This includes "nice"d processes, such as 
commflagging or transcoding.

XvMC isn't a help; on my system, if I try to use XvMC on HD content, playback 
doesn't work at all. XvMC works fine with SD content, though.

-- 
Rod Smith