[mythtv-users] HD on SDTV playback question

[Rod Smith]

On Sunday 25 February 2007 12:26, Steven Adeff wrote:
> On 2/25/07, Paul Bender <pebender at san.rr.com> wrote:
> >
> > Most of the frontend's power is used to decode the video. Therefore, the
> > power needed depends on the resolution of the content much more than the
> > resolution of the display. Of course, by today's standards, a below
> > average computer system can decode and display HDTV.
>
> If the other frontends have an FX5200 or better you can use XvMC to do
> the decoding if the CPU isn't fast enough.

Up to a point, yes, but XvMC isn't a magic bullet. On my system (an Intel 
Celeron D 3.06GHz), using XvMC with an nVidia MX4000 and nVidia's drivers 
produces smooth playback with CPU loads of around 50-60% for mythfrontend 
when playing back HD content on an SD screen (800x600 resolution), with X 
chewing up another 5-10%. Disabling XvMC but using the same card kicks CPU 
use up to around 80%, IIRC, with playback remaining smooth -- but it gets 
unsmooth quickly whenever you do anything (skip back, etc.). Swapping in a 
video card based on a SiS Xabre chipset, which can't do XvMC, produces choppy 
playback with higher CPU loads for both mythfrontend and X (around 80% for 
mythfrontend and 20% for X, IIRC). Cutting playback speed down to 60-90% of 
normal typically produces smoother playback. Based on this, I'd guesstimate 
that XvMC reduces HD playback CPU load by 25-50%. This is good, of course, 
but it's nowhere near as much of a reduction as you see when using hardware 
encoding cards vs. simple frame grabbers.

That said, results are likely to vary from one card to another. My 
understanding is that the cards with the most hardware acceleration delivered 
by XvMC are those based on the nVidia 5200FX series.

-- 
Rod Smith
http://www.rodsbooks.com