[mythtv-users] Where's the playback bottleneck?

[Jerry Rubinow]

On 1/24/06, John P Poet <jppoet at gmail.com> wrote:
> On 1/24/06, Jerry Rubinow <jerrymr at gmail.com> wrote:
> > If I playback an HD resolution file, I get stuttering, pick a codec, any codec.
> >
> > If I play it back with mplayer (-vo xv -vc ffmpeg12), also stuttering.
> >  If I play it back with mplayer (-vo xvmc -vc ffmpeg12mc), plays back
> > pretty much ok.
> >
> > In all cases, myth and mplayer, CPU is between 20% and 50% (this is on
> > a combined front/backend, P4 2.8 Pundit, with onboard VGA disabled,
> > using a PCI FX5200).
> >
> > The fact that I can playback the file in mplayer with xvmc, tells me
> > it's not a disk issue, and the fact that mplayer cannot play it back
> > smoothly with xv tells me it's probably not a myth issue.
> >
> > Is it a bandwidth problem in the video card or the bus?  Some BIOS
> > setting incorrect?  Does anyone have any suggestions for how to narrow
> > it down and tell for sure where the problem might be?
> >
> > Is anyone using an original (sis-based) Pundit for HD playback successfully?
> >
> > Thanks.
> >
> > -Jerry
>
> I have never tried it myself, but I remember a discussion about a year
> ago which came to the conclusion that a PCI based video card did not
> have enough bandwidth for HD.
>
> John

John, your memory serves you correctly, thanks.

Here:

http://www.gossamer-threads.com/lists/mythtv/users/122696?search_string=hd%20pci%20bandwidth;#122696

I found this quote, attributed to the DScaler people:

"PCI can reliably
continuously transfer up to about 600-700 megabits per second *in one
direction only* out of a possible 1056 megabits per second (132
megabytes per second) due to overhead."

1280x720x24bits (or is it 24 bits plus 8 for alpha?  Anyone know
what's being send to the video card?) = ~22Mbits, at 30 frames a
second, that's 660Mbits/sec, which means I'm probably bumping up
against that limit every time something decides to do something on the
bus.  So basically, I'm screwed and stuck with some useless hardware.

Or, more positively, this means I have no choice but to go and buy
more stuff to build the even greater frontend that will come after and
finally answer the question.  Surely the wife will see the logic in
this.

If anyone sees any flaws in my calculations, now's the time to speak
up, before I spend a bunch of money I don't have to.

-Jerry