[mythtv-users] Preserving HDTV AC3 audio stream whilst transcoding

[Isaac Richards]

On Thursday 18 November 2004 04:02 pm, Joe Barnhart wrote:
> --- Isaac Richards <ijr at po.cwru.edu> wrote:
> > Current CVS myth, a gf4 (4200), 66.26 drivers, not
> > using XvMC (slower than Xv
> > without a better card), no deinterlacing/filtering,
> > doesn't matter what
> > display res.  'Extra audio buffering' is turned on.
> >
> > 720p plays perfectly on this machine.
> >
> > Of course, none of that is with the OSD displaying
> > anything during playback -
> > I don't have enough cpu to blend in software. =)
>
> I'm puzzled by the statement that the cpu usage
> "doesn't matter what display res."
>
> A simple calculation shows that a stream played at
> 1080i with 24-bit color pushes about 1.5Gbps through
> the video card (1920x1080 x30fps x24bpp).  A 720p
> stream pushes about 1.3Gbps.  If your output device is
> 720x480i, you're looking at about 250Mbps.  I would
> think the difference in these OUTPUT resolutions would
> have a profound effect on system performance.

Two major things wrong with your reasoning.
 - Scaling is done by the video card, not the CPU.  The video buffer size sent 
by myth to the video card doesn't change based on what the final display 
resolution is.
 - The video we're talking about is 12bpp, which halves your numbers.

Isaac