[mythtv-users] CPU Performance required three dumb capture cards (WORKING!)

[Brian Foddy]

On Saturday 27 March 2004 09:27 am, Adam Felson wrote:
> > From: "Gregorio Gervasio, Jr." <gtgj at pacbell.net>
> >
> >         What's the usage like with 480x480?  I think your system
> > should be able to handle that.  I have two Bt878 cards on an
> > Athlon XP 2500+ (1.83GHz), nForce2 system capturing 480x480 MPEG4,
> > 2000 bitrate (1500 eff.), 4MV and HQ disabled, adjust and quickdnr
> > filters enabled.  CPU usage is 60-75% when recording two programs and
> > playing one more at the same time.  I don't transcode
>
> Thanks for the heads-up. I went back to doing mpeg-4 on the fly with no
> transcoding and I set the resolution to 480x480.  It was the transcoding
> that was choking the system and causing it to crash;  sometime's I'd see
> 5-6 transcode processes and perhaps they had used up all the physical
> memory and caused the system to thrash.
>
> Right now I have two capture cards doing RT mpeg-4 @ 480x480.  With two
> cards recording and tvout playing a stream, I get 20% idle on the athlon
> 2400.  And the system doesn't crash any more!   (at least it made it
> through the night for the first time in a week)  Another thing I did was
> quit using a low latency pre-emptive kernel.  I'm now running
> linux-2.4.25 straight from kernel.org without any of mandrake's
> finaglry.
>
> I much prefer dumb capture cards over smart ones.  They're half the
> cost, mpeg-4 compression ROCKS (<1gb/hr vs. 4gb/hr for mpeg2), and
> there's no need to transcode everything.
>
> When I get the third capture card I want to decidate it to TV only use
> and use rtjpeg compression.  If the system can't keep up, getting an
> athlon XP 3000 is cheap enough.


Don't forget to figure out your PCI bus budget.  The big advantage I
found with the PVR cards is the relatively limited PCI bus bandwidth
goes much farther.  You get a max of 133MB/sec.  I used to figure
640(hres) x 480(vres) x60(frames/sec) x3(bytes/pix) = 54MB/sec each;
the PCI bus sees the raw format, plus the compressed stream.
Now on top of that, you need to write those compressed streams to disk
and read 1 stream from disk, plus the audio, plus a little more.
Look at your lspci to see which devices are on what bus, maybe your
HD won't count towards this.  But usually all PCI devices are on the
same bus, unless you get an expensive MB.
I found 2 cards plus a PVR was just over the max, and created
lots of skips, sound drop outs, and other problems.

I ended going 3 PVR cards and its been much better.

Brian