[mythtv-users] pci bandwidth: how many tuners and video (vdpau) cards?

Michael D. O'Brien obrienmd at gmail.com
Sat May 15 01:33:44 UTC 2010

> I asked in another thread but I thought perhaps this question deserved
> it's own thread.
> I have been avoiding a VDPAU video card because the 8400GS PCI is the
> only option for my FE/BE.  But that machine currently has 4 tuners in it
> already (on 3 cards).  Having frequently read warnings about stuffing
> more tuners into a machine that it has bandwidth I had always just
> dismissed a PCI video card without doing the math.
> But now that I have done the math, it seems there is plenty of room on
> the PCI bus.  Here's my calculations:
> My (Hauppage PVR) recorded files are 2.17GiB/h, which by my math equals
> 633KiB/s or 4.9Mib/s.  So, my guess is that to have 4 recording and 1
> playing through a VDPAU capable video card, I am using 24.7Mib/s.
> Does that sound about right?  Or perhaps it's in fact double that
> because there is the PCI card -> memory bandwidth and then memory ->
> disk, for recording, or disk -> memory and then memory -> VDPAU video
> card.  That's most obvious for a disk I/O card in a PCI slot.  Does that
> also count for a disk on a SATA bus?  Do they use PCI bandwidth?
> But even at 50Mib/s, that seems to be waaaaaaay shy of the 133MB that
> wikipedia (http://en.wikipedia.org/wiki/Conventional_PCI) says PCI can
> run at.  In fact 133MB/s is on the order of 1Gib/s which is 20x the
> bandwidth I am talking about here.
> Have I got my numbers all messed up somewhere?

You should be fine.

> I wonder why anyone has even thought of mentioning bus limitations in
> reference to putting too many PVR style cards into a machine.  Maybe my
> memory is failing me here on that.
> b.
> P.S.  The wiki docs say that a PCI card cannot handle uncompressed HD
> video (i.e. if your video card won't do VDPAU for a given HD stream PCI
> won't be enough to carry it uncompressed).  Just what is the math there
> that it doesn't fit inside of 133MB/s?

1920(h)x1080(w)x24(bpp)x60(fps)=about 3Bbps, or about 350MBps

I have a third monitor (1920x1080) on a PCI video card, and it's
extremely slow in desktop interaction (typing lags, etc).

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