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

Brian J. Murrell brian at interlinx.bc.ca
Fri May 14 19:53:39 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?

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.


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?

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