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Dual 5500s saturating PCI bus?
PostPosted: Mon Jan 22, 2007 2:07 pm Reply with quote
Joined: 11 Jan 2007
Posts: 3
Location: Portland, OR

I built a brand new system for my Myth box. It's got a Pentium D 3 Ghz, 2 gigs of RAM, an NVidia 7800 PCI express video card, a Seagate SATA HD, and two HD-5500s. I'm running a brand new install of Gentoo linux with the DVB and V4L drivers compiled as modules. I didn't install the drivers off the CD. I just used what was in portage.

I don't subscribe to HD, so I'm only concerned with NTSC over cable (channels 2 through 70-something) at the moment. The capture cards are set up as V4L devices. Eventually I may subscribe to HD (or just wait for it to become mandatory).

I get a signal just fine with tvtime, and with only one capture card installed and using the passthrough cable for audio the system worked beautifully. I switched to the PCI based audio and with one card that worked too. The problems started when I installed the second card. I've only got the onboard audio, so I figured I needed to use the PCI audio because there's only one line in jack.

So once I got it all set up, I tried to watch live TV. The video and audio stutters and is basically unwatchable. In the logs I'm seeing a ton of the prebuffering error messages that everybody seems to get. I searched around, but couldn't find anyone who seemed to get the prebuffering problems and had a setup similar to mine. The system seems to record a single broadcast alright, but with both cards recording the recordings seem to get bad sound and video.

My question is whether it's likely that the PCI bus is getting saturated with so much going on... video and audio from two cards, PCI express traffic to the video card, traffic to the onboard audio card, traffic to and from the disk. Seems like a lot of stuff going on to me. I read somewhere that it might also be some kind of hardware conflict with the two 5500 cards since I'm not using the DVB drivers. Could there be any truth to that?

If the PCI bus is getting saturated, would it help to use both audio passthrough cables and try installing a PCI sound card?
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PostPosted: Tue Jan 23, 2007 6:00 pm Reply with quote
Joined: 14 Aug 2006
Posts: 2

Any NTSC recording in mythtv uses software encoding. So, it could use a fair amount of cpu depending on what your recording profile is.

I am guessing that your cpu is pretty loaded. Thus, causing the prebuffereing pauses in mythtv. I know my 3800x2 with two 5500 card system gets pretty loaded when I setup 2 NTSC recordings.

Did you try any of the tips mentioned in the forum for disabling the dvb_frontend shutdown timeout or modifying the cx88_alsa driver?

My NTSC/QAM mythtv box works pretty well after using these tips.
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PostPosted: Wed Jan 24, 2007 1:18 pm Reply with quote
Joined: 11 Jan 2007
Posts: 3
Location: Portland, OR

The cpu usage doesn't seem to get over about 60%. I tried different kernels with different schedulers and they didn't seem to make much of a difference.

I had the cards configured as analog V4L devices, because I can't get it to work using the DVB driver. I set the analog options and scan for channels, but when I try to tune to a channel it can't get a signal lock (since it's trying to find a digital signal, not an analog one). I haven't been able to figure out how to get it to look for analog channels.

I tried the patches to the cx88_alsa driver and the dvb_frontend_shutdown timeout, but they didn't seem to change anything (probably due to the digital/analog problem I just mentioned). I am starting to suspect that the cx88_alsa driver just sucks at converting analog sound to digital sound, because I tried switching back to that configuration and still got messed up audio sometimes. I think I just got lucky before.

The only configuration I have been able to use successfully is one HD-5500 with the sound cable plugged into my line-in on my onboard sound configured as an analog V4L device.

I was thinking that when I get some free time later this week I should pull apart all three of my machines, rearrange a bunch of hardware, and put the box back together with onboard sound plus a secondary sound card. I'll plug both HD-5500's into the line-in ports on the sound devices and try it that way. Does anybody know for sure if that does or doesn't work?
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Dual 5500s saturating PCI bus?
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