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Old 08-02-10, 08:34 PM   #3
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Join Date: Nov 2008
Posts: 8
Default Re: Another GT-240 Audio/video Issue

Originally Posted by Stephen Warren View Post
When capturing audio and video, each frame of video and packet of audio is marked with a capture timestamp. During playback, SW attempts to have video and audio frames/packets with the same timestamp play back at the same time to maintain AV sync. However, this only works if the clock source used to create those timestamps at recording time are the same, and the clock source used for both video and audio during playback are the same.

I imagine the HDHR has a HW design that uses the same clock for recording both video and audio, and hence the timestamp streams correctly align in the encoded file.

With your new HD-PVR setup, I'm not sure whether the HD-PVR is a box that encode both the audio and video, or whether it's a PCI/PCI-E card that just captures the video, while you're using a PC sound-card to capture the audio? In the latter case, different clocks are used, so the timestamps will drift apart, causing the kind of issue you're seeing. Even if the HD-PVR is a single all-in-one device, it may have been incorrectly designed and hence produce a timestamp drift that the HDHR doesn't.

Finally, in the PC architecture, the PC's system clock, the video scanout clock, and the audio clock are all completely different clocks, and will suffer some drift. This in itself can produce the kind of issue you're seeing too.

Summary: Getting AV sync correct on a PC without dropping/duplicating either video frames or short segments of audio is impossible.

That said, the apparent rate of frame drops in your log seems excessive. To find the real cause, some significant analysis of the video files would be required. I'd suggest asking in a MythTV forum about this though.
I need to make an important correction. Namely, I am seeing the same behavior (log messages) when I playback content from both the SiliconDust HDHomeRun AND the Hauppauge HD-PVR. Originally, I thought this was strictly related to the content captured by the HD-PVR (H.264). However, it is also observed for the HDHR (mpeg-2) as well. Maybe this is a good thing since it points to something in my configuration that is equally affecting playback of content from both devices.

To clarify, the HD-PVR is an H.264 encoder that captures both audio and video from a STB (over a USB connection).

Again, if I completely disable audio in the myth setup, the messages stop (and so does the sound).

I'll keep looking. Tips appreciated.


Last edited by kennelm; 08-03-10 at 07:29 AM. Reason: edit
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