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Old 08-02-10, 10:25 AM   #2
Stephen Warren
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Join Date: Aug 2005
Posts: 1,327
Default Re: Another GT-240 Audio/video Issue

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.
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