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Old 25th October 2008, 19:17   #1  |  Link
lchiu7
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How to handle CRC errors in AC-3 streams when muxing?

Not sure where to put this question but will try here since it's primarily and audio question.

Been capturing DVB-T streams from our FTA broadcasts which are H.264 and AC-3 HD broadcasts. Quality is mainly excellent.

The captured videos play fine on a Popcorn Hour with no apparently glitches etc., also on a PC using PowerDVD8 but not as smoothly since the video encoding uses MBAFF or PAFF (not sure which) which can tax a CPU without a graphics card with a GPU.

Anyway, that's not the issue.

The files are pretty large (5G/hour) and need to be compressed. But I am finding that when the compressed video is muxed back with the original audio using tsremux (de-muxed with tsremux or dgavcindx) often tsremuxer will complain about CRC errors in the audio and say, audio sync lost or something like that. And the resultant stream will go slowly out of sync (usually starts fine)

This is a bit puzzling since assuming the de-mux introduced no errors, how does the original stream play fine, while just demuxing the audio out and back again, causes this problem?

I bumped into delaycut and it seemed like the way around this problem but whether I choose skip, silence or fix as the processing option, and there mux back has no CRC errors, the sync problem still occurs.

I am guessing the problems originate from micro dropouts in the UHF transmissions but can't work out why they don't affect the recorded signal, only after processing.

Any help or insigh greatly appreciated. Thanks

Larry
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Old 26th October 2008, 00:52   #2  |  Link
tebasuna51
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Try this little app SyncAc3.exe

Usage:

SyncAc3 input.ac3

You obtain a input_syn.ac3 with less than 32 ms of delay (and duration). At problematic points can have clicks.

You can also use drag&drop ac3 files over SyncAc3.vbs or over a ShortCut created on Desktop.
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Old 29th October 2008, 01:50   #3  |  Link
lchiu7
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Thanks but it didn't work. When I watch the original broadcast I can see the pixellation and image breakup that coincides with the AC-3 CRC errors. Took the original AC-3 stream and passed it through this program and muxed it back with the original video. It still goes out of sync at that point.
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Old 29th October 2008, 14:34   #4  |  Link
tebasuna51
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Yeah, if there are missing frames we can recover the sync. You need insert a Delay manually at these points.
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