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Old 26th February 2009, 17:23   #1  |  Link
belloq
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Join Date: Mar 2007
Posts: 10
HDTV AC3 audio not "linear" (SciFi captures)

Hi all,

I've lurked around doom9 for a while, but haven't posted much at all. I am a Mac user, but can use a PC if I need to. I have a Motorola DVR from Comcast which I have regularly (literally years) captured live TV and from the DVR via FireWire to my Mac. My editor has been MPEG Streamclip and HandBrake my final encoder. Typical final target is my AppleTV.

Recently, I have started capturing reruns of the first season of LOST which is running on SciFi HD. It's 1080i compared to the original ABC airings @ 720p. (The Blu-Ray version is 1080p, FYI.) The programs play live with no issues, and also play back from DVR to my TV and receiver with no noticable aural flaws.

However, what I have found is that after capturing and pulling to streamclip, the audio is filled with all sorts of gaps, pops, clicks and chirps. I also detect that audio plays out of place compared with video, as if the audio is encoded non-linearly. For example: audio of one character speaking is interrupted by the next character to speak, before they appear onscreen, and the audio then jumps back to the original speaking character. Then, the other character comes on the screen, but the word (or portion of a word) that interrupted the previous character, is either dropped and replaced with a gap, or the audio and video skips where that word (or portion of a word) would have been. It's quite difficult to explain in words. If I feed even the raw .m2t file to HandBrake, it reports things like "sync: audio 9 time went backwards 0 ms, dropped 3 frames" or "sync: video time didn't advance - dropped 2 frames" and "sync: adding 78ms of silence to audio 0".

Now, I am not looking for streamclip or HB support. I know where to go for that. However, these issues simply do not exist on current captures from ABC (current LOST) or FOX (24). The editing with streamclip and encoding with HB for those programs is fast, painless, and nearly error free. It seems to me that there is something going on with the stream from SciFi.

Whether this is just plain errors or planned, however is my question. Is SciFi doing something to the streams, audio specifically, which the cable box/DVR is able to deal with on the fly, or has received a recent firmware update to deal with specifically? It seems that this could be on purpose, but why they wouldn't just us the "Copy Once" or "Never" flag to discourage pirating, I don't know.

So, anyone with any thoughts or insight or experience with this?

Thanks!

Last edited by belloq; 26th February 2009 at 17:29. Reason: Added more text around "non-linear" audio
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Old 5th March 2009, 21:16   #2  |  Link
d1g1ta7
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Join Date: Apr 2006
Posts: 19
You should probably check the cable signal level going into your cable box. Those errors are (probably) resulting from signal errors.
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