[mythtv-users] Archiving Shows to DVD
directhex at apebox.org
Mon Aug 1 15:00:27 UTC 2005
Michael T. Dean wrote:
> Cory Papenfuss wrote:
>> On Mon, 1 Aug 2005, Fredrik Karlsson wrote:
>>> On 8/1/05, Cory Papenfuss <papenfuss at juneau.me.vt.edu> wrote:
>>>> I just looked through this a bit. I must admit I haven't
>>>> it, but as far as I can tell from reading through the code, it
>>>> starts out
>>>> by demuxing/remuxing the MPEG stream. That will break audio sync on
>>>> arbitrary ivtv captures.
>>> Ok, I had not realized this.. although I have acctually used these
>>> scripts and my recordings are all IVTV captures, so I am not sure how
>>> big a problem this is..
>>> Changing strategy would probably be wise though..
>> It's a very insidious problem. The ivtv chip will hiccup once in
>> awhile, and generated A/V offsets in the capture that break standard
>> demux/remux utilities. It doesn't happen most of the time. It does
>> seem to happen more often from tape captures (where I found it).
> Agreed. Also, on some analog cable feeds (i.e. the one in my area).
>> Unfortunately, all of the cut and/or burn scripts (nuvexport as a
>> big example) use this technique. It works 99% for 90% of the people
>> out there who never capture tapes. It works probably 60% of the time
>> for the 8% of the people who don't do tapes, but have marginal
> (which includes users of analog cable and probably OTA)
>> It works 10% of the time for the 2% who capture from tapes.
>> In other words, most people don't notice the problem... but some
>> day they likely will... :)
> To give you an example, I had a 2-hour show recorded from the History
> Channel using the local cable company's analog feed. I've done a lot
> of work to ensure the best signal quality possible (proper choice of
> cables, connectors, etc., and proper placement/use of amplifiers,
> splitters, etc.), but the same happens when taking the cable feed
> directly to the PVR-250 without any amps, splitters, or anything
> else. When I cut commercials in avidemux2, I ended up with a A/V sync
> being off by about 5 seconds. When I used replex before cutting (to
> "average" out the sync offset), the greatest offset I saw was about 3
> seconds, but there was always an offset. Using ProjectX resulted in
> proper sync throughout--except for small sections up to about 30
> seconds in length with a tiny, but noticeable offset--but audio
> artifacts at several locations (i.e. words clipped when truncating the
> audio stream or clicks and pops added to the audio).
> Fortunately, this doesn't happen with my DISH video source, so I do
> everything I can to ensure I never have to archive anything from the
> cable video source.
> The best approach is to add code to avidemux2 (and probably
> mplex/replex/transcode/whatever) to handle varying A/V sync within the
> stream. Since most players (i.e. xine, MPlayer, and even Myth) can do
> so, it should be possible--the hard part is finding someone willing to
> make the time to make the changes. (I'll admit I'm being selfish with
> my time since DISH provides a workaround for me.)
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> mythtv-users at mythtv.org
Does anyone know to what degree this desync issue affects DVB-T recordings?
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