[mythtv-users] Archiving Shows to DVD
Michael T. Dean
mtdean at thirdcontact.com
Mon Aug 1 14:56:30 UTC 2005
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 tried
>>> 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 reception.
(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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