[mythtv-users] Archiving Shows to DVD

Jo Shields 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 
>>>> 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.)
> Mike
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users

Does anyone know to what degree this desync issue affects DVB-T recordings?

--Jo Shields

More information about the mythtv-users mailing list