[mythtv] Mytharchive - bad subsid-track mp2 to ac3 conversion

John Pilkington J.Pilk at tesco.net
Mon Feb 11 17:33:27 UTC 2008

John Pilkington wrote:
> Paul Harrison wrote:
>> John Pilkington wrote:
>>> I just installed the latest ATrpms build: 0.21-175_trunk_r15741.el5.i386
>>> As suggested by the wiki I have Mytharchive converting the two native 
>>> mp2 audio channels to ac3.  The subsidiary track is garbled in the dvd 
>>> image - Donald Duck effect.   Same in mplayer and internal .iso player.
>>> With the earlier 15447 build I had noticed that the two audio streams 
>>> before the remux were both the same size.  I now find that the 
>>> subsidiary track is garbled on a DVD burned with that build.
>>> I don't think I really need to do the mp2 to ac3 thing - it's just a 
>>> workaround to help players that can't handle both, and the title music 
>>> is supplied as ac3.
>>> For the original recording as input after mythtranscode:
>>> $ ffmpeg -i 3009_20080207205900.mpg
>>> Input #0, mpeg, from '3009_20080207205900.mpg':
>>>    Duration: 00:58:59.3, start: 0.298589, bitrate: 3569 kb/s
>>>      Stream #0.0[0x1e0]: Video: mpeg2video, yuv420p, 720x576 [PAR 64:45 
>>> DAR 16:9], 15000 kb/s, 25.00 tb(r)
>>>      Stream #0.1[0x1c0]: Audio: mp2, 48000 Hz, stereo, 256 kb/s
>>>      Stream #0.2[0x1c1]: Audio: mp2, 48000 Hz, mono, 64 kb/s
>>> and in the working directory:
>>> ffmpeg -i VTS_02_1.VOB
>>> Input #0, mpeg, from 'VTS_02_1.VOB':
>>>    Duration: 00:38:45.1, start: 0.160000, bitrate: 3694 kb/s
>>>      Stream #0.0[0x1e0]: Video: mpeg2video, yuv420p, 720x576 [PAR 64:45 
>>> DAR 16:9], 15000 kb/s, 25.00 tb(r)
>>>      Stream #0.1[0x80]: Audio: liba52, 48000 Hz, stereo, 192 kb/s
>>>      Stream #0.2[0x81]: Audio: liba52, 48000 Hz, mono, 192 kb/s
>>> Thanks to Paul H for the recent native-mode patch.
>>> John P
>> I've just tried to reproduce this and it worked OK for me. I had a hard
>> time finding a recording where the secondary audio track was actually
>> being used but did eventually find a film with an audio described track
>> for the second track mono, mp2, 64kb/s just like your original. I chose
>> not to re-encode the file but did chose to re-encode the audio to ac3
>> which looks to be what you did and both audio tracks played fine in the
>> internal player, mplayer and with xine.
>> If you still have the work directory intact you could look in there for
>> the ac3 sound track and see if it plays OK.
>> Paul H.
> The work directory is cleaned up in the version that I have after an 
> apparently successful run, but I have a 100Kb .ac3 sample stripped out 
> of the .VOB file, and that shows the effect when played in Mplayer.  My 
> commercial DVD player does too, on the burned disk.
> I see that the re-encoding is done by ffmpeg.  I'm using the ATrpms 
> stable build > ffmpeg-0.4.9-25_r11517.el5.i386
> John P
I edited mythburn.py to retain the intermediate sound files and 
processed last Monday's 'Life in Cold Blood' (BBC1).  The subsidiary 
channel is silent for the first minute or so, but then the .ac3 version 
is mangled.  The other three files play ok in Mplayer and, when remuxed, 
in the internal .iso player.

> ls -l stream*.*
> -rw-rw-rw- 1 mythtv mythtv   84400128 Feb 11 16:24 stream0.ac3
> -rw-rw-rw- 1 mythtv mythtv  112533504 Feb 11 16:21 stream0.mp2
> -rw-rw-rw- 1 mythtv mythtv   84420096 Feb 11 16:25 stream1.ac3
> -rw-rw-rw- 1 mythtv mythtv   28140288 Feb 11 16:21 stream1.mp2
and others.

The commands used are all like:

> ffmpeg -v 0 -y -i '/pathto/work/1/stream0.mp2' -f ac3 -ab 192k -ar 48000 '/pathto/work/1/stream0.ac3'


John P

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