[mythtv-users] How to Transcode BBC HD and keep AC3 sound intact

[Bob Sully]

> Message: 12
> Date: Wed, 9 Sep 2009 17:45:03 +0100
> From: Another Sillyname <anothersname at googlemail.com>
>
> I've noticed assorted postings on the list previously about
> transcoding BBC HD material and problems users had keeping the AC3
> sound intact and synced.
>
> With the recent slight change to the transmission method of some BBC
> HD material I decided to allocate a bit of time to trying to work it
> out.
>
> This is NOT a script and needs to be run from the command line, I'll
> look to see if there's some way to incorporate this stuff into
> nuvexport, I'll also have a look at ITV HD to see whether it works for
> that.
>
> You need pretty up to date versions of ffmpeg and mencoder installed
> on the machine you intend to transcode with, how to install them is
> outside the scope of this so you'll have to google elsewhere.
>
> Firstly create a directory on the machine you intend to perform the
> transcode on.
>
> Open a command session and change to the directory you just created
> (if you intend to run multiple transcodes simultaneously you need your
> command session to be in a different directory for each transcode to
> accommodate the 2 pass file log else they interfere with each other).
>
> The following line will do the first pass....
>
> ffmpeg -i /path/to/input/file/and/its/name -vcodec mpeg4 -r 25 -map
> 0:0 -map 0:3 -ss hh:mm:ss.000 -t hh:mm:ss.000 -acodec copy -b 3600k -s
> hd720 -pass 1 /output/file/path/and/name.avi
>
> -vcodec is self explanatory
> -r 25 forces the frame rate to 25fps - This should always be correct
> for BBC material
> -map 0:0 This takes the first stream and outputs it to the output
> file, 0:0 is nearly always the video stream
> -map 0:3 This takes the fourth stream and outputs it to the output
> file, 0:3 is usually the AC3 audio stream
> -ss Is where you want to create the new file from, as well as
> hh:mm:ss.000 format you can just use seconds
> -t Is how long you want the new file to run for, as well as
> hh:mm:ss.000 format you can just use seconds
> -acodec is self explanatory - I would suggest doing nothing to the stream
> -b 3600k Is the bitrate for the recording, 3600k produces a file about
> 1.1GB for a 40 minute(ish) program - You can dabble here to your own
> taste.
> -s hd720 Resizes the output file dimensions to 1280x720 You can have
> the output file any dimensions you want, use man ffmpeg for other
> valid dimensions
>
> When the first pass has finished check the output file for the correct
> dimensions, start position and end position, IGNORE timeclock and skip
> related issues at this point, I'll fix them shortly.
>
> If you're happy that the new file starts where you want and ends where
> you and is the right dimensions then run pass 2
>
> ffmpeg -i /path/to/input/file/and/its/name -vcodec mpeg4 -r 25 -map
> 0:0 -map 0:3 -ss hh:mm:ss.000 -t hh:mm:ss.000 -acodec copy -b 3600k -s
> hd720 -pass 2 /output/file/path/and/name.avi
>
> You'll be asked to confirm you want to overwrite the existing file, say
> yes.
>
> Once this is finished you should have a good file, however it reports
> the end time incorrectly.
>
> The last stage is to re-index the file so that audio and video report
> the correct file length.
>
> mencoder /path//to/output/file/above.avi -oac copy -ovc copy -forceidx
> -o /path/to/new/output/file.avi
>
> You should now have a fully working transcode of the BBC HD material,
> resized to 720p and with AC3 sound working and in sync.
>
> As an example a 40 minute BBC recording takes about 9-10GB in the
> original format, without any noticeable loss of quality (subjective I
> realise) this comes down to about 1GB.
>
> I've tried this now on about 6 different recordings and it seems to
> work OK, if anyone has problems please post in the thread.


Thanks VERY much for this!!!

I've adapted this to work with US ATSC HD recordings by modifying your
parameters slightly (to work with a framerate of 59.94 rather than 50) and
tweaking things a bit:

ffmpeg -i 4071_20090802205900.mpg -vcodec mpeg4 -r 29.97 -map 0:0 -map 0:1
-ss 00:00:00.000 -t 02:03:08.090 -acodec copy -b 3000k -s hd720 -pass 1
DG090802.avi

0:1 seems to be the 5.1 AC3 stream in the recordings I've tried so far.
To attempt to decrease the file sizes a bit, I've gone with 3K bitrate
rather than 3.6.

Interestingly, the SECOND pass doesn't always seem to work - sometimes
aborts.  If there's something I'm doing wrong there, please clue me in, 
However, I get excellent quality even with the first pass only.

After the mencoder step, I edit out the commercials using avidemux and
save with Video/x264 (two pass with approx 1GB file size per 40-42 minute
episode with commercials removed), Audio/copy, and filetype .mkv. (or, if
the .avi file size is OK, just save with Video/copy, Audio/copy and AVI) 
So far, the two files I've created have turned out quite nicely.

Thanks again - now, if we can get this to work with h.264 files from the
HD-PVR, I'm in nirvana.

Bob

-- 
________________________________________
Bob Sully - Simi Valley, California, USA
http://www.malibyte.net
http://www.malibyte.com