[mythtvnz] nuvexport of HD recordings

[Tony Sauri]

On Tue, 16 Feb 2010 22:59, Robin Gilks wrote:
> Looking forward to it...
>
>
> --
> Robin Gilks

Well I promised so I had better get started however there is a chance this 
will be a bit disjointed as I am on holiday and away from my main pc and my 
notes and more importantly WAF dictates that I can not spend all day polishing 
this message.

Where to start?

First my environment

My Backend is a 3GHz P4 with 1 GB RAM.  I have installed Mythbunto 9.04 and 
then then I installed JYA's fixes for VDPAU (even though my F/E does not have 
NVidia componetnts) as I thought at the time this was the easiest way to get 
Paul Kendall's LATM patch for FFMPEG

The Frontend is an IBM T23 laptop (1.13GHz)also running Mythbuntu 9.04 with 
JYA's fixes.

After absorbing the quite compelling message imparted by trying to watch DVB-T 
HD tv on an under-powered platform I decided that the way forward would be to 
downrate the recorded files to DVD quality   that would play adequately on my 
frontend platform while retaining all the functionality of the Myth platform.

My first experiments were with FFMPEG that I have used quite extensively over 
the last few years and it is my transcoder of choice.  
About this time I realised that JYA's fixes for LATM were only in the private 
Myth FFMPEG library and not in the general FFMPEG software.  So I built a 
local version of FFMPEG that had PK's patch applied

My first problems were trying to keep audio and video in sync after much 
experimentation I found that the -itsoffset parameter was crytical and -async 
1 parameter seemed to add aditional benefit although I am not fully convinced 
about -async. 

I then started searching for how to calculate with precision the time 
adjustment to give -itsoffset.  My quest led me to 2 pieces of software a) 
MEDIAINFO from http://mediainfo.sourceforge.net/en and b) FFPOCKET from 
http://members.hellug.gr/djart/ffpocket/index.html.  By using information 
from both programmes I seem to have struck a formula which works for me.

I then saw it as important to seamlessly integrate the transcoded file back 
into the Myth system.

I have had to regenerate the entries in the markup table to facilitate 
navigation and comm flagging.  To do this I had to understand how Myth 
generates the markup table entires in the first place.  They are created in 
the nupplevideo recorder function  and I understand that they are generated a 
fixed number of frames apart irrespective of the encoding of the incoming 
datastream.  I thought this was going to be the end of my journey until 
reading the myth code I discovered that the strict fixed number of frames per 
gop did not apply to the avi container.

After determining that Myth seemed to process an playback an avi file 
correctly i used this container as my standard so now my FFMPEG command line 
looks a mlittle like this:

nice -n 19 /home/mythtv/bin/ffmpeg $itsoffset_term -i $recordedfile -acodec 
libmp3lame -ab 128k -ac 2 -ar 48000 -vol 384 -vcodec mpeg4 -b 4500k -r 25 -s 
720x576 -croptop 2 -cropbottom 2 -padtop 2 -padbottom 2 -f avi -y -async 1 -g 
15 $deinterlace_term $transcodedfile 

The four variables would look like:
$itsoffset_term		=	-itsoffset -234
$recordedfile		=	/var/lib/mythtv/recordings/1001_20100217221000.mpg
$deinterlace_term	=	-deinterlace 	or 	"blank"
$transcodedfile		=	/var/lib/mythtv/recordings/1001_20100217221000.mpg.tmp

Once the transcoded file has been created the only things left to do are to 
rename the files to their new locations, cleanup the database by deleting, 
blanking certain fields in the recorded table, deleting entries in the 
recordedmarkup and recordedseek tables followed by the use of mythcommflag to 
a) rebuild the recordedseek entries and then if appropriate use mythcommflag 
to flag for commercials.

All this is setout in the attached perl script.

Note: this is not my current production script and contains much hardcoded 
stuff.  As said before I am on holiday and do not have access to my usual 
system at the moment.

When I get back home ... end of next week I will be willing to post the 
current script for your info and use.

I hope this has been of some use to Robin and others.

Regards

Tony Sauri

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