[mythtv-users] specific channel seek issue and acts like poor signal

[Stephen Worthington]

On Fri, 12 Apr 2019 08:54:02 +0100, you wrote:

>I've a long standing issue with one of my channels (UK, 4Seven via DVB-S2) where historically the seek will be out. If you play it till the first advert break, then skip forward, then the seek process will jump back a few minutes. almost like the video is playing back faster than a counter that the seek system uses.  AV sync is fine. I've asked about this specific problem maybe a year ago and iirc was sugested to rebuild the seek table but that gave no solution. i think the thread died after that.
>However, since i've upgraded to myth30 this channel is also acting like I've weak signal - heavy blocking rendering it almost impossible to watch. mythtv UI doesnt highlight it in yellow to denote a damaged recording.
>Problem exists in liveTV as well as recorded.
>
>I figured I'd try to download the file and playback locally via vlc just to see what happens. playback works perfect. no blocking or corruption so the problem seems to be with mythtv specifically. issue only occurs with that channel and all others work perfect.
>
>any ideas on what i could try to fix this specific channel? I've put serious consideration into wiping the DB and starting from scratch with it all just in case there's something bad in the db. If that feeling is true, restores would obviously carry the problem forward. I know the db carrys a number of tables related to channel config so manually cleaning does have a good chance of missing something in the cleanup op and causing further issues.

There is no point in playing around with the database, since the
recording file plays fine in other players.  So it is recording
properly, and the playback is the problem.

Recording of a digital channel is a very simple operation - the data
for the channel is selected out of the full set of data for all the
channels on the mux.  That one channel's data is written directly to
disk - it is not modified, so MythTV's recording process can not cause
a problem such as you are seeing.  If the channel is not transmitting
powerfully enough for your tuner to capture it and it is therefore
missing bits of data, that would cause blockiness and other artifacts.
But all the channels on the same mux would record equally badly.  If
they do not, then there is no problem with the recording process.  And
as you can play your "bad" recordings properly with vlc, there is no
clearly no problem with the recording process.

One thing to try is to put the file (or a copy of it) in your videos
storage group and try playing it from there.  If it plays better from
there, then there is a problem with the seek table data that
mythbackend has created for the recording.  If not, then the problem
is in mythfrontend's playback code.

It is possible to set up an alternate player program that can be used
from the videos menus - I use mplayer, but vlc should work too.  Then
you can easily select to play with your alternate player for any video
file that has playback problems with mythfrontend's builtin player.
But for recordings, there is no similar option, so you would need to
move recordings over to the videos area to make use of that, or use
one of the bits of third party software that can make understandable
links to your recording files so you could play them that way.
Mythlink is an example:

https://www.mythtv.org/wiki/Mythlink.pl

I have a problem similar to the one you originally had where the
playback skips back to the start of the file if I skip forwards.  It
only happens with some older .wmv files, but it was the reason I set
up mplayer as an alternate player program.  There are some other less
common problems with playing some other older file formats, such as
RealMedia, which using mplayer usually helps with.

What would be good is if you are able to cut a section of the file
that demonstrates the problem, but is small enough to post somewhere
the developers can download it from.  Then you can create a ticket
about the problem and the devs may be able to fix it.