[mythtv-users] padding question

Stephen Worthington stephen_agent at jsw.gen.nz
Thu Dec 7 03:50:21 UTC 2017


On Wed, 06 Dec 2017 19:09:27 -0800, you wrote:

>beginning somewhere in the updates to .28 i started getting a strange
>padding issue on my ota recordings. i had 2 hdhomerun dual tuners and
>just now replaced them with one hdhomerun quattro unit, but the same 4
>tuners. each set to multirec of 2 per tuner to allow for subchannel and
>main channel on one tuner. i have no padding set in any recording rule
>and none set under setup/video/advanced where the option is also
>provided.
>
>what happens is this: if i record a program running from say 8pm-9pm on
>a channel and then have a program scheduled from 9pm to 10pm on the
>same channel. the recording of the first goes over around 30 seconds.
>the second show is correct, unless there is a third show on the same
>channel to be recorded, then that goes over also. the scheduler will
>almost always assign a different tuner to the 2nd show. if i have all
>four tuners being used, it was that there was no padding. now i notice
>that even with all four being used i still get the padding, this may be
>only since i switched out to the quatro. I have a feeling that it
>somehow has to do with multirec, but this is something that never
>happened for the six years i have been using myth. 
>
>if anyone has any ideas how to avoid please let me know. also when i
>would force the back to back shows on same channel to the same tuner
>number, it did not happen, but i don't know if that is currently the
>case. it's not a real big thing, but i would like to correct the
>behavior if possible. i am running on arch linux, on their current
>mythtv packages. which is now 29.0-b4e4ddcf97
>
>thanks.

There are two different types of preroll and postroll settings that
are applied to recordings.  One comes from the recording rule and is
always applied.  The others come from two global settings: (v29) Setup
> Video > General > General (Advanced) > Time to record before start
of show and Time to record after end of show.  These two settings are
RecordPreRoll and RecordOverTime respectively in the database settings
table, and I think default to 60 seconds each.  When they are used,
the preroll and postroll they do is added onto the preroll and
postroll that is done from the recording rule settings.  However, when
there are back-to-back recordings on the same multirec tuner, the
RecordPreRoll and RecordOverTime settings are not used, as that would
mean that MythTV had to record from the same multitrec tuner to two
different recordings at once, which it can not do.

With the preroll and postroll settings from a recording rule, the
times are used to extend the recording time at both ends and the
extended recording time is the one used in selecting which multirec
tuner is free to be used for a recording.  So if those preroll or
postroll settings mean that the next recording will overlap with one
that is currently using a multirec tuner, then a different multirec
tuner will be used for the recording.

So your setting up of multirec tuners is likely what is causing the
difference.  MythTV has not changed how it works, but the extra tuners
available means that it can now use options it was unable to before.

However, you have only set up two multirec tuners per physical tuner.
You probably should set up at least five multirec tuners per physical
tuner, as there are often several channels on the same multiplex that
you will want to be able to record from at the same time, and it is
best to use the same physical tuner to record all of them as that
leaves the other physical tuners free to be used on other multiplexes.
With only two multirec tuners per physical tuner, two back-to-back
recordings with overlap due to pre- or postroll will use up both
multirec tuners, and a recording from another channel on the same
multiplex will be forced to use another physical tuner.

The only reason not to have as many multirec tuners as possible per
physical tuner is that the scheduler can then record many more
programs at once when they happen to be on the same multiplex, and if
you do not have enough disk drives it can record too many at once to
one drive and cause data loss in the recordings due to the disk head
movement being too slow.


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