[mythtv-users] padding question

glen glenb at glenb.us
Thu Dec 7 05:33:08 UTC 2017

On Thu, 2017-12-07 at 18:08 +1300, Stephen Worthington wrote:
> On Wed, 06 Dec 2017 20:05:28 -0800, you wrote:
> > thanks for the reply. again i have no padding set, preroll or
> > postroll
> > on any of the rules themselves or in the
> > setup/video/general/advanced.
> > they are all zero. i still don't understand given what you are
> > saying
> > why the earlier show would run over. i have not changed anything in
> > the
> > setup of tuners / mulitrecs since i set it up years ago, and it
> > never
> > did this before. it changed somewhere in an update at .28. on the
> > ota,
> > it is rare that i would want to record off of a single channel with
> > sub
> > channels as the quality and content of those subs is not what i am
> > usually recording, i set it at two as that is the most i have ever
> > needed. this is not qam, this is ota channels. i appreciate the
> > explanation. but still don't understand why with no front or back
> > padding the earlier shows on only the same channel are running
> > over.
> You are working under a misapprehension about subchannels.  They are
> not actually "sub" channels, they are just channels.  The main and
> sub
> channel names are something that seems to be used in the American
> market, but ATSC transmissions (like DVB-T) can broadcast as many
> channels on one frequency (one multiplex) as they can fit in the
> available bandwidth.  A multiplex is just a stream of digital data,
> divided up into packets.  Each packet is tagged to say what it is,
> audio, video, subtitles or something more esoteric, and which channel
> that packet stream belongs to.  So to record a channel, the software
> simply looks for all the packets tagged as belonging to that channel
> and stores them to disk in the recording file.  For one TV channel,
> there will normally be a stream of audio packets and a stream of
> video
> packets, and sometimes some more streams (subtitles, audio for
> alternate languages, 5.1 or 7.1 audio, teletext, ...).
> The total bandwidth used by all the streams for all the channels on a
> multiplex can not ever instantaneously exceed the available bandwidth
> for the multiplex.  But within that bandwidth, the transmitter can
> vary the bandwidth used by each stream as it likes.  So a channel can
> be transmitting at high bandwidth because it is broadcasting an HD
> program, and then it might switch to a lower bandwidth for a while
> during an ad break as the ads being transmitted are only SD format.
> During that period, when that channel is low bandwidth, it would be
> possible for another channel on the multiplex to switch to a higher
> bandwidth (eg to do HD ads), as long as the total bandwidth used is
> still within the limit.
> It is not usual to switch bandwidths like that, but it is supported
> by
> the ATSC (and DVB-T) standards.  What is usual is that on any
> multiplex there will be one or two channels that normally broadcast
> in
> high bandwidth in order to transmit HD programs, and several other
> channels that normally broadcast in lower bandwidth and only transmit
> SD programs.  But the actual bandwidth used by each channel can vary,
> so if there are two HD channels, one might be broadcasting at say
> 9000
> kbyte/s and one might be only broadcasting at 8000 kbytes/s.  The
> same
> applies to SD channels - if they want to squeeze in an extra channel,
> they can reduce the quality of the video on the other channels on the
> multiplex by doing heavier compression, and fit in the extra channel.
> That does reduce the visual quality of the other channels, and when
> that happens, they may well get complaints about it.
> In the USA, the right to use one multiplex seems to be assigned to
> one
> company, and only they can transmit on that frequency.  They
> typically
> seem to have one main flagship channel, broadcasting in HD, and
> several other channels that they transmit, including possibly a +1
> hour copy in SD of their HD channel.  Some of the other channels can
> be produced by that company, or they may have a deal with other
> providers to transmit channels for them.  From that seems to come the
> naming of "main" and "sub" channels, with the flagship HD channel
> being called the "main" channel and the others called the "sub"
> channels.  But there is no difference in how each channel is
> transmitted over the multiplex.  Here in New Zealand (DVB-T), we have
> several multiplexes that broadcast "main" channels from multiple
> different companies, as well as two multiplexes that work similarly
> to
> the US model.
> As for your padding question, the usual thing that happens when you
> have more tuners available is that there is no need for back-to-back
> recordings to take place on the same multirec tuner.  So if the
> recordings that used to have to be back-to-back are now recorded on
> two different multirec tuners, the RecordPreRoll and RecordOverTime
> settings that used to be ignored will now be applied, and you will
> get
> 60 seconds of preroll and 60 seconds of postroll (or whatever you
> have
> those set to).  Where there are not enough multirec tuners available,
> the scheduler will have to do back-to-back recordings on the same
> multirec tuner and will not apply those settings between the two
> recordings, although the RecordPreRoll will be applied to the start
> of
> the first recording and RecordOverTime will be applied at the end of
> the last recording.  If this is not how you want it to work, you can
> adjust those two settings to 0 so that the only pre- and postroll
> that
> happens will be from the recording rules.
> Since you are saying that you do not have any pre- or postroll
> settings at all, then the above is not an explanation, although I
> would suggest that you check the settings again just to be sure there
> are none on the affected recordings.  However, there is one other
> thing that mythbackend does that might explain it.  Tuners take a
> little while to be started up.  That timing varies between tuners,
> and
> also between multiplexes and channels within a multiplex.  So
> mythbackend starts a tuner up a bit earlier than it is needed, I
> think
> about 60 seconds early, so that it will be ready when the time for
> the
> recording to start comes along.  I think it actually starts the
> recording as soon as the tuning process completes, rather than
> waiting
> until the exact moment that it is scheduled to start recording.  So
> you can get somewhere between 60 and 0 seconds of preroll from that.
> There is no corresponding late stopping - recordings get stopped when
> they are scheduled to be stopped, taking into account all the
> postroll
> settings.
> It would be unusual for your RecordPreRoll and RecordOverTime
> settings
> to be 0 unless you have set them to be that at some time in the past.
> There are good reasons for them to be defaulted to 60 seconds each.
> It
> is possible that a MythTV upgrade has changed those settings, so it
> would pay to check them.  The mythbackend code has default values for
> settings that it uses when there are no values in the database for
> those settings.  So if those defaults were 0 and a MythTV upgrade
> added settings for RecordPreRoll and RecordOverTime to the database,
> that could have changed how things work.
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thanks again, yes i have checked those settings both at zero and always
have been since i set them years and they remained. i understand what
multirec is. i am only saying that in the us, in my market, it is very
infrequent that i would want to record a second channel on the same
frequency (channel). i realize that there are more than one broadcast
streams on a tuned frequency. but in my area, there is only one hd
broadcast stream per frequency and the others, what i am call sub-
channels, are not content that i would be recording usually, with a
rare exception of an old movie here and there and in those cases it is
not likely i am looking for something on the hd channel (stream) that
is on that frequency. i am in los angeles / usa. 

my problem is not preroll, i am only getting 30 seconds tacked on to
the end. if i missed a setting it would happen on every recording. this
only happens on same channel when recordings are back to back, nothing
else. also i can also see in the hdhomerun_config app on a computer
when the tuner starts, and in my case on a hdhomerun, it comes up /
tunes the channel right at the time it is scheduled, these hdhomeruns
tune almost instantly. clearly you know a lot more about this than i
do, but nothing in your explanation explains what happens, it is a
minor thing. i may, just to see, disable all multirecs and go to just 4
tuners 1 encoder each tuner  and see if it continues to happen. 

thanks a lot for the help.
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