[mythtv-users] schedule howto

James Linder jam at tigger.ws
Fri Dec 8 21:18:34 UTC 2023



> On Dec 9, 2023, at 00:27, Mike Perkins <mikep at randomtraveller.org.uk> wrote:
> 
> On 08/12/2023 15:56, Stephen Worthington wrote:
>> On Fri, 8 Dec 2023 20:55:31 +0800, you wrote:
>>> G’day all
>>> playing with the rules I can solve once, but everytime is a pain …
>>> 
>>> Aprogram (news on Australian ABC) scheduled to record from 1900 to 2000 (ish)
>>> Bprogram scheduled 1930 to 2230
>>> 
>>> Aprogram (daily) can be on any combination of 2, 20, 21, 22, 23
>>> My hardware causes some virtual channels to sometimes record broken recordings (badly pixelated) as a result I’ve set <no> virtual channels.
>>> As suggested I put “prefer HD”
>>> 
>>> Now Aprogram records on tuner one, two, three, four
>>> One recording of News is [red] conflict
>>> One recording of Bprogram is [red] conflict
>>> 
>>> Wunce-upon-a-time I recall a rule “record 1 showing only”. That with “Prefer HD” would completely solve my problem. What is the solution? Better tuners, they are Haupauge USB dual tuners.
>>> 
>>> James
>>> 
>>> PS I can't give prority to any 2, 20-23, that channel may not carry Aprogram
>> How sure are you that the pixelation is caused by virtual tuners? Most
>> people do not have that problem, and there are plenty of users of
>> Hauppauge USB tuners.  The way that virtual tuners work is just to
>> allow more than one set of streams to be recorded at the same time
>> from the same tuner.  So the tuner hardware gets told to send streams
>> a,b, and c to the PC for one channel and streams f, g, and h for
>> another channel.  It sends all those streams at once to the CPU.  The
>> CPU sorts out which streams are for which channel and writes them to
>> separate recording files.  The only problem you could get from getting
>> the tuner to send more streams is if the total bit rate exceeded the
>> maximum for USB 2.0, which is not possible for a DVB-T transmitter,
>> especially an Australian one that operates on 7 MHz bandwidth where
>> the rest of the world mostly uses 8 MHz.
>> When you set virtual tuners to 0, all you are doing is telling
>> mythbackend to record from effectively one virtual tuner, in other
>> words, record all the streams for one channel only from the tuner. But
>> the same code is used as when multiple virtual tuners is specified.
>> So your problem is more likely to be at the other end of things - if
>> you write too many recordings at once to a hard drive, you will
>> eventually get to the point where the heads will not be able to move
>> between the file locations fast enough to keep up, and mythbackend's
>> recording buffers will overflow before all the data can be written to
>> disk.  Which causes gaps in the data, which you see as pixellation and
>> missing bits, and it also causes glitches in the sound.  If this is
>> happening, you should be getting bad recordings for all the recordings
>> going to the overloaded hard disk during the time it was overloaded. I
>> have been there and done that.
>> So how many recordings are you writing to disk when you are getting
>> pixilated recordings?  How many disks are they going to?  How fast are
>> the disks?
>> If this is what your problem is, then the solution is normally just to
>> had another hard drive to the PC.  I have 7 recording drives in order
>> to prevent this problem.  To be completely sure not to get overloaded
>> hard drives, I work on the basis that there should be only 2
>> recordings at once going to one hard drive.  That then allows for
>> playback of another recording from the same drive while those two
>> recordings are in progress.  In theory, the faster more recent hard
>> drives should be able to cope with 3 recordings at once plus playback,
>> or maybe more, but I have never tested that.  I still have 2 old hard
>> drives that probably can not do that, but I keep them as they are very
>> quiet compared to more modern ones, so I can use them for the
>> relatively few overnight recordings and not have the hard drive noises
>> keep me awake.
>> Along with recordings and playback, there can be other hard disk
>> activity for commercial skip processing.  Modern CPUs can normally
>> cope with doing commercial skip processing in real-time from the
>> in-memory recording buffers on the basis of one core per recording.
>> Such processing does not cause any extra hard disk activity.  If you
>> are making more recordings at once than you have CPU cores, you
>> normally set the number of commercial skip processes to the number of
>> cores, and the extra recordings get scheduled for later processing. In
>> which case, the commercial skip processes will be reading back those
>> recordings from hard disk at a speed significantly greater than for
>> playback or recording.  And that can interfere with any recordings
>> going on at the time as the heads will be moving back and forth quite
>> a bit.  I do not have this problem as I do not do any commercial skip
>> processing - it does not work usefully on any New Zealand channels
>> that I know of.
> There is a possible confusion here between virtual /tuners/ and virtual /channels/. I agree that whether a tuner is 'virtual' or not is going to make any difference, it is all the same code.
> 
> However, virtual /channels/ could be a different problem, depending on specific language. In the US I know they call those 'subchannels' and it is likely that some or most have lower bitrates to cram more subchannels in. I don't know if the same applies in AU, but that could be the problem.
> 
> Alternatively, it is simply possible that the signal is not strong enough and sometimes drops out. Perhaps a tweak to the antenna direction, check all the joints for soundness and/or water, etc? That might go a long way to solving the problem.
> 

How I wish sane logic would prevail.

Here be what happens:

With Virtual Tuners about 1 recording a week is ‘damaged’ with bad pixelation
With my bizare setting I’ve seen NO damaged recordings in 5 months.
All the storage is SSD
OS and all the associated stuff is on a samsung 980 M2 (2T)
All myth including Videos is on a samsung QVO SATA (4T)
CPU is Ryzen 7 5700U (8 core) with 24G ram (I use this box as as workstation during the day)

The Oz TV (successfully !) channels defeat commercial skipping by changing the algorithm even for a single show  so I disable all commercial skipping.

Please wax lyrical on virtual channels. How could I tell a virtual channel from a mux?

Definitely there is some fiddling:
The usual bitrate on 1080 is cica 1000 Kb/s
Last year ABC had a live documentary of coral spawning where they raised the bitrate to over 2mb/s
I did not check what they did on other channels to compensate

[diversion] coral spawning is special: on 1 moonlit night per year at the right time ALL the coral spawn. On the barrier reef that means simultainous spawning over 1000s km.

There is no logic on damaged channels. It could be the sole recording being made.
I have tested 8 simultainous recordings all without any problem.

I use an external antenna some 10 km line-of-sight to the transmitter farm. We have various ‘filler’ (low power) transmitters round Perth. I carefully pruned them from my list.

James




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