[mythtv-users] Tuning problems

Stephen Worthington stephen_agent at jsw.gen.nz
Sat Dec 29 06:44:32 UTC 2018


On Fri, 28 Dec 2018 16:11:59 +0000, you wrote:

>Hi all,
>
>I've had tuning issues for months now, but we mainly watch downloaded 
>content, so I've not worried too much about it.
>
>However, there's not much point using MythTv without the TV bit! :-) And 
>I know how great it can be when it's working, so I'd like to sort this 
>out if I can.
>
>Here's the issue ...
>
>To keep things simple, I've got 2 SD tuner sticks installed at the 
>moment. I've got a HD one waiting to be used too, but I'm not too 
>worried about HD TV. I'd rather just get the 2 SD sticks working again.
>
>I'm in the UK and should be (at least we used to be!) in the Mendip area.
>
>Here are the details hoe how Mendip 'should' tune ...
>https://ukfree.tv/transmitters/tv/Mendip
>
>The settings are different to what they were a few years ago, but to me 
>it looks like I the main channels are at 562000000Hz, 578000000Hz.
>
>When I do a full scan, with MythTV, it gives me transports like ...
>698000000 Hz,
>738000000 Hz
>690000000 Hz
>721833330 Hz
>
>Auto turn always results in finding channels, but they are temperamental 
>and unreliable.
>
>This wouldn't be a problem as MythTV always seemed to tune to the wrong 
>transports. The problem is that if I edit the transports, like I have in 
>the past, to mathc up frequencies, then the system fins nothing!
>
>Back when things used to work, manually editing the transports would 
>solve everything and I'd have rock solid tuning. however, that's not 
>been the case for along time now.
>
>Any ideas how to get to the bottom of this?
>
>I should mention that if I need to go to the command line, my level is 
>'ls' and 'top'! :-) I can follow instructions, but have no idea what I'm 
>doing in there.
>
>One final thing to mention is a new error message that I'm noticing when 
>I go into the Multiplex editor. It's stated saying:
>"Sorry, the Transport Editor can only be used to edit transports which 
>are connected to a card input."
>
>However, it then seems to work fine.
>
>Hope some of you geniuses can help! :-)
>
>Cheers,
>Damian

First, you have a slight misconception as to SD and HD tuners.  There
is no such thing.  I am guessing that you mean DVB-T when you say SD
and DVB-T2 when you say HD, but that is not correct.  To broadcast an
HD channel requires more bandwidth than an SD channel, but it is
entirely possible to do it on a DVB-T transmitter - you just wind up
being able to broadcast usually only one HD channel on the frequency,
with only one or two more SD channels.  That is what is done here in
New Zealand.  The DVB-T2 standard allows higher bandwidth transmission
using the same amount of radio spectrum as a DVB-T transmitter,
allowing multiple HD channels on the same transmitter.  In the UK, I
believe that when the HD channels were added, they were all put on new
DVB-T2 transmitters, and they were done as duplicates of the same SD
channels on the old DVB-T transmitters.  It was done that way so that
people who had DVB-T only receivers (older TVs and set top boxes)
would still be able to receive the SD channel.  In New Zealand, we do
not have as many channels so we just kept using DVB-T and added two
more frequencies and moved some SD channels on to them to allow HD to
be broadcast on the original DVB-T frequencies.  So we do not have
duplicate SD and HD versions of the HD channels, just the HD versions,
but all the older receivers can receive the HD channels as they are
broadcast using DVB-T.

Next, exactly what are you using for your DVB-T tuners?  There are
plenty of DVB tuners that work with Linux, and many that do not work
at all, but there are also quite a few that sort-of work with Linux
but cause problems.  If you have one of the problematic tuners, then
you may need a fixed driver, but if that is not available, then you
would be better off getting a tuner that is in the category of "just
works" with Linux.  Your new DVB-T2 tuner might be one of them, so if
you are having problems with your DVB-T tuners, using the new DVB-T2
one might be the solution.  All DVB-T2 tuners are in fact DVB-T/T2
tuners - they will receive DVB-T or DVB-T2 signals.  The driver
normally has to control the selection between DVB-T and DVB-T2, except
for some of the very first DVB-T2 tuners where the firmware inside the
tuner automatically tried both standards.  The automatic switching was
actually a violation of how the Linux drivers were supposed to work,
but it made the early DVB-T2 tuners work with DVB-T only drivers.

According to the web page you cited, these are the frequencies you
should be receiving, sorted in frequency order:

562000000 DVB-T
570000000 DVB-T
578000000 DVB-T
586000000 DVB-T2
594000000 DVB-T
690000000 DVB-T
714000000 DVB-T
746000000 DVB-T
754000000 DVB-T

The frequencies above are in Hz, as should be stored in your
mythconverg database's dtv_multiplex table.  When you do a scan, the
scan should pause on each of those frequencies (except possibly the
DVB-T2 one) while it locks on and reads the data to tell it what
channels are present.  It may also pause on other frequencies if there
are other transmitter sites it can receive.

If the scan is not seeing those frequencies, there are some settings
to check.  In mythtv-setup, go to Capture Cards and then for each of
your tuners check that Signal Timeout is set to 1000 ms (or more).  If
I remember correctly, that is the default value, but check anyway.
Then check the Tuning Timeout.  That will likely be set too low for
the way things work now.  Around the world, there seem to be some
channels that now need a rather longer timeout.  Here in New Zealand,
we need over 7 seconds for one of our channels ("Three").  I have mine
set to 10000 ms (10 seconds).  Until a couple of versions of MythTV
ago, having this setting too low did not matter too much, as when
mythbackend was tuning a channel, if it timed out there were several
retries and it usually tuned on one of the retries.  Now, if it times
out on this timer, there are no retries and tuning fails.  I am not
sure if this timer affects scanning, but it is best to have it set
properly.

The other thing to check is the frequency table being used for the
scan.  This is able to be set globally and also for each video source,
and is a list of all the frequencies used in a country or region.  It
also sets the bandwidth used for each frequency.  The standard
bandwidth for DVB-T and DVB-T2 in most countries is 8 MHz, but some
countries (Australia for example) use 7 MHz and fit fewer channels on
each frequency, but have more frequencies available.  The bandwidth is
a critical setting for tuning as if it is set wrongly, the tuner will
not tune at all even if the frequency it is tuning to is correct.

The global setting for the frequency table is found in General >
Locale Settings > Channel frequency table.  For the UK, I think it
needs to be set to "europe-west", but I am not sure.  I hope some UK
user on this list can confirm that.  The per video source setting is
found in Video Sources > (your source) > Channel frequency table.  It
is normally set to "default", in which case the global setting gets
used, but you can override the global setting by changing it to
something else.  In New Zealand, the values in the frequency table
have had to be updated a couple of times as new frequencies have
started to be used.  If your channel table is not up to date, then it
is possible the scan my miss some (or all) frequencies.  If you think
that is happening, you can try changing the "Channel frequency table"
settings to "try-all".  That is supposed to check all the DVB-T/T2
frequencies used world wide, so it makes a scan take much longer.  I
am not sure if it checks with both 8 MHz and 7 MHz bandwidth settings
(making it take twice as long), but I would hope that is the case.


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