[mythtv] mythtv-setup - DVB channel scanning

John Pilkington J.Pilk at tesco.net
Fri Mar 3 22:17:12 UTC 2017


On 03/03/17 21:03, Roger James wrote:
> On 3 March 2017 2:28:37 pm David Matthews <dm at prolingua.co.uk> wrote:
>
>> On 02/03/2017 11:31, roger wrote:
>>>
>>>
>>> On 01/03/17 18:19, John Pilkington wrote:
>>>> On 01/03/17 15:30, David Matthews wrote:
>>>>> On 27/02/2017 12:41, roger wrote:
>>>>>>
>>>>>>
>>>>>> On 27/02/17 10:22, Stuart Auchterlonie wrote:
>>>>>>> On 23/02/17 14:34, roger wrote:
>>>>>>>> I have been delving into DVB-T channel scanning in mythtv.
>>>>>>>>
>>>>>>>> When using "full scan (tuned)" and the transport port scans with
>>>>>>>> "search
>>>>>>>> new transports" with a DVB_T tuner we often see the scanner
>>>>>>>> trying to
>>>>>>>> tune transports that are not physically receivable by the
>>>>>>>> hardware. I
>>>>>>>> looked into this and found that the code was not honouring the
>>>>>>>> other_frequency_flag in the TerrestrialDeliverySystemDescriptor
>>>>>>>> contained in the per transport stream loop in the Network
>>>>>>>> Information
>>>>>>>> Table (NIT). This flag means that this particular transport
>>>>>>>> stream is
>>>>>>>> also available on frequencies other than the one specified in the
>>>>>>>> TerrestrialDeliverySystemDescriptor itself. Information on these
>>>>>>>> other
>>>>>>>> frequencies information should be found a
>>>>>>>> FrequencyListDescriptor for
>>>>>>>> this transport. The ETSI EBU standards mandate that this flag
>>>>>>>> must be
>>>>>>>> set if a ChannelListDescriptor is present but not that a
>>>>>>>> ChannelListDescriptor must be present if this flag is set, which
>>>>>>>> seems a
>>>>>>>> little weird.
>>>>>>> Not information but some theories. Since the flag is meant to
>>>>>>> indicate
>>>>>>> that the mux is available on other frequencies, but it doesn't
>>>>>>> include
>>>>>>> the frequencies, then it's probably trying to signal that this mux
>>>>>>> could
>>>>>>> be seen on multiple frequencies when doing a full scan.
>>>>>>>
>>>>>>> I'm guessing it is trying to give the receiver a heads up that it
>>>>>>> needs
>>>>>>> to do some differentiation and decide on the "best" mux. Although
>>>>>>> why it
>>>>>>> would need a flag to do that i dunno....
>>>>>
>>>>> My feeling is that a tuned scan is right for DVB-S but not for
>>>>> DVB-T in
>>>>> the UK and that the only way to pick up all the multiplexes is a full
>>>>> scan over all the frequencies.  In particular the definition of the T2
>>>>> delivery system descriptor in the NIT allows the frequency to be
>>>>> optional and last time I looked it wasn't being included.  That means
>>>>> that a receiver has to do a scan in order to find T2 multiplexes and
>>>>> can't find them from the NIT.
>>>>>
>>>>> See
>>>>> https://www.dvb.org/resources/public/standards/a38_dvb-si_specification.pdf
>>>>>
>>>>>
>>>>> table 129.  The bandwidth, guard_interval and frequency information
>>>>> are
>>>>> only included if descriptor_length > 4.
>>>>>
>>>>> David
>>>>
>>>> A comment as a user:
>>>>
>>>> My pctv-290e autoswitching tuner, given only basic parameters for the
>>>> main DVB-T2 mux from Waltham (East Midlands), automatically tunes all
>>>> the DVB-T muxes but not the second DVB-T2 mux used for BBC FOUR HD.
>>>> That needs to be specified separately. Now I see why.
>>>>
>>>> But sometimes I relocate and use the Sandy Heath transmitter. The
>>>> signal I get there isn't usually as good and that procedure has
>>>> occasionally yielded 0-byte recordings.  A full scan followed by
>>>> manual mux selection seems to give more reliable results there.
>>>>
>>>> Mostly I record SD channels.
>>>>
>>>> John P
>>>>
>>> Hi John,
>>>
>>> Looking through some old docs. I found that the frequency list
>>> descriptor had actually been removed from UK radiated DVB-T signal in
>>> the 2007 switchover. At a guess that was because it was not really
>>> proving useful in the UK DVB set up, and they wanted STB and DTV
>>> manufacturers to switch to full frequency scans by default.
>>>
>>> The old version 7 D book, which you can find online, also has useful
>>> information in it.
>>>
>>> A idea for myth might be to add a geographical scan option and use the
>>> ofcom dtv transmitter data table
>>> https://www.ofcom.org.uk/spectrum/information/transmitter-frequency.
>>> <https://www.ofcom.org.uk/spectrum/information/transmitter-frequency>
>>> to scan for frequencies in  a particular radius.
>>>
>>> This should easy to implement. (But not by me!)
>>>
>>> Roger
>>
>> I have wondered in the past about improving the support for DVB-T scans.
>>   At present if there are duplicate entries the code seems to just
>> select the first it finds.  It would be better if it could select the
>> entry with the stronger signal based on S/N ratio or signal strength.
>>
>> I've had problems with scanning myself in the past.  I can pick up most
>> multiplexes from both Craigkelly and Black Hill but the current aerial
>> gets a better signal from Craigkelly.  It's further complicated because
>> Craigkelly appears to just relay everything from Black Hill and, from
>> what I recall, that includes the NIT.  So the multiplex frequencies in
>> the NIT from Craigkelly are actually those for Black Hill.  Perhaps this
>> is what John was finding with Sandy Heath.
>>
>> All this suggests that for DVB-T in the UK the only way to do a scan is
>> by a full scan and the NIT should be completely ignored.  I'm not
>> convinced that geographical information would be very helpful because
>> there can be low-power relays that cover very localised areas and in
>> hilly areas such as Scotland the choice of transmitter to use can depend
>> on which side of the hill one is located.
>>
>> David
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>
> Probably the best way here in the UK to set up multiplex frequencies is
> the to use the digitaluk postcode checker.
>
> http://www.digitaluk.co.uk/operations/about_the_coverage_checker
>
> This uses a sophisticated transmission path analysis model, described here.
>
> http://downloads.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP048.pdf
>
> Anybody want to try hacking up an app that takes a post code and scrapes
> the coverage  checker to find the best multiplex. Then pulls the latest
> transmitter data from ofcom and puts this directly into the Mythtv
> multiplex table.
>
> Roger

I can't estimate how difficult this would be to code but I doubt that 
it's worth much effort;  there's too much variability in the users' 
aerial characteristics and regional preferences for a unique approach. 
Full scan with editing would still need to be an option.

I would like a working 'import previous settings' facility, but that 
probably wouldn't help many people.  A full scan takes only a few 
minutes and choosing the 'best' muxes might be needed only once.

... but don't let me stop anyone who wants a project :-)

John P




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