[mythtv] mythtv-setup - DVB channel scanning

[Roger James]

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