[mythtv] mythtv-setup - DVB channel scanning

Stuart Auchterlonie stuarta at squashedfrog.net
Fri Mar 3 10:45:21 UTC 2017

On 02/03/17 16:24, Roger James wrote:
> Are you referring to the ones here.
> https://www.linuxtv.org/downloads/dtv-scan-tables/

Yep, although i would expect most mythtv systems to
have them already installed under /usr/share/dvb


> Roger
> On 2 March 2017 3:23:18 pm Stuart Auchterlonie
> <stuarta at squashedfrog.net> wrote:
>> On 02/03/17 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!)
>> I would like to be able to pull the dvb-scan data files and use those
>> as data sources for scans, primarily for satelliite users where a
>> "full scan" isn't an option. You have to know where to find at least
>> one frequency, and then process the NIT to find others.
>> Although given the various different formats between the different
>> versions, it may be simplest to convert those files to a bunch of
>> xml/json which the channel scanner can then load which would provide
>> a good set of frequencies to use.
>> Regards
>> Stuart
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