[mythtv] mythtv-setup - DVB channel scanning

David Matthews dm at prolingua.co.uk
Wed Mar 1 15:30:23 UTC 2017

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.

table 129.  The bandwidth, guard_interval and frequency information are 
only included if descriptor_length > 4.


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