[mythtv] mythtv-setup - DVB channel scanning

John Pilkington J.Pilk at tesco.net
Wed Mar 1 18:19:23 UTC 2017

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

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