[mythtv] mythtv-setup - DVB channel scanning

[roger]

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.
>>
>> I plan to implement the following algorithm. I would appreciate comments
>> on this.
>>
>> 1. If the other_frequency flag is present then (conditional on the usual
>> stuff)  then go to step 2. otherwise use the centre frequency from the
>> TerrestrialDeliverySystemDescriptor.
>>
>> 2. If the frequency I am actually tuned to is the same as the centre
>> frequency from the TerrestrialDeliverySystemDescriptor then use it
>> otherwise go to step 3.
>>
>> 3. If a FrequencyListDescriptor is present then go to step 4 otherwise
>> use the centre frequency from the TerrestrialDeliverySystemDescriptor.
>>
>> 4. Add all the frequencies from the list to the list of new transports.
>> Need to check if I can just do this blindly without ending up with
>> duplicates when a new transports is tuned.
>>
>>
>> I may have to extra work for this to work here in the UK. I have checked
>> the streams coming from my local transmitter. They have the
>> other_frequency_flag set but do not contain any
>> FrequencyListDescriptors. I know that they used to. Because I have seen
>> them in traces. Has anyone got any information on this?
>>
>>
> 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....
>
>
> Regards
> Stuart
>
>
>
Hi Stuart,

I subsequently found a reference to this in an obscure old document I 
have attached to this message.

FrequencyList descriptors were removed from  the UK radiated SI in 2007.

So now here in the UK the flag tells us that the freqeuncy in the TDS 
descriptor is probably wrong if you are tuned to a local relay 
transmitter. But now you have no hints to what the correct one is. So 
now you probably have to use a geographical location and a database. 
However some cheeky blighter has managed to get a patent on that 
process, https://www.google.com/patents/WO2011161582A1?cl=en. 
<https://www.google.com/patents/WO2011161582A1?cl=en>

Roger
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