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<div class="moz-cite-prefix">On 01/03/17 18:19, John Pilkington
wrote:<br>
</div>
<blockquote
cite="mid:f51bffd6-7a02-909e-842a-90f62cc9d0dc@tesco.net"
type="cite">On 01/03/17 15:30, David Matthews wrote:
<br>
<blockquote type="cite">On 27/02/2017 12:41, roger wrote:
<br>
<blockquote type="cite">
<br>
<br>
On 27/02/17 10:22, Stuart Auchterlonie wrote:
<br>
<blockquote type="cite">On 23/02/17 14:34, roger wrote:
<br>
<blockquote type="cite">I have been delving into DVB-T
channel scanning in mythtv.
<br>
<br>
When using "full scan (tuned)" and the transport port
scans with
<br>
"search
<br>
new transports" with a DVB_T tuner we often see the
scanner trying to
<br>
tune transports that are not physically receivable by the
hardware. I
<br>
looked into this and found that the code was not honouring
the
<br>
other_frequency_flag in the
TerrestrialDeliverySystemDescriptor
<br>
contained in the per transport stream loop in the Network
Information
<br>
Table (NIT). This flag means that this particular
transport stream is
<br>
also available on frequencies other than the one specified
in the
<br>
TerrestrialDeliverySystemDescriptor itself. Information on
these other
<br>
frequencies information should be found a
FrequencyListDescriptor for
<br>
this transport. The ETSI EBU standards mandate that this
flag must be
<br>
set if a ChannelListDescriptor is present but not that a
<br>
ChannelListDescriptor must be present if this flag is set,
which
<br>
seems a
<br>
little weird.
<br>
</blockquote>
Not information but some theories. Since the flag is meant
to indicate
<br>
that the mux is available on other frequencies, but it
doesn't include
<br>
the frequencies, then it's probably trying to signal that
this mux could
<br>
be seen on multiple frequencies when doing a full scan.
<br>
<br>
I'm guessing it is trying to give the receiver a heads up
that it needs
<br>
to do some differentiation and decide on the "best" mux.
Although why it
<br>
would need a flag to do that i dunno....
<br>
</blockquote>
</blockquote>
<br>
My feeling is that a tuned scan is right for DVB-S but not for
DVB-T in
<br>
the UK and that the only way to pick up all the multiplexes is a
full
<br>
scan over all the frequencies. In particular the definition of
the T2
<br>
delivery system descriptor in the NIT allows the frequency to be
<br>
optional and last time I looked it wasn't being included. That
means
<br>
that a receiver has to do a scan in order to find T2 multiplexes
and
<br>
can't find them from the NIT.
<br>
<br>
See
<br>
<a class="moz-txt-link-freetext" href="https://www.dvb.org/resources/public/standards/a38_dvb-si_specification.pdf">https://www.dvb.org/resources/public/standards/a38_dvb-si_specification.pdf</a>
<br>
table 129. The bandwidth, guard_interval and frequency
information are
<br>
only included if descriptor_length > 4.
<br>
<br>
David
<br>
</blockquote>
<br>
A comment as a user:
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
Mostly I record SD channels.
<br>
<br>
John P
<br>
<br>
</blockquote>
Hi John,<br>
<br>
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.<br>
<br>
The old version 7 D book, which you can find online, also has useful
information in it.<br>
<br>
A idea for myth might be to add a geographical scan option and use
the ofcom dtv transmitter data table<br>
<a
href="https://www.ofcom.org.uk/spectrum/information/transmitter-frequency">https://www.ofcom.org.uk/spectrum/information/transmitter-frequency.</a><br>
to scan for frequencies in a particular radius.<br>
<br>
This should easy to implement. (But not by me!)<br>
<br>
Roger<br>
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