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An exemplary explanation of the Split NIT problem on UK DTTV.

From a usenet post by GlynM.

Newsgroup: uk.tech.broadcast



MPEG/DVB transport streams not only contain the digitised program components
but also carry a number of data tables that tell the receiver about services
and service components contained in the received stream and any associated
streams.

One of these tables is the network information table, or NIT.

The NIT gives details of all the streams (muxes) being radiated from a main
transmitting station and its daughters. For UK DTT it contains:


The network name - which is the name of the main station

Then for each mux:

Terrestrial delivery descriptor giving the frequency and modulation
parameters.

Frequency list descriptor which lists the alternate frequencies that any
daughter Tx relays use for this mux.

Service list descriptor which lists the service id numbers for all the
services on the mux.

A UK specific Logical Channel Number, LCN, descriptor that defines the EPG
service numbers for the services on this mux.

A UK specific service attribute descriptor which defines if any of the
services on this mux are hidden from the EPG.


The NIT therefore contains information about all the muxes from that main Tx
station and therefore the same NIT is carried by each mux.


Now lets get to the problem:

MPEG/DVB tables are divided up into sections. For the NIT the maximum size
of the section is 1024 bytes. The NIT table can be different sizes in
different regions dependant on the number of relays and the number of
services which vary between England and the Nations.

Up until recently all the NITs fitted into one section. However a couple of
years ago the expansion of the platform caused some regions NITs to exceed
1024 bytes so expanded into a 2nd section. This is a split NIT, and
highlighted problems with some non-DVB compliant receivers. (Multi section
NITs are completely MPEG/DVB compliant but were not being radiated when some
receivers were being developed.)

I believe that all the NITs have now been forced to span two sections to
bring the receiver issues to a head. Without these issues being addressed
expansion of the platform is very constrained.

Some receivers can't cope with the NIT spread across more than one section.
Some just use the first section they see, others overwrite the first section
with the second section. Receivers with the Setpal firmware refuse to work
at all with multi section NITs.

The NIT is split on a transport stream, mux, boundary and at the moment
probably has 5 muxes in the first section and the 6th mux in the second
section. Therefore receivers that overwrite the sections have problems with
the data for the last defined mux or the first five depending on which way
they fail. This usually manifests itself as messing up the logical channel
numbers for either the services on one mux or five muxes.

I hope this makes sense.

Glyn



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