Hi Ralf,
Since the Viterbi Decoder has to wait for a tag to start (and is thus a general block), I've used TPP_DONT as its tag propagation policy. Furthermore, we also send a tag whenever a new frame begins under "normal operation" (see https://github.com/git-artes/gr-isdbt/blob/56b2556c14ecc5d710070f969fda7a2deae65d8b/lib/viterbi_decoder_impl.cc#L815).
I've checked https://github.com/gnuradio/gnuradio/blob/main/gr-dtv/lib/dvbt/dvbt_viterbi_decoder_impl.cc and it seems that GNU Radio's current code does not proceed similarly. Maybe that's the problem?
best
El jue, 20 feb 2025 a las 5:30, Ralf Gorholt (<ralf.gorholt@gmx.de>) escribió:
Hi Federico,
I can imagine that you are very busy, so don't worry :-)
In most cases reception is fine. However, I have noticed difficulties (already in the past) that are not related to the recognition of the OFDM symbols. It might be that I have now found the reason why but I don't know GNU Radio well enough yet. Perhaps you or somebody else knows someone who could help me? Let me explain the problem in a few words.
The Viterbi Decoder needs the "superframe_start" tag to start decoding. When it detects a superframe start, it creates this tag again (although it should to my knowledge be propagated to the downstream blocks by default). The next block in the chain, the Convolutional Deinterleaver, also needs this signal. That's where the problem arises.
Sometimes it happens that the Convolutional Deinterleaver gets a "superframe_start" somewhere in the middle when there is no start of a superframe (I have been able to trace this using Tag Debug blocks). That causes the reception to freeze immediately. When I provoke a complete reset then (e.g. by changing the frequency back and forth), a re-synchronization of all blocks is triggered and everything is fine again. Unfortunately, when I change the tag propagation policy to TPP_DONT in the Viterbi Decoder, only the first synchronization works (at offset 0).
Of course I can open a bug report for GNU Radio, but in this case I would prefer being able to present a solution (if possible). This is a very special case that is difficult to reproduce and trace because it can happen after some seconds or even hours. I also doubt that many users use GNU Radio for DVB-T reception. It might be only some friends of mine and me who would like to do this in Amateur Radio.
Kind regards,
Ralf
Am 19.02.2025 um 21:30 schrieb Federico 'Larroca' La Rocca:
Hi Ralf,
It's great to see that our work is useful and you're willing to undertake its migration to DVB-T. Sadly, I'm afraid I won't have time to check the code. However, if you're still receiving a nice constellation after a sampling error rate (using a channel model) the code should be just fine.
BTW, what would make me very happy is to migrate the whole gr-isdbt into the core of GNU Radio, if that's interesting for the project. I'm not sure how to proceed if we were to undertake this migration. Maybe someone on the list can provide me with pointers?
bestFederico
El jue, 6 feb 2025 a las 18:09, Ralf Gorholt (<ralf.gorholt@gmx.de>) escribió:
Dear all,
I have just released a very first version of the source code of my OOT
module gr-dl5eu. This is the first time that I publish my work, so
please forgive me if it does not correspond to or if I have not
respected all written or unwritten rules ;-)
This module contains two blocks, dvbt_ofdm_synchronization and
dvbt_tps_decoder. They are based on two blocks that Federico La Rocca
has developed for ISDB-T.
gr-dl5eu is work under development and I encourage particularly radio
amateurs who are interested in DVB-T reception with GNU Radio to test
them. There is no documentation yet, but there are comments in the code.
Here is just a short description of what the blocks do.
The first block, dvbt_ofdm_synchronization, recognizes the OFDM symbols
of a DVB-T signal and delivers them to the second block. Sampling rate
differences between the transmitter and receiver are interpolated so
that reception should be stable (at least when the received signal is
stable).
The second block, dvbt_tps_decoder, decodes the TPS information that is
transmitted within the DVB-T signal and outputs the payload carriers
(that are fed into the block DVB-T Demap). dvbt_tps_decoder can send
messages containing the detected signal parameters to other blocks.
You will find two simple sample flowgraphs in the examples directory.
Please note that I am no DSP guy and that I actually haven't fully
understood how the blocks work exactly. I had to copy parts of the code
from the GNU Radio tree because it was either inaccessible or needed
some modifications.
The reason why I have developed these blocks is that friends of mine are
looking for a simple and cheap solution for DVB-T reception with low
bandwidth in amateur radio, However, I would be happy if others also
find my work useful.
Federico, when you have the time, would you take a look at my code?
Without your work I would not have been able to create my blocks!
Kind regards,
Ralf
