Re: Adaptation of Federico La Roccas ISDB-T blocks for DVB-T

[Federico 'Larroca' La Rocca]

Hi, 

I'd be more than happy to help. A couple of things that come into my mind. 

The OFDM Synchronization block is a combination of our "old" OFDM Symbol acquisition (for a while now it's been part of GNU Radio) and Sync and Channel estimation blocks (which performed equalization and integer frequency correction) . The most important difference is that OFDM Synchronization includes a loop with the estimated channel gains, which in turn is used to estimate the sampling error (plus fine frequency errors). It also indicates some events downstream via tags, just like the older blocks. This new "DVB-T OFDM Synchronization" block should then be a combination, if I'm not mistaken, of OFDM Symbol Acquisition plus Demod Reference Signals (I'm sure Ron will know more on this). 

Anyhow, my point is that you should take a look at the OFDM Symbol Acquisition and Demod Reference Signals blocks in GNU Radio, and check which tags are used and when. Maybe this lack of tags is generating an unforeseen situation on the downstream blocks which generate the segfault? Furthermore, if I'm not mistaken, the pilots in DVB-T (in particular continuous pilots) are not exactly the same as in ISDB-T. Another possibility is that the Demod Reference Signals block is not equivalent to our Sync and Channel estimation block, and further processing is needed for it to be ready for the DVB-T Demap...

best

Federico

El vie, 10 dic 2021 a las 9:55, Ralf Gorholt (<ralf.gorholt@gmx.de>) escribió:

Hi Vasil,

thank you for your message. As I have no experience with GNU Radio and
command line debugging, your hints may be really helpful. I have
attached the gdb and valgrind output to this email.

In the gdb output thread 27 that receives the SIGSEGV is the DVB-T
"Symbol Inner Interleaver" that comes with GNU Radio, not one of my blocks.

As far as valgrind is concerned, it tells me for my block OFDM
Synchronization: "Conditional jump or move depends on uninitialised
value(s)". I will see if I can find out which variable is uninitialized
and how I can get rid of this problem.

Kind regards,

Ralf

Am 10.12.2021 um 12:35 schrieb Vasil Velichkov:
> Hi Ralf,
>
> On 10/12/2021 11.52, Ralf Gorholt wrote:
>> Unfortunately, when I deactivate the original flowgraph, it does no
>> longer work and I get a -11 return code.
> The "-11" value means that you got a segmentation fault and the process was kill with signal 11 (SIGSEGV)
>
> https://docs.python.org/3.8/library/subprocess.html#subprocess.CalledProcessError.returncode
> https://www.man7.org/linux/man-pages/man7/signal.7.html
>
> In my opinion the easiest way to debug segfaults is to run the flowgraph under gdb and valgrind. Open your flowgraph in the gnuradio-companion and then Generate (F5) but do not Execute (F6). The open a terminal, go where the flowgraph python (.py) file was generated and execute
>
>   gdb -ex run --args /usr/bin/python3 test.py
>
> and then when it stops execute `bt` command in the gdb's shell and provide the full output. To run it under valgrind execute
>
>   valgrind --tool=memcheck /usr/bin/python3 test.py
>
> Adjust the path to your python interpreter and its version if needed.
>
> Regards,
> Vasil