Hello Federico,
after a lot of thinking I have finally managed to adapt your OFDM
Synchronization and TMCC Decoder blocks for DVB-T :-) The problem was
that the frame and symbol indexes in the tags sent by the TPS decoder
were not correct.
Although I can see the video now, I am not sure that I have really done
everything right, because I have still some problems of understanding.
Perhaps you can bring some light into the dark?
I have connected GUI number sinks to the freq. error and samp. error
outputs of the OFDM Synchronization block to display the values and I
have noticed the following:
1) With interpolation in the OFDM Synchronization block turned on, when
I adjust the frequency of the source block (frequency correction ppm) so
that the freq. error output value is near to zero, this value drifts
very slowly in one direction, from higher to lower values, crossing zero
(i.e. from positive to negative values).
2) The value at the samp. error output is zero and constant (at least I
have seen no change during about 30 minutes).
Is this the correct behaviour? If not, how should it be?
Thank you very much for your help and kind regards,
Ralf
Am 11.12.2021 um 16:11 schrieb Ralf Gorholt:
Hello Federico,
I have a lot of difficulties to generate the output and the tags for
the blocks that follow my TPS Decoder in the flowgraph. That's why I
had the idea to take a different approach.
The OFDM Synchronization block outputs the payload carriers (1705 in
2K mode) whereas the Demod Reference Symbols block that normally
follows the FFT expects 2048 carriers. My question sounds certainly
silly but would it be possible to fill in the missing carriers with
zero carriers (amplitude and phase == 0) to get 2048 carriers again
and thus be able to use the original DVB-T blocks? As far as I know,
in DVB-T the carriers at the borders are all set to zero (and perhaps
the one in the middle too). Perhaps this would be too simple to be
true :-)
Regards,
Ralf
Am 10.12.2021 um 14:41 schrieb Ralf Gorholt:
Hi Federico,
indeed, the "symbol_index" tag that is normally sent for each OFDM
symbol is missing. This might cause an unexpected situation for the
following deinterleaver block, "Access not within mapped region"
according to valgrind.
I will see how I can generate the symbol index for each symbol. The
"Demod Reference Signals" block uses the dvbt_pilot_gen object to
parse the input data and to generate symbol and frame indices.
As far as I have understood, in your block OFDM Synchronization you
have combined the symbol acquisition and the FFT. Then you connect a
TMCC decoder that eliminates the TMCC and auxiliary carriers,
generates tags and outputs the data carriers. My idea was to do the
same for the TPS signals in DVB-T.
To my surprise, in principle what I have done seems to work. If not,
my TPS decoder would not be able to correctly decode the TPS
information that changes accordingly when I change settings in the
transmitter, for instance the modulation scheme.
I will focus on the tags and see what is missing.
Regards,
Ralf
Am 10.12.2021 um 14:25 schrieb 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.gorh...@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
