Dear all,
please apologize my long email but I cannot explain my
problem and what I have done so far in three words.
I am currently working on a DVB-T receiver project to
receive transmissions on 434 MHz with 2 MHz bandwidth or
less using GNU Radio and an RTL-SDR stick. The flow
graph is based on the examples in gr-dvbt. The
transmitter is a HiDes model HV320E. Reception works,
but the video stops after one minute or so while the
constellation diagram is still active (dots are moving).
I am no expert and have only few knowledge of DSP and
DVB yet, but to me the problem seems to be in the OFDM
symbol acquisition block.
Conditions:
Linux Mint 19.3 in a virtual machine (VMware)
GNU Radio 3.7.11 if I am right (I would need to check at
home)
What I have done so far:
I have created a flow graph for a DVB-T receiver based
on the examples in gr-dvbt. The signal source is an
RTL-SDR stick and the sample rate is 16/7 MHz (=
2.285714 MHz) to get 2 MHz bandwidth. The signal sink is
a TCP server sink to which I connect with VLC media
player to display the received video transport stream.
This works, but after one minute or so the video stops
while the constellation diagram is still active (dots
are moving). I am currently using QPSK, code rate 3/4
and guard interval 1/8 but I have also tried 16QAM, code
rate 1/2 and guard interval 1/8 and have the same
problem.
To track down the source of the problem, I have created
a file outside of GNU Radio that I can use in a file
source instead of the RTL-SDR source of my flow graph.
This allows me to make tests with an input signal that
does not change between different tests.
The data of this file are sent to the OFDM symbol
acquisition block and the output of this block is stored
in a second file. When the input signal, the algorithm
and the parameters do not change between different tests
I expect the generated file to be always the same.
However, this is not the case. The files that are
generated with the output of the OFDM symbol acquisition
block differ from each other. But when I send the
content of one of those generated files to the rest of
the receiver and do this several times, I always get the
same transport stream at the output. I have verified
this by using a file sink and comparing the files.
That’s why I suspect the OFDM symbol acquisition block
to be the culprit.
When I was searching the internet for information
concerning my problem, I found an email from 2015 in the
archive of this mailing list where someone wrote that
the OFDM symbol acquisition block of gr-isdbt should be
used because it worked better than the one in gr-dvbt. I
have done so and installed gr-isdbt from git and
replaced the block. However, there are two such blocks
in gr-isdbt and none of them solves my problem. They
perform even worse than the original block in gr-dvbt
and after some time the program terminates without a
message.
I hope that my explanations are clear enough.
Is there anybody who can tell me what might be wrong
here or what I am doing wrong? Why is the output of the
OFDM symbol acquisition block not always the same when
the start condition of every run is the same? Am I
missing something here?
Thank you very much for your help.
Kind regards,
Ralf