Hello Ron,
do you think it would be possible to use your block that detects the
drift to adapt the sample rate of the source block automatically and if
yes, how would I have to do that? When the position of the CP increases,
the sample rate must be decreased and vice versa. Could it be done by a
kind of feedback? This way it would not be necessary to implement the
algorithm for DVB-T. Even if I would like to do this, due to my lack of
knowledge I don't think that I would be able to.
Sample rate correction solves my problem but in order to be useful, it
would have to be done automatically.
Thank you very much and kind regards,
Ralf
Am 02.03.2020 um 13:42 schrieb Ron Economos:
Did you read the README.md of gr-dvbt? It says:
/Late note: As of 2015, I donated the gr-dvbt project to gnuradio. It
is now integrated in the mainline of gnuradio/gr-dtv./
The OFDM symbol acquisition block in gr-dtv has been upgraded with the
fixes from the ISDBT team. See this commit:
https://github.com/gnuradio/gnuradio/commit/761b62d4660a121c78b6a7ad17fd7b08badcbb88#diff-aa5858d955a31c6be8746db56ea13c6a
However, there is still an issue with the block. It can't tolerate a
sample rate difference between the transmitter and receiver. If the
difference is large, the block will fail fairly quickly (minutes).
I have a test OFDM acquisition block that prints out the drift. It can
be found here:
https://github.com/drmpeg/gr-dvbtx
You may have to go back one commit to make it compile with the latest
version of GNU Radio 3.7.
Ron
On 3/2/20 03:58, Ralf Gorholt wrote:
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
