Hi, Although I don't have an answer to Ralf's question, if the sampling rate seems to be a problem, gr-isdbt's OFDM Synchronization block (with "Interpolate" set to "yes") corrects the sampling rate; see https://iie.fing.edu.uy/publicaciones/2016/LFGGB16/ for an explanation of the algorithm (although it expects different tags and parameters, so that's probably why it won't work out of the box with the rest of the gr-dtv blocks or a DVB-T transmission). As Ron wrote, the rest of the block is mostly the same as in gr-dtv. best Federico
El lun., 2 mar. 2020 a las 9:44, Ron Economos (<w...@comcast.net>) escribió: > 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 > >
