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Re: [Discuss-gnuradio] DVBT project receiver probelm


From: Marcus Müller
Subject: Re: [Discuss-gnuradio] DVBT project receiver probelm
Date: Thu, 26 Nov 2015 18:39:37 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

Hi Frederico, hi Ihab

to add a little background Vector to Stream / Stream to Vector advice:
That's a pretty good trick to add buffering between your hardware and your decoder, because the stream to vector has to accumulate as many samples as fit in one vector, and the buffer between that and the vector to stream will usually hold at least two items, which in this case contain a huge amount of samples. The buffer downstream of stream to vector is going to be huge, too, because stream to vector tells the GNU Radio runtime his big output multiple. That's three large buffers at once.

Another trick you can try is set a "min output buffer" in your hardware source block's "advanced" tab.

Notice that both these tricks actually need large circular buffers, which means they will consume shared memory, which on Linux doesn't grow on trees -- If you get a ENOSPACE (Error, no space left on device), it's not because some filesystem is full, but because you actually ran out of shared memory buffers. You can increase the available shared memory, though:

sysctl -w kernel.shmmax $(( 2**30 ))

Best regards,
Marcus

On 26.11.2015 18:22, Federico Larroca wrote:
Hi Ihab,
Those "restart aquisition" are generated by the ofdm_sym_acquisition block when it does not find a peak on the correlation function he uses to detect the beginning of the OFDM symbol. You should verify there is indeed a DVB signal precisely at 570 MHz. To do this you may use our "Sym Align OFDM" block (not a very nice name) available with gr-isdbt (https://github.com/git-artes/gr-isdbt) and check that you see a "triangle" (please refer to the readme, the examples, and even the issues on our git). The symbol alignment is totally equivalent between DVB-T and ISDB-T.
Moreover, I see several "O"s, meaning that somewhere samples are being dropped. That does not help either. If you do not have a very fast machine, you may use at the beginning of the flowgraph a "Stream to Vector" followed by a "Vector to Stream" with a very big "Num Items" (some millions).
best
Federico

2015-11-26 12:59 GMT-03:00 Ihab Zine <address@hidden>:

Hi,

I’m trying to receive a live signal from an antenna on the roof through a cable connected to my SDR (hackrf one ).The bandwidth of the DVB-T in ireland is 8 Mhz , and the parametetrs Here in ireland as following:

Bandwidth: 8 MHz
Stream code rate (hi prio): FEC 2/3
Stream code rate (lo prio): FEC 2/3
Modulation: QAM 64
Transmission mode: 8k mode
Guard interval: 1/32
Hierarchy: none

net_data_rate = 24.128 Mbits/s

I used dvbt_rx_demo_8k_QAM64_rate78.grc flow graph with the following changes :

  • frequency of hackrf Mhz, sampling rate = ((8*8e6)/7).

  • Cyclic Prefix to (8192/32, is this right?)

  • Code Rate HP : 2/3

  • Code Rate LP : 2/3

  • Guard Interval : 1/32

The problem i have is that no output data coming out after the demod_reference_signals BLOCK(no cancellation map), and got the following:

restart aquisition

OOrestart aquisition

d_freq_offset: -5

Od_freq_offset: -5

d_freq_offset: -5

d_freq_offset: -5

d_freq_offset: -5

d_freq_offset: -5

d_freq_offset: -5

d_freq_offset: -5

restart aquisition

OOd_freq_offset: -5

d_freq_offset: -5



so I had to add a signal source to shift the signal. After shfting the signal i got the following :

restart aquisition

restart aquisition

restart aquisition

restart aquisition

Orestart aquisition



I have read a lot about DVB , mapping ,ofdm and others, i have spent weeks on this but still the same problem, if it is synchronization problem what i should do?

So please i need a some help with this problem .



Thanks

Ihab



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