On 27/10/2022 07:36, Marco Spanghero wrote:
Hi,
I am currently facing a problem using the C API with a USRP N210 r4. I
am trying to stream a int16_t interleave IQ file. When using the
included C++ tx_samples_from_file demo, the baseband is streamed
without interruptions or other issues. The BB is received at the DUT
and the symbols seem correct.
Unfortunately, integration with a bigger software requires usage of
the C API. What is unclear is how does the C and C++ API compare in
handling the samples? Seems to me there is no direct mapping between
the std::complex<short> and interleaved int16_t for IQ. When analyzed
with a spectrum analyzer, the baseband streamed look completely
different.
Details on the implemented functions and problem:
Issue Description
I am using the C API to stream a binary data file. The file is saved
as interleaved int16_t [I][Q] samples. I am using an N210r4 with the
latest UHD and fpga image. MTU on the NIC is configured at 3000,
buffer size is 10000 samples. The same file, when streamed with the
included tx_samples_from_file example works fine and the baseband is
received correctly. When the equivalent code is written in C, the
baseband is not correct.
*Setup Details*
_Implemented C code streaming loop:_
while (1) {
if (stop_signal_called)
break;
uhd_tx_metadata_make(&md, false, 0, 0.1, false, false);
size_t read = fread(buff, sizeof(int16_t), samps_per_buff, file);
I think your problem might be ^^^^^^^^^^^^^^^
You probably meant "sizeof(int16_t)*2"
for(int i = 0; i < read; i++){
printf("%d \n", buff[i]);
}
if(read > 0){
uhd_tx_streamer_send(tx_streamer, buffs_ptr, read, &md,
0.1, &num_samps_sent);
total_num_samps += num_samps_sent;
}
else
break;
if (verbose)
printf("\n Sent %ld - from file %ld\n ", total_num_samps,
read);
}
buff containes the data block to stream and is defined as buff =
malloc(samps_per_buff*sizeof(int16_t));
_C metadata_
uhd_stream_args_t stream_args = {
.cpu_format = "sc16",
.otw_format = "sc16",
.args = "",
.channel_list = 0,
.n_channels = 1};
_Reference C++ streaming loop:_
void send_from_file(
uhd::tx_streamer::sptr tx_stream, const std::string& file, size_t
samps_per_buff)
{
uhd::tx_metadata_t md;
md.start_of_burst = false;
md.end_of_burst = false;
std::vector<samp_type> buff(samps_per_buff);
std::ifstream infile(file.c_str(), std::ifstream::binary);
// loop until the entire file has been read
while (not md.end_of_burst and not stop_signal_called) {
infile.read((char*)&buff.front(), buff.size() *
sizeof(samp_type));
size_t num_tx_samps = size_t(infile.gcount() / sizeof(samp_type));
md.end_of_burst = infile.eof();
const size_t samples_sent = tx_stream->send(&buff.front(),
num_tx_samps, md);
if (samples_sent != num_tx_samps) {
UHD_LOG_ERROR("TX-STREAM",
"The tx_stream timed out sending " << num_tx_samps <<
" samples ("
<< samples_sent << " sent).");
return;
}
}
infile.close();
}
_Reference C++ metadata_
uhd::stream_args_t stream_args("sc16", "sc16");
channel_nums.push_back(boost::lexical_cast<size_t>(channel));
stream_args.channels = 0;
uhd::tx_streamer::sptr tx_stream = usrp->get_tx_stream(stream_args);
Expected Behavior
I would expect the two samples to perform exactly the same. The
baseband should be identical
*Actual Behaviour*
Once shown on a spectrum analyzer, the C example shows a much larger
gain an the baseband appears fragmented. I don't understand how the C
api handles the buffer streaming. According to the code the C function
wraps the exact same behavior of the send call.
Steps to reproduce the problem
I can provide source code for the two examples and binary file. Both
samples are executed at 2.5Msps
*Question*
What am I missing here to correctly stream the baseband? My
understanding is that once the data type is fixed in the streaming
metadata, we call uhd_tx_streamer_send with the number of samples that
we want to stream (which is what the C++ example does using as type
std::complex<short>. In the C case, how do we achieve the same behavior?
I would really appreciate any support in this matter.
Best regards
Marco Spanghero
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