[Discuss-gnuradio] UHD transmission timing control is quite tricky yet.

[Sangho Oh]

I am trying to achieve two synchronized transmissions from two USRP2s.

Both USRPs are connected to each host PC and feed with 5PPS clocks, without 10Mhz reference clock.

But I cannot make it happen yet.

1. I have synchronized two USRP2s using clocks from GPS pps, and wait for 1 sec

   sdev->set_time_next_pps(uhd::time_spec_t(0.0));

   boost::this_thread::sleep(boost::posix_time::seconds(1));

2. Trigger the transmission of a stream of packets (10M samples = 500 packets) for both usrps

    -  with time delay 3 secs

 

   for (size_t i = 0; i < num_packets; i++){

       md.start_of_burst = true;     
       md.end_of_burst   = (i == num_packets-1);  
       md.has_time_spec  = (i == 0);              

       md.time_spec = uhd::time_spec_t(uhd::time_spec_t(3.0));
       size_t samps_to_send = std::min(total_num_samps - samps_per_packet*i, samps_per_packet);
       size_t num_tx_samps = dev->send(&buff.front(), samps_to_send, md,uhd::io_type_t::COMPLEX_FLOAT32,uhd::device::SEND_MODE_FULL_BUFF, seconds_in_future + 0.1);

However, the two transmissions are not synchronized well. There is a significant timing offset between the two transmissions, which is hundreds of milliseconds.

I can hardly control the transmission timing, and the concept on the timing control is pretty obscure to me yet. 

- I used a shell scrip from another host to trigger the transmissions for 2 usrps. Timing delay between the two executions is < 0.2-0.3 sec