[Discuss-gnuradio] if uhd_transmitter instantiated after receiver, TX works, otherwise RX works

[George Nychis]

Hi all,

I'm observing some odd behavior that I can't seem to figure out.  This is in (your favorite script) tunnel.py

Here, self.sink is instantiated as a uhd_transmitter() and then self.source is a uhd_receiver().  Both are connected respectively to their part of the flow graph:

https://github.com/gnychis/gnuradio-3.5.0-dmr/blob/b3b9be2380553706f152fbf4ffee6804aef8525c/gr-digital/examples/ofdm/tunnel.py#L54

The weird thing that I am observing is that whichever (uhd_transmitter or uhd_receiver) is instantiated _last_ is the part of the flow graph that operates properly.  If the receiver is instantiated last, the receiver is able to receive packets (from another device) without a problem, but the transmitted packets are unable to be decoded by a standalone receiver (a different device running a receiver only).  It is actually transmitting (we see power out of the antenna), but whatever it is transmitting is undecodable by our receiver.

If you instantiate the transmitter before the receiver, then the transmitter works just fine and our standalone receiver can receive its packets.  But, this breaks the uhd_receiver() running on the transceiving device.

Has anyone else observed this?  I have double checked that both are setting the sampling rates, independent of which is instantiated first (bandwidth is the same)... so I'm a little confused what would make the difference here. 

Thanks a bunch!

- George