[Discuss-gnuradio] Manually decoding output from ZMQ PUB Sink

[Kjetil Wormnes]

Hi everyone,

Firstly, just a disclaimer. I am  particularly familiar with GnuRadio at all. However I need to interface with a GnuRadio project.

I am trying to manually decode the output of a ZMQ PUB Sink. In other words, I have, in a completely different non-gnuradio project, a bit of code that uses the standard python-zmq library to subscribe to the zmq socket.

The ZMQ PUB Sink block is sitting just before the USRP block in the Tx chain. And what I eventually want to do is effectively replicate the spectrum display from Gnu Radio in a web app.

In my long google searches someone said the trick to decode things from gnuradio is to use the PMT library. Now from what I could see I should use pmt.deserialize_string(). However that does not seem to work.

>>> pmt.deserialize_str(d)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/pmt/pmt_swig.py", line 3373, in deserialize_str
    return _pmt_swig.deserialize_str(str)
RuntimeError: pmt::deserialize: malformed input stream, tag value = : 243

I thought perhaps there would be an alignment issue, but it doesn't seem to matter where (in d) I start the deserialize command.

By the way, d above is a recording of what I saw come out of that ZMQ PUB Sink block. I created a python-zmq socket and connected it to the ZMQ Sink Block port and recorded everything. You can find the file here:
https://www.dropbox.com/s/9r6390lwd3kdc9o/port_5551.bin?dl=1

Any pointers would be most appreciated. Thanks in advance.

Kjetil