How to change PSK roll-off during runtime (or an arbitrary parameter)

Hi all!

Thank you for contributing to gnuradio and providing new users like me
with help.

I have read and worked through some of the guided tutorials. However I
come to a point now where it seems that own Python code has to be
written. Unfortunately I am more like a C++ guy and would appreciate it
if someone could give me some advice.

Let us please take a look at the QPSK transmitter example of tutorial 6
[1]. It gives a nice example and you can play around with the parameters
of the PSK Mod block in gnuradio-companion. What we would like to do is
to change the roll-off factor during runtime using a Qt GUI Range
element (i.e. a slider control) to demonstrate its effect. As the
associated parameter is not underlined in the properties dialogue I
wonder how this can be done during runtime.

When looking at the generated sourcecode it can be seen that an instance
of digital.psk.psk_mod is created in __init__ method and stored in
element self.digital_psk_mod_0. I tried to add code in the slider’s
callback function def set_excess_bw_slider(self, excess_bw_slider).
But it does not show changes in constellation and spectrum plot though
printing debug messages like RRC roll-off factor: 0.15 on the command
line.

I tried:

  • to access the excess_bw property of the digital.psk.psk_mod directly
    to be modified
  • to create a new instance of digital.psk.psk_mod in the callback
    function and overwriting the original one
    (in C++ this would be bad practice probably leading to memory leaks
    and might be the same for Python)

I did not really find the documentation of the digital.psk.psk_mod
class, except at [2]. And in the sources themselves [3].

A short hint where to take a closer look would be great!

Thank you in advance
Matthias

[1]
https://raw.githubusercontent.com/gnuradio/gr-tutorial/master/examples/tutorial6/gr-tutorial-qpsk-tx.grc
[2] http://www.reynwar.net/gnuradio/sphinx/digital/blocks.html
[3]
http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-digital/python/digital/psk.py

Matthias,

I can see how this would be a pretty cool demo. Off the top off my head,
some thoughts:

  • The RRC is implemented as a FIR in C++ (a pfb_arb_resampler_ccf, to be
    precise). But this doesn’t have a method to update the taps :confused:
  • An update-able filter would be interesting, though. If you want to
    take a crack at it, people here would surely be interested.
  • You’d need to consider how to handle history (what happens when you
    change the number of filter taps?). Not sure what the best way is here.

M

On 17.08.2015 07:38, Matthias W. wrote:

Let us please take a look at the QPSK transmitter example of tutorial
the slider’s callback function `def set_excess_bw_slider(self,
I did not really find the documentation of the digital.psk.psk_mod

[2] http://www.reynwar.net/gnuradio/sphinx/digital/blocks.html

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