Noutput_items

GNU Radio
1

Hi,
I’m confused about what is the return value of the work function if I
have
multiple output ports?
I use produce() method to determine how many items produced on each
port,
so what does it mean to return again a number from work if I produced
different number of items for each output port??

best,
Mostafa

2

Hi Mostafa,

a few things:

  1. work() only is meaningful for sync and interpolator/decimator blocks;
    then, you always have a fixed relation between consumed and produced
    items, and you can’t have a different number of output items on
    different ports.
  2. noutput_items is the parameter of the work function that tells you
    how many items you can produce maximally. It’s not the number you
    produced; in the (performance-wise) best case, you return noutput_items
    to tell that your block consumed and produced everything GNU Radio asked
    it to. If you produced less, you return exactly that amount.
  3. When using produce(), you should return the special value
    WORK_CALLED_PRODUCE.
  4. If you’re having different output item numbers on different ports,
    you’re bound to use a general block with general_work. Then you’ll have
    to consume() at each input the amount of items that you’ve used and
    produce them at each output. Also, you should implement a forecast()
    method just to give GNU Radio an idea how many items you need on which
    input port if you have to produce a certain number of output items on
    different ports.
  5. Most of the time, blocks described by 4. are complicated. I used to
    write blocks like that, but nowadays I rarely do, because the message
    port API of GNU Radio fits my designs’ needs better, most of the time.
    When dealing with things that are basically packets of digitally
    modulated data, take a look at the Tagged Stream / PDU infrastructure as
    described in the GNU Radio doxygen.
  6. I’ve seen some screenshots of defunctional flowgraphs where the
    developer tried to implement correction loops (e.g. phase correction
    loop) by outputting an error/correction estimate on one output port and
    the corrected samples on the other, feeding back the correction to a
    multiplier upstream. Don’t fall into that trap! In GNU Radio, you can’t
    have sample loops, since this violates causality requirements (a block
    can’t produce output without input, but within a loop there is no input
    without output). Just mentioning this because it can lead to a lot of
    frustration :wink:

Greetings,
Marcus

3

Thank you so much for your complete explanation. Just the term
maximally
and WORK_CALLED_PRODUCE works for me :slight_smile:

best,