Dynamic parameter modification in a custom block

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1

Hello all,

I am dealing with the problem of finding a way to dynamically give
values
to a parameter of a custom block that I 've made in gnuradio
by the output of another custom block.
The solution of function probe that I have found in previous
conversations
in the forum does not fit me as the value to be changed dynamically is
entered ad-hoc by the user.
I will try to simplify my problem to the rest of the community by giving
the following example
let us assume the following flowgraph implementation

signal_source -> custom_block(arg amplitude) -> file_sink (1)

file_source -> custom_block2 -> file_sink (2)

Would it be possible to use as the amplitude argument in (1) the output
of
the custom_block2 in (2).

Thanks
-George

2

Hi George - I believe there are 2 possible solutions, though I’ve never
used or tested either myself. If you just want to verify that what you
want to do works, I think (1) should be quite straight forward.

  1. This method will certainly work, but it’s not very robust : Make your
    “custom_block2” take an argument which is of the “custom_block1” smart
    pointer type, such that the amplitude can be set any time the former
    performs the computation to do so. Instantiation of the “custom_block2”
    would take a smart pointer to the “custom_block1”, and then store that
    reference for future use; because this is a C++ object, just use “.” or
    “->” to access the object’s variable or method.

  2. I don’t know if stream tags are in place well enough yet, nor if they
    would work in the manner you require. The idea is that there’s a tag
    plane (sort of like a control plane) that “overlays” (or is in parallel
    with) the graph (data-flow) plane, where tags can be inserted – a
    timestamp, or a value that might get used downstream. Since
    “custom_block1” isn’t downstream from “custom_block2”, tags might not
    work. Maybe there’s a branch for this sort of non-downstream async
    control?

Hope this helps; good luck! - MLD

3

Any ideas guys?

---------- Forwarded message ----------
From: George S. [email protected]
Date: Mon, Jun 4, 2012 at 12:08 AM
Subject: Dynamic parameter modification in a custom block
To: [email protected]

Hello all,

I am dealing with the problem of finding a way to dynamically give
values
to a parameter of a custom block that I 've made in gnuradio
by the output of another custom block.
The solution of function probe that I have found in previous
conversations
in the forum does not fit me as the value to be changed dynamically is
entered ad-hoc by the user.
I will try to simplify my problem to the rest of the community by giving
the following example
let us assume the following flowgraph implementation

signal_source → custom_block(arg amplitude) → file_sink (1)

file_source → custom_block2 → file_sink (2)

Would it be possible to use as the amplitude argument in (1) the output
of
the custom_block2 in (2).

Thanks
-George

–
Sklivanitis Georgios
Postgraduate Student
Telecommunications Division
Technical University of Crete
ECE Dept.
Tel. : 28210 59257

4

Thank you so much for the response Michael.
I will try to figure out what’s going on with both ways and I will post
updates to the list too.

Thanks,
George

5
  1. I don’t know if stream tags are in place well enough yet, nor if
    they would work in the manner you require. The idea is that there’s
    a tag plane (sort of like a control plane) that “overlays” (or is in
    parallel with) the graph (data-flow) plane, where tags can be
    inserted – a timestamp, or a value that might get used downstream.
    Since “custom_block1” isn’t downstream from “custom_block2”, tags
    might not work. Maybe there’s a branch for this sort of
    non-downstream async control?

:slight_smile:

Heres the work:

And a little coding guide:

-josh

6

On Mon, Jun 4, 2012 at 5:30 PM, Michael D. [email protected]
wrote:

Hi George - I believe there are 2 possible solutions, though I’ve never used or
tested either myself. If you just want to verify that what you want to do works, I
think (1) should be quite straight forward.

  1. This method will certainly work, but it’s not very robust : Make your
    “custom_block2” take an argument which is of the “custom_block1” smart pointer
    type, such that the amplitude can be set any time the former performs the
    computation to do so. Instantiation of the “custom_block2” would take a smart
    pointer to the “custom_block1”, and then store that reference for future use;
    because this is a C++ object, just use “.” or “->” to access the object’s variable
    or method.

George,
That would work, but it’s very unsafe to do something like that. For
testing or a simple program, sure, but don’t rely on it as a method.
If something goes out of scope at the wrong time or you close one
object that is sharing state with another, you’ll find yourself in
trouble.

  1. I don’t know if stream tags are in place well enough yet, nor if they would
    work in the manner you require. The idea is that there’s a tag plane (sort of like
    a control plane) that “overlays” (or is in parallel with) the graph (data-flow)
    plane, where tags can be inserted – a timestamp, or a value that might get used
    downstream. Since “custom_block1” isn’t downstream from “custom_block2”, tags
    might not work. Maybe there’s a branch for this sort of non-downstream async
    control?

Hope this helps; good luck! - MLD

I’m not sure stream tags are right for this application because they
are in two different flowgraphs. Stream tags start at one end of a
flowgraph and move downstream.

There is a message passing interface where a block has a message
handler function. The trick you’re going to have to deal with is how
to tell one block about another. This has been a problem for a while
that I hope we can sort out this year in a way that’s robust (i.e.,
thread safe and won’t suffer from scope issue if one block has a
pointer to another).

Tom