Re: [Discuss-gnuradio] Variable rate output blocks

[Eric Blossom]

On Fri, Jul 04, 2003 at 08:33:20PM +0100, S?rgio Rui Silva wrote:
> Hi all,
> 
> I want to create a block that accepts a fixed rate input and
> generates a variable rate output. For example if I have 1000
> input samples each time the work function is called, the block
> will generate a random number of output samples that can go from
> 0 to 1000. What is the correct and best way to do this with GNU
> Radio.
> 
> S?rgio Rui Silva 

As long as your work method calls ...

   sync (output.index)

... you may return less than the number of samples requested.

You'll want to create a forecast method that indicates how much input
has been consumed.  Without a custom forecast method, the input and
output rates are identical.

E.g.,

class GrMyClass : public <foo> {

  ...
  VrSampleIndex                         d_next_input;
  ...

};

GrMyClass::GrMyClass ()
{
  ...
  d_next_input = 0;
  ...
}

int
GrMyClass::forecast (VrSampleRange output,
                     VrSampleRange inputs[]) 
{
  for(unsigned int i=0;i<numberInputs;i++) {
    inputs[i].index=next_input;
    inputs[i].size=output.size;
  }

  return 0;     // OK
}  

int 
GrMyClass::work (VrSampleRange output, void *ao[],
                 VrSampleRange inputs[], void *ai[])
{
  float  *in = ((float **) ai)[0];
  float *out = ((float **) ao)[0];

  sync (output.index);  // force in-order computation

  int  number_of_outputs_to_compute;
  int  r = some_random_function ();
  int  k;

  number_of_outputs_to_compute = std::min (output.size, r)

  for (k = 0; k < number_of_outputs_to_compute; k++){

    out[k] = <the next output value>;

  }

  d_next_input += <number of input samples consumed>;

  return k;
}


Take a look at gnuradio/src/gnu/lib/dtv/GrAtscBitTimingLoop2.{h,cc}
for a complete example.

Eric