Re: [Discuss-gnuradio] Block with N inputs and M outputs (M>=N)

[Michael Dickens]

Achilleas - I did not have the gr_sync_interpolator code in front of  
me when I wrote originally, so it was a guess ... which was not  
entirely correct (the interpolation must be an -integer- value, while  
yours is not guaranteed to be so).  Thus, yes, you will need to  
inherit from "gr_block" and, since M and N are reasonably small, you  
can use "set_output_multiple(M)" to guarantee a multiple of N input  
items when you use write "forecast()" to handle this.  For example,  
if "d_M" and "d_N" are the values of M and N at instantiation, then ...

1) In your instantiator, include:
+++
// Set the approximate output rate / input rate
  set_relative_rate (1.0 * d_M / ((double) d_N));
// Set the output multiple to guarantee a multiple of M
  set_output_multiple (d_M);
+++
2) In forecast (), include:
+++
// Check that the number of requested output items is a multiple of M
  assert (noutput_items % d_M == 0);
// find the number of input items required to
// generate the requested number of output items
  int ninput_items =  d_N * noutput_items / d_M;
// set all streams' input requirements
  unsigned ninputs = ninput_items_required.size ();
  for (unsigned i = 0; i < ninputs; i++)
    ninput_items_required[i] = ninput_items;
+++
3) In general_work (), include:
+++
// Check that the number of requested output items is a multiple of M
  assert (noutput_items % d_M == 0);
// find the number of input items required to
// generate the requested number of output items
  int ninput_items = d_N * noutput_items / d_M;
+++
to get the number of input items as make sure it really is a multiple  
of N (via that the # of output items is a multiple of M).  You can,  
of course, choose other means for doing this which are more in your  
coding style.  And the asserts can be dropped (I use #if's) once  
you're happy that it's all working as you want.  Hope this is clear!  
- MLD