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Re: [Discuss-gnuradio] frequency modulating block.

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] frequency modulating block.
Date: Mon, 29 May 2006 14:04:44 -0700
User-agent: Mutt/1.5.9i

On Mon, May 29, 2006 at 04:09:17AM -0700, seph 004 wrote:
> Hi

> I just had a few questions regarding the frequency modulatig
> block. The current input sample is used in conjuction with the
> sensitivity parameter to determine the new phase value. Is this
> correct?


> I'm just a bit confused as to how the phase calculations and the
> sensitivity parameter relate to the instantaneous frequency output.

The instantaneous frequency would be d(d_phase)/dt.

If you feed the modulator a constant value, you'll get a constant
frequency out.

gr_frequency_modulator_fc::work (int noutput_items,
                                 gr_vector_const_void_star &input_items,
                                 gr_vector_void_star &output_items)
  const float *in = (const float *) input_items[0];
  gr_complex *out = (gr_complex *) output_items[0];

  for (int i = 0; i < noutput_items; i++){
    d_phase = d_phase + d_sensitivity * in[i];
    float oi, oq;
    gr_sincosf (d_phase, &oq, &oi);
    out[i] = gr_complex (oi, oq);

  // Limit the phase accumulator to [-16*pi,16*pi]
  // to avoid loss of precision in the addition above.

  if (fabs (d_phase) > 16 * M_PI){
    double ii = trunc (d_phase / (2 * M_PI));
    d_phase = d_phase - (ii * 2 * M_PI);

  return noutput_items;

> Secondly, I suppose this is a more general question about the vector
> and file source blocks. At what rate are they producing data? For a
> non repeating vector source of n values, how can I detemine the
> exact amount of time it takes for all the values to be used?

They produce data at whatever rate the downstream blocks consume it.
No part of GNU Radio has any a priori knowledge of sample rate.  It's
just samples in / samples out.  Sources or sinks that are tied to
external hardware such as an audio card or USRP will consume or
produce at the rate that they're configured to run at.


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