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/* -*- c++ -*- */
/*
* Copyright 2002 Free Software Foundation, Inc.
*
* This file is part of GNU Radio
*
* GNU Radio is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2, or (at your option)
* any later version.
*
* GNU Radio is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with GNU Radio; see the file COPYING. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street,
* Boston, MA 02110-1301, USA.
*/
#include <GrAtscBitTimingLoop2.h>
#include <algorithm>
#include <atsc_consts.h>
#include <stdio.h>
#include <assert.h>
static const int DEC = 2; // nominal decimation factor
static const unsigned AVG_WINDOW_LEN = 256;
static const float TIMING_RATE_CONST = 1e-5; // FIXME document interaction with AGC
GrAtscBitTimingLoop2::GrAtscBitTimingLoop2 ()
: VrDecimatingSigProc<float,float> (1, DEC),
next_input(0), dc (0.0002), mu (0.0), last_right(0), use_right_p (true)
{
history = 100; // spare input samples in case we need them.
#ifdef _BT_DIAG_OUTPUT_
fp_loop = fopen ("loop.out", "w");
if (fp_loop == 0){
perror ("loop.out");
exit (1);
}
fp_ps = fopen ("ps.out", "w");
if (fp_ps == 0){
perror ("ps.out");
exit (1);
}
#endif
}
//
// We are nominally a 2x decimator, but our actual rate varies slightly
// depending on the difference between the transmitter and receiver
// sampling clocks. Hence, we need to compute our input ranges
// explictly.
int
GrAtscBitTimingLoop2::forecast(VrSampleRange output,
VrSampleRange inputs[]) {
/* dec:1 ratio with history */
for(unsigned int i=0;i<numberInputs;i++) {
inputs[i].index=next_input;
inputs[i].size=output.size*decimation + history-1;
}
return 0;
}
inline float
GrAtscBitTimingLoop2::filter_error (float e)
{
return e; // identity function
}
int
GrAtscBitTimingLoop2::work (VrSampleRange output, void *ao[],
VrSampleRange inputs[], void *ai[])
{
iType *in = ((iType **)ai)[0];
oType *out = ((oType **)ao)[0];
// Force in-order computation of output stream.
// This is required because of our slightly variable decimation factor
sync (output.index);
// We are tasked with producing output.size output samples.
// We will consume approximately 2 * output.size input samples.
unsigned int ii = 0; // input index
unsigned int k; // output index
// We look at a window of 3 samples that we call left (oldest),
// middle, right (newest). Each time through the loop, the previous
// right becomes the new left, and the new samples are middle and
// right.
//
// The basic game plan is to drive the average difference between
// right and left to zero. Given that all transitions are
// equiprobable (the data is white) and that the composite matched
// filter is symmetric (raised cosine) it turns out that in the
// average, if we drive that difference to zero, (implying that the
// average slope at the middle point is zero), we'll be sampling
// middle at the maximum or minimum point in the pulse.
iType left;
iType middle;
iType right = last_right;
for (k = 0; k < output.size; k++){
left = right;
iType middle_raw = produce_sample (in, ii);
iType middle_dc = dc.filter (middle_raw);
middle = middle_raw - middle_dc;
iType right_raw = produce_sample (in, ii);
iType right_dc = dc.filter (right_raw);
right = right_raw - right_dc;
if (use_right_p) // produce our output
out[k] = right;
else
out[k] = middle;
}
#ifdef _BT_DIAG_OUTPUT_
float iodata[8];
iodata[0] = 0;
iodata[1] = out[k];
iodata[2] = 0;
iodata[3] = 0;
iodata[4] = 0;
iodata[5] = mu;
iodata[6] = 0;
iodata[7] = 0; // spare
if (fwrite (iodata, sizeof (iodata), 1, fp_loop) != 1){
perror ("fwrite: loop");
exit (1);
}
#endif
last_right = right;
next_input += ii; // update next_input so forecast can get us what we need
return output.size;
}
/*!
* Produce samples equally spaced in time that are referenced
* to the transmitter's sample clock, not ours.
*
* See pp 523-527 of "Digital Communication Receivers", Meyr,
* Moeneclaey and Fechtel, Wiley, 1998.
*/
GrAtscBitTimingLoop2::iType
GrAtscBitTimingLoop2::produce_sample (const iType *in, unsigned int &index)
{
iType n = intr.interpolate (&in[index], mu);
index++;
return n;
}
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