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/*!
* Fixed-point tone generator.
*
* The code here implements a simple fixed-point tone generator that uses
* integer arithmetic to generate a sinusoid at a fixed sample rate of
* 16kHz.
*
* To set the initial state of the state machine, you specify a frequency
* and duration using tone_reset. The corresponding C file embeds a
* sinusoid look-up table. The total number of samples is computed for
* the given time and used to initialise 'remain', 'time' is initialised
* to 0, and 'step' gives the amount to increment 'time' by each iteration.
*
* The samples are retrieved by repeatedly calling tone_next. This
* advances 'time' and decrements 'remain'. The tone is complete when
* 'remain' is zero.
*
* Author Stuart Longland <me@vk4msl.id.au>
* Copyright (C) 2015 FreeDV project.
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License version 2.1,
* as published by the Free Software Foundation. This program 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 Lesser General Public
* License along with this program; if not, see
* <http://www.gnu.org/licenses/>.
*/
#include "tone.h"
/*! Fixed-point shift factor */
#define TONE_SHIFT (12)
/*! Static compiled quarter-sinusoid. */
static const int16_t partial_sine[] = {
830, 2488, 4140, 5781, 7407, 9014, 10598, 12155,
13681, 15171, 16623, 18031, 19394, 20707, 21967, 23170,
24314, 25395, 26411, 27360, 28238, 29043, 29774, 30429,
31006, 31503, 31919, 32253, 32504, 32672, 32756
};
/*! Length of quarter-sinusoid in samples */
#define TONE_PART_SINE_LEN (sizeof(partial_sine)\
/sizeof(partial_sine[0]))
/*! Total length of sinusoid */
#define TONE_SINE_LEN ((TONE_PART_SINE_LEN*4)+4)
/*!
* Generate a sine from the quarter-waveform.
*/
static int16_t tone_sine(uint8_t sample)
{
/* Key points */
if ((sample % (TONE_SINE_LEN/2)) == 0)
/* Zero crossings */
return 0;
if (sample == TONE_SINE_LEN/4)
/* Maximum */
return INT16_MAX;
if (sample == (3*TONE_SINE_LEN)/4)
/* Minimum */
return INT16_MIN;
if (sample < TONE_SINE_LEN/4)
/* First quarter of sine wave */
return partial_sine[sample-1];
if (sample < (TONE_SINE_LEN/2))
/* Second quarter */
return partial_sine[(TONE_SINE_LEN/2)-sample-1];
if (sample < ((3*TONE_SINE_LEN)/4))
/* Third quarter */
return -partial_sine[(sample-3) % TONE_PART_SINE_LEN];
if (sample < TONE_SINE_LEN)
/* Final quarter */
return -partial_sine[TONE_SINE_LEN-sample-1];
/* We should not get here */
return 0;
}
/*!
* Re-set the tone generator.
*
* @param tone_gen Tone generator to reset.
* @param freq Frequency in Hz, 0 = silence.
* @param duration Duration in milliseconds. 0 to stop.
*/
void tone_reset(
struct tone_gen_t* const tone_gen,
uint16_t freq, uint16_t duration)
{
if (freq)
/* Compute the time step */
tone_gen->step = (((2*freq*TONE_SINE_LEN) << TONE_SHIFT)
/ ((2*TONE_FS) + 1) + 1);
else
/* DC tone == silence */
tone_gen->step = 0;
/* Compute remaining samples */
tone_gen->remain = (uint16_t)(
((uint32_t)(TONE_FS * duration)) / 1000);
/* Initialise the sample counter */
tone_gen->sample = 0;
}
/*!
* Retrieve the next sample from the tone generator.
* @param tone_gen Tone generator to update.
*/
int16_t tone_next(
struct tone_gen_t* const tone_gen)
{
if (!tone_gen)
return 0;
if (!tone_gen->remain)
return 0;
if (!tone_gen->step) {
/* Special case, emit silence */
tone_gen->remain--;
return 0;
}
/* Compute sample index */
uint16_t sample_int = ((tone_gen->sample) >> TONE_SHIFT)
% TONE_SINE_LEN;
/* Advance tone generator state */
tone_gen->sample += tone_gen->step;
tone_gen->remain--;
return tone_sine(sample_int);
}
/*!
* Retrieve the current time in milliseconds.
*/
uint32_t tone_msec(const struct tone_gen_t* const tone_gen)
{
uint64_t ms = tone_gen->sample;
ms *= 1000;
ms /= TONE_FS;
return ms >> TONE_SHIFT;
}
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