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Diffstat (limited to 'stm32/src/tone.c')
| -rw-r--r-- | stm32/src/tone.c | 151 |
1 files changed, 151 insertions, 0 deletions
diff --git a/stm32/src/tone.c b/stm32/src/tone.c new file mode 100644 index 0000000..ab56792 --- /dev/null +++ b/stm32/src/tone.c @@ -0,0 +1,151 @@ +/*! + * 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 <[email protected]> + * 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; +} |